Pelvis 11

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11.1

Patient Preparation, Positioning, and Coil Selection

Magnetic resonance (MR) imaging is established as the investigation of choice for most neurological and mus- culoskeletal imaging. Its role in body imaging is less well established, but up-to-date machinery allows good quality, and reproducible images can be obtained from most parts of the body. In general, MR imaging of the pelvis is well-researched, presents few technical problems, and is rapidly gaining acceptance as the technique of choice in many clinical situations, particularly in urological and gynecological oncology. The pelvis is sufficiently distant from the diaphragm that respiration artifacts are minimal. Bowel peristalsis may be modi- fied pharmacologically, and positioning maneuvers and bladder filling can help remove the small bowel from the field-of-view. The major vessels in the pelvis rarely cause clinically distracting artifacts. Demonstration of disease within the pelvis depends on good quality images using sequences that are reliable and reproducible. Therefore, up-to-date instruments operating at high field strength (1.0–1.5 T) are ideally suited to pelvic imaging. Nevertheless, clinically adequate images can often be obtained with low-field systems, usually by extending the imaging times. With a cooperative patient, clinically useful images may be obtained with machines operating at a field strength as low as 0.2 T.

Patient selection for pelvic MR imaging rarely caus- es problems, as few patients are confused or agitated.

Some patients with severe pelvic pain may require shortening of the protocol, but some images are obtain- able in virtually all patients. Whether the images are clinically useful depends on the degree of cooperation between the radiologist and the referring physician, and protocols may vary with the clinical question.

Details of surgical procedures, e.g., cystectomy and gut

Pelvis 11

D. MacVicar, P. Revell

Contents

11.1 Patient Preparation, Positioning,

and Coil Selection . . . 335

11.2 Sequence Protocols . . . 337

11.2.1 General Considerations . . . 337

11.2.2 Nodal Survey . . . 338

11.2.3 Uterus and Cervix . . . 338

11.2.4 Ovary . . . 339

11.2.5 Vagina . . . 339

11.2.6 Prostate . . . 340

11.2.7 Urinary Bladder . . . 340

11.2.8 Anorectal Region . . . 341

11.2.9 External Genitalia . . . 342

11.3 Clinical Applications of Pelvic MR Imaging . . . 343

11.3.1 Uterus and Cervix . . . 343

11.3.1.1 Anatomy . . . 343

11.3.1.2 Congenital Anomalies . . . 344

11.3.1.3 Benign Pathology . . . 344

11.3.1.4 Malignant Disease . . . 346

11.3.2 Parametrium and Ovaries . . . 349

11.3.3 Urinary Bladder . . . 350

11.3.3.1 Anatomy . . . 350

11.3.3.2 Benign Bladder Pathology . . . 351

11.3.3.3 Bladder Carcinoma . . . 351

11.3.3.4 Diffuse Disease of the Bladder . . . 353

11.3.4 Prostate . . . 354

11.3.4.1 Anatomy . . . 354

11.3.4.2 Benign Disease . . . 354

11.3.4.3 Malignant Disease . . . 355

11.3.5 Anorectal Region . . . 357

11.3.5.1 Anatomy . . . 357

11.3.5.2 Benign Conditions . . . 358

11.3.5.3 Malignant Disease of the Anorectal Region . . . . 358

11.3.6 Male External Genitalia . . . 361

11.3.6.1 Penis and Scrotum . . . 361

11.3.6.2 Benign Conditions . . . 362

11.3.6.3 Malignant Conditions . . . 362

11.3.7 Female External Genitalia . . . 363

11.4 Pediatric Pelvic MR Imaging . . . 363

Further Reading . . . 363

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resection, are particularly important and may alter the way in which the patient is prepared for the investigation. Unless they are particularly anxious or claustro- phobic, patients for pelvic MR imaging need no seda- tion. The urinary bladder should not be emptied prior to the investigation, as it is a useful and immediately recognizable anatomical landmark. Ideally, the patient should be instructed, at the time of scheduling, to empty the bladder 2–3 h before the investigation and, subsequently, drink normally. Overfilling of the bladder is also to be avoided, as this can generate patient dis- comfort, particularly if imaging times are protracted.

The usefulness of bowel-marking contrast agents is less clear for pelvic MR imaging than it is for upper abdominal studies. We do not use oral or rectal gut-marking agents on a routine basis. However, if the patient is thin and, therefore, considered unlikely to have adequate natural contrast within the pelvis and lower abdomen to identify bowel loops clearly, oral contrast is sometimes given. Likewise, if the upper-pelvic and retroperitoneal nodal groups are of paramount clinical importance, as is frequently the case in the investigation of gynecological malignancy, contrast agents may be given. The agents used may be positive contrast agents, which are designed to increase the signal return from the bowel on T1-W sequences, or negative contrast agents, which reduce the signal from the bowel. An example of a positive contrast agent is gadolinium-die- thylene triamine penta-acetic acid (Gd-DTPA; Oral Magnevist, Schering). This preparation is largely water based and contains mannitol. The advantages of this preparation are ease and speed of administration, as well as the production of a fairly reliable contrast column, which reaches the pelvis within 30 min under normal circumstances. However, it may induce brisk peristalsis, necessitating administration of a gut-relaxing agent. Several other gadolinium-based preparations are available. An alternative approach is to coat polymer balls of suitable size with a super-paramagnetic iron- oxide preparation, which reduces signal from the bowel, particularly on T2-weighted imaging (T2-WI). An example of this is Abdoscan® (Nycomed). This preparation has a 2-h administration time, but produces a well-distributed contrast column. It is tolerable but unpleasant to drink, and care is needed in sequence selection, as some gradient-echo sequences may be asso- ciated with susceptibility artifacts around the contrast.

Antiperistaltic gut-relaxing agents are given routinely in our department for all pelvic studies that involve

structures above the prostate, including those that involve a nodal survey of the pelvis. The only studies exempt from gut-relaxing agents are those directed to the external genitalia, perineum, or anal canal.

Hyoscine butylbromide (Buscopan, Boehringer Ingel- heim) is a quaternary ammonium compound with anti- muscarinic action causing smooth-muscle relaxation.

Parenteral injection of 20 mg by the intravenous or intramuscular route will reduce peristalsis for 30–40 min. Reflex hyperperistalsis may then ensue, and further doses may be necessary if imaging times are prolonged. Side effects such as dry mouth, impairment of visual accommodation, and hesitant micturition are common. The drug should be avoided in patients with a history of glaucoma. The drug is extremely inexpen- sive.

An alternative muscle relaxant is glucagon, which may be given by subcutaneous, intramuscular, or intravenous injection in a dose of 1 mg (1 mg/ml). The antiperistaltic action lasts longer than hyoscine (usually over 1 h). Nausea and vomiting occur rarely, and hyper- sensitivity reactions have been described, although these are also rare. In the UK, the dose costs approximately 30 times as much as hyoscine, but remains inex- pensive when the cost of the total examination is considered.

Positioning of the patient is influenced by individual preference. In our unit, patients are routinely scanned lying supine. With the bladder adequately filled, the small bowel is displaced from the pelvis, allowing visu- alization of the pelvic organs. Respiratory artifacts are rarely distracting. Some investigators prefer to use the prone position on the basis that the anterior abdominal wall movement during respiration is reduced. In addition, the gut is squashed upward away from the pelvis.

The disadvantage of the prone position is that many patients are less comfortable, and thus patient movement is more likely.

A variety of coils can be used for imaging the pelvis.

In most up-to-date high-field systems, the body coil gives adequate images and is used for surveying the pelvis and retroperitoneum for nodal enlargement in cases of pelvic malignancy. More detailed pelvic anatomy and pathology can be demonstrated by a variety of surface coil designs. Most manufacturers use a system that involves anterior and posterior surface coils. Phased- array electronics are available on many, and the signal- to-noise ratio, spatial resolution, and contrast resolution are excellent. The quest for increased signal-to-

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noise ratio and the convenient placement of pelvic ori- fices has led to the development of a number of intra- cavitary coils, including endorectal coils, endoanal coils, and endovaginal coils. Of these, only the endorectal coil, most frequently used for prostate imaging, is widely used. Some manufacturers offer integrated endorectal and pelvic phased-array coils. The practical utility of increasingly sophisticated coil design will be discussed individually, with reference to the clinical setting, later in the chapter.

11.2

Sequence Protocols 11.2.1

General Considerations

As a general rule, sequence selection in the pelvis involves a compromise between maximizing the signal- to-noise ratio, contrast resolution, and spatial resolution while keeping imaging times to a minimum. The anatomy of the pelvis lends itself to MR imaging, as there are easily recognizable anatomical landmarks separated in most individuals by some fat. Spin-echo (SE) sequences and spoiled gradient recalled echo (GRE) sequences form the basis of most protocols. T1- W SE sequences and T2-W turbo spin-echo (TSE) sequences can be completed on high-power machines with acquisition times of 1–5 min. Since motion artifacts are less problematic than in the upper abdomen, the reliance on very fast scanning techniques is reduced. A wide range of pathologies is found in the pelvis, but malignant tumors and inflammatory conditions, including infection, form the majority of the case load. These conditions generate edema and excess free water within the pathological tissues, and the resulting prolonged relaxation times alter the signal characteristics, increasing disease conspicuity, especially on T2-W sequences. In cases where doubt exists as to whether pathology is present, very high contrast techniques, such as short tau inversion recovery (STIR), will also function satisfactorily in the pelvis. The STIR sequence has the added advantage of suppressing signal from fat.

Other sequences suppressing fat signal include chemical-shift fat-saturation techniques. These are usually performed with T1-W and can be repeated following intravenous gadolinium administration to increase the sensitivity for pathological tissue.

Some pelvic organs, notably the cervix and body of the uterus and the prostate gland, have a zonal anatomy that is clearly demonstrable on T2-W sequences.

SE/TSE sequences also demonstrate urine and other pelvic fluid collections as areas of high signal. As a result, they are frequently the most important single sequence, and if imaging time is limited, for example, by patient claustrophobia, the T2-W sequence is carried out first. The suggested sequence protocols for individual organs within the pelvis may be varied according to the pathology under investigation. Some general advice is given in this section regarding basic imaging techniques, and in the following clinical section, some refinements for specific pathologies are suggested.

In all the sequences where a field-of-view of less than 200 mm is used, a reduced bandwidth is used to main- tain an adequate signal-to-noise ratio. The effect of this produces a chemical-shift misregistration of one pixel between the water and the lipid image. This must be considered in the interpretation of the resultant images.

If the bandwidth is increased to 210 Hz/pixel or greater (at 1.5 T), the water and lipid signals will fall within the same pixel, but the signal-to-noise ratio will be reduced.

The use of spatial presaturation produces significant improvements in the quality of the images obtained in the pelvic region. This is especially true if a phased- array surface coil is being used. In these cases, correct adjustment of the normalization filter (if available) is also necessary.

Imaging sequences in the transverse plane require spatial presaturation of tissue, both proximal and distal to the imaging volume, to saturate the spins of inflow- ing blood and to prevent a variation in the signal returned from blood vessels passing through the image stack. If the sequences are acquired within a breath- hold, this is sufficient; if not, the image quality can be improved by the use of spatial presaturation of the anterior and posterior abdominal-wall subcutaneous fat. In sequences using a small field-of-view, these areas of presaturation should extend from the edge of the imaging volume to beyond the skin surface.

In sagittal acquisitions, the use of anterior and posterior spatial presaturation produces improvement in image quality. As in the transverse plane, the areas should be prescribed up to the limit of the imaging volume or, if necessary, slightly beyond it.

In the coronal plane, spatial presaturation is only necessary if large blood vessels are flowing into the

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imaging volume. Signal normalization should not be used in the coronal plane.

11.2.2 Nodal Survey

A common indication for pelvic MR imaging is staging urological or gynecological malignancy. An integral part of this investigation is surveying the pelvis and retroperitoneum for enlarged nodes (Table 11.1). It is wise to complete this part of the investigation first, as gut relaxation is maximized early in the procedure, and if oral contrast agents have been used, the contrast column is less likely to have broken up. Images of the pelvis are obtained using T2-W and T1-W spoiled GRE (breath-hold) sequences, in the axial plane, from the aortic bifurcation to the inguinal region just below the femoral canal. Slice thickness should be 8–10 mm using a 0-mm to 4-mm gap. This may be varied to accommo- date patients of varying heights. Subsequently, a survey of the retroperitoneum should be carried out. If the pelvis is clear of enlarged nodes, the incidence of metasta- sis to the retroperitoneal nodes from the low pelvic tumors (cervix, bladder, prostate, and rectum) is very low, but metastatic lymph node spread from the body of the uterus or ovaries may occur directly to the retroper-

itoneum. If the pelvis is clear, a single sequence through the retroperitoneum using T1-W in the coronal plane will usually suffice. This should be included routinely in the protocol if there are no suitably trained personnel available to review the pelvis at the time of imaging.

Following this node survey, the local staging of the malignancy should be pursued using the appropriate protocol for the primary site.

11.2.3

Uterus and Cervix

Initially, a localizing sequence should be obtained in the axial plane, using a rapid-acquisition technique that is able to identify the uterus. If a node survey has been completed, one of the axial images of the pelvis may be used as a localizer. Following this, T2-W SE/TSE sequences form the basis of imaging of the cervix and body of the uterus, as the sequences demonstrate the zonal anatomy, sometimes in exquisite detail (Table 11.2). The zonal anatomy is difficult to appreciate on T1-W sequences. A sagittal sequence should be acquired first, from which the orientation of the uterus can be ascertained. Subsequent T2-W images should be obtained in a plane perpendicular to the long axis of the uterus. This will usually be paraxial, but in patients with

Table 11.1. Pulse sequence recommendations for pelvic nodal survey. (Breath-held sequences, each of two interleaved acquisitions) Sequence WI Plane No. of TR TI TE Flip Echo Slice Matrix FOV recFOV Band- No. Acq.

slices (ms) (ms) (ms) angle train thickness (%) width of time

length (mm) acq. (min:s)

GE T1 ax 16 163 – 4.1 75 1 5 256×128 300 62.5 260 1 0:13

TSE T2 ax 16 5000 – 120 – 65 5 256×128 300 62.5 557 1 0:21

Turbo-IR T2 cor 15 4800 150 60 – 11 6 256×176 230 100 130 2 4:38

Table 11.2. Pulse sequence recommendations for uterus and cervix

Sequence WI Plane No. of TR TI TE Flip Echo Slice Matrix FOV recFOV Band- No. Acq.

TSE T2 sag 13 3500 – 120 – 15 5 256×192 180 100 130 2 3

TSE T2 ax 15 4000 – 120 – 15 4 256×192 160 100 130 2 3:46

TSE T1 ax 15 700 – 12 – 3 4 256×192 160 100 195 3 5:02

Imaging plan will vary with orientation of the anatomical structures

Abbreviations: WI weighted image; TR repetition time; TI inversion time; TE echo time; Matrix matrix (phase×frequency matrix); FOV field of view (mm); recFov % rectangular field of view; Acq number of acquisitions

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extreme anteversion of the uterus, this may be closer to a true coronal plane. Occasionally, the uterus may be anteflexed, i.e., there is a considerable angle between the cervix and body of the uterus. In these circumstances, the main clinical question should be addressed, and if the procedure is being carried out, for example, for the staging of a cervical carcinoma, then the imaging plane should be perpendicular to the long axis of the cervix.

If parametrial spread of cancer is suspected, a T1-W sequence perpendicular to the long axis of the uterus can demonstrate tumor spread. If this is equivocal, a fat- suppression sequence, such as STIR, may be helpful;

alternatively, fat suppressed (FS) T1-W images may be acquired before and after gadolinium enhancement.

Coronal plane imaging is occasionally helpful, particularly if disease spread to the vagina is suspected on the initial imaging sequences. Investigation of benign conditions of the uterus, such as leiomyoma and adenomyosis, will usually require a shorter protocol than a preoperative staging procedure for malignancy.

The best quality images of the uterus and cervix are obtained using TSE sequences in a machine operating at 1.0–1.5 T. SE sequences generally perform better than GRE sequences, giving greater clarity of anatomy. Older

and lower power machines using standard SE sequences can still give reasonable image quality, but at a cost of relatively long imaging times.

11.2.4 Ovary

The ovary and adnexal structures are relatively difficult to image with any technology. Ultrasound is frequently the first technique used, but MR is capable of imaging masses and cysts in the adnexa. Once again, T2-W and T1-W sequences will usually demonstrate the pathology adequately (Table 11.3). The axial plane is of paramount importance, and coronal imaging is often helpful in clarifying the relationship of masses to the uterus and vessels. The sagittal plane is of limited use.

11.2.5 Vagina

T2-W SE/TSE images once again form the mainstay of the imaging technique (Table 11.4). Small field-of-view Table 11.3. Pulse sequence recommendations for ovary

TSE T2 ax 15 4000 – 120 – 15 5 256×128 239 75 220 2 3:46

TSE T1 ax 15 700 – 14 – 3 5 256×192 230 75 220 3 5:02

Turbo-IR T2 ax 15 4800 150 60 – 11 6 256×176 230 75 130 1 5:24

TSE T2 cor 11 4000 – 120 – 15 5 256×192 230 100 130 2 3:01

Table 11.4. Pulse sequence recommendations for vagina

TSE T2 sag 13 3000 – 120 – 15 4 256×192 200 100 130 2 3

TSE T2 cor 13 3000 – 120 – 15 4 256×192 200 100 130 2 5:24

TSE T2 ax 13 3000 – 120 – 15 6 256×192 180 100 130 2 3:46

TSE T1 sag 13 600 – 12 – 3 4 256×192 200 100 195 2 3:54

TSE T1 cor 13 600 – 12 – 3 4 256×192 200 100 195 2 3:54

TSE T1 ax 13 600 – 12 – 3 6 256×192 180 100 195 4 5:02

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high-resolution images can sometimes differentiate layers of the vaginal wall. Axial, coronal, and sagittal planes are all of use in demonstrating the relationship of the vagina to the adjacent organs. Coronal images demonstrate the relationship to the levator ani muscle to its best advantage, while axial and coronal images demonstrate the relationship to the rectum and bladder. Tampons should preferably be removed, particularly if soiled, as hemorrhagic debris may obscure diag- nostic detail.

11.2.6 Prostate

The zonal anatomy of the prostate is well-demonstrated by T2-W SE/TSE sequences (Table 11.5). Good contrast can be demonstrated between the inner gland, which is predominantly transition zone, and the outer gland,

which is predominantly peripheral zone. The zonal contrast is lost on T1-W sequences, but pathological entities can be seen to enhance to a greater degree than normal tissue with gadolinium on T1-W sequences. T2- W SE/TSE sequences should be obtained in the axial plane initially. Coronal and sagittal images are useful in staging malignancy. Best results are obtained using a local surface coil, such as a pelvic phased-array coil. We use a small field-of-view high-resolution technique for imaging the prostate and seminal vesicles (Table 11.6).

The impact of endorectal coil imaging on patient man- agement remains under investigation.

11.2.7

Urinary Bladder

A variety of techniques have been employed to study the bladder. T2-W SE/TSE sequences are particularly Table 11.5. Pulse sequence recommendations for prostate

TSE T2 ax 13 4000 – 120 – 15 4 256×162 182 75 130 4 4:20

TSE T2 cor 13 3600 – 120 – 15 4 256×162 160 100 130 4 4:07

Table 11.7. Pulse sequence recommendations for bladder

Sequence WI Plane No. of TR TI TE Flip Echo TD Slice Matrix FOV recFOV Band- No. Acq.

slices (ms) (ms) (ms) angle train (s) thickness (%) width of time

length (mm) acq. (min:s)

TSE T2 sag 15 4000 – 120 – 15 – 5 256×192 180 100 130 2 3:01

TSE T1 sag 15 800 – 14 – 3 – 5 256×192 180 100 195 2 3:54

The imaging plane for investigations of the bladder is dependent on the position of the lesion within the bladder. Sagittal or coronal planes are most commonly used

Abbreviations: WI weighted image; TR repetition time; TI inversion time; TE echo time; TD time delay; Matrix matrix (phase×fre- quency matrix); FOV field of view (mm); recFov % rectangular field of view; Acq number of acquisitions

Table 11.6. Pulse sequence recommendations for prostate (endorectal receiver coil)

TSE T2 ax 15 6000 – 112 – 15 3 256×240 120 100 130 3 4:49

SE T1 ax 15 600 – 17 – – 3 256×256 120 100 130 2 5:10

TSE T2 cor 15 6000 – 112 – 15 3 256×240 120 100 130 3 3:41

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useful for demonstrating the extent of malignant disease within the bladder wall (Table 11.7). T1-W SE/TSE sequences are often critical in demonstrating perivesical spread of tumors when treatment options are being contemplated. The most useful imaging plane can often only be established once the site of disease within the bladder has been identified. We commence imaging with an axial T2-W sequence, unless there is cystoscop- ic evidence that there is a small tumor lying at the bladder base or at the dome, where it is unlikely to be adequately visualized by axial imaging. Fat-suppression sequences can be extremely useful for demonstrating perivesical spread. It should be noted that the bladder is particularly susceptible to chemical-shift artifacts, because of the markedly different signal characteristics of urine, bladder wall, and perivesical fat, and sequence selection should reflect this fact.

11.2.8

Anorectal Region

Two main indications for MR imaging of the anorectal region have developed in recent years (Tables 11.8, 11.9). The first is imaging of inflammatory disease and fistula formation. Axial and coronal images can be used to demonstrate the muscles of the pelvic floor, and the coronal plane is particularly important in demonstrating the extent of inflammatory conditions such as fistulae. Penetration of the pelvic floor by a fistula is most reliably detected by MRI. A variety of inversion recovery sequences have been tried, and STIR images are the most widely used, supplemented by T1-W SE sequences. Some authors recommend the use of gadolinium to demonstrate fistula extent.

The second major indication is in the preoperative staging of rectal carcinoma. Initial imaging is in the sagittal plane, from the perineum to the sacral promon- tory. Most rectal tumors will be visible, and once local-

Table 11.8. Pulse sequence recommendations for anorectal region

TSE T2 sag 13 4000 – 120 – 15 4 256×162 160 100 130 3 3:01

TSE T2 cor 13 4000 – 120 – 15 4 256×162 160 100 130 4 5:24

TSE T2 ax 13 4000 – 120 – 15 5 256×162 160 100 130 4 4:20

TSE T1 ax 13 750 – 14 – 3 5 256×192 160 100 195 3 4:01

Turbo-IR T2 cor 13 4000 150 60 – 11 4 256×162 160 100 130 2 3:40

Turbo-IR T2 ax 13 4000 150 60 – 11 5 256×162 160 100 130 2 3:40

Table 11.9. Pulse sequence recommendations for the rectum

Sequence Plane WI No. of TR TE Flip TI Echo Slice Matrix FOV recFOV Band- slices (ms) (ms) angle train thickness (%) width

length

TSE Trans. T1 25 600 11 – – 3 8 512×384 380 75 130

TSE Trans. T2 25 5000 130 – – 15 8 512×400 380 75 220

TSE Sagittalsag T2 19 5000 130 – – 15 3 512×358 300 60 130

TSE Trans. T2 26 6600 130 – – 15 3 256×192 140 100 130

Abbreviations: WI weighting of images; TR repetition time; TI inversion time; TE echo time; Matrix phase × frequency matrix; FOV field of view (mm); recFOV rectangular field of view

For high-resolution demonstration of local spread of rectal tumors, the transverse scans should be oriented perpendicular to the long axis of the tumor, and may therefore be paraxial or even paracoronal

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ized should be imaged using a T2-W sequence. Thin slice (3–4 mm), small field-of-view, high image matrix scans should be obtained in a plane perpendicular to the long axis of the tumor. The exact imaging plane will depend on the orientation of the rectum at the site of the tumor. For example, if the tumor is close to the rec- tosigmoid junction, a plane perpendicular to the axis of the tumor may be closer to coronal than true axial.

Using this technique, it should be possible to resolve the tumor within the wall, and nodules in the perirectal fat.

It is also possible to see the mesorectal fascia and pre- dict the likelihood of successful removal by modern surgical procedures such as total mesorectal excision (TME). Sagittal and coronal imaging planes are useful in demonstrating the proximity of the tumor to the anal sphincter complex, and give an indication of whether the sphincter can be preserved and reanastomosis effected.

Satisfactory imaging is usually achievable using the body coil, but high quality, small field-of-view images are best obtained using pelvic phased-array coils.

Tumor staging has been performed using endorectal coils. As with prostate cancer, its place in clinical practice is not yet established. Endoanal coils are being developed that are capable of identifying the individual muscle groups of the anal sphincter, and these are likely to take on a role in the assessment of incontinence (for example following obstetric trauma) and perianal fistulae.

11.2.9

External Genitalia

The male external genitalia are very suitable for MR imaging (Table 11.10). The corpus spongiosum and cor- pora cavernosa are separated by layers of fascia, and the urethra runs through the corpus spongiosum in the ventral compartment of the penis. Tumors of the penile urethra and a variety of inflammatory and traumatic conditions can be demonstrated. T2-W images yield excellent contrast, and T1-W images following gadolinium administration are useful in demonstrating tumors. A pelvic phased-array coil is ideal, and attention to detail when positioning the male organ can result in greater ease of interpretation of images. A small local surface coil can be used that will yield a good signal and reduce the need to oversample, in con- trolling aliasing artifacts.

The scrotal contents may be imaged using T2-W and T1-W images. The axial plane is useful to orientate the testes and epididymis, and coronal images are also useful. A wide variety of pathologies, including tumors and trauma, are demonstrable by MRI, but the technique has not replaced ultrasound for most clinical indications.

The female external genitalia may also be imaged using a pelvic coil. Indications are limited, but it is useful for the staging of vulval carcinoma, and tumors of the urethra may be demonstrated.

Table 11.10. Pulse sequence recommendations for external genitalia

TSE T2 sag 13 4000 – 120 – 15 3 256×192 160 100 130 3 3:01

TSE T1 sag 13 750 – 14 – 3 3 256×192 160 100 195 2 3:54

TSE T2 ax 15 4500 – 120 – 15 5 256×192 160 100 130 2 3:46

TSE T1 ax 15 800 – 14 – 3 5 256×192 160 100 195 4 5:02

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11.3

Clinical Applications of Pelvic MR Imaging 11.3.1

Uterus and Cervix 11.3.1.1

Anatomy

The uterus is divided into three segments: the fundus lies above the cornua, the body or corpus uteri lies between the fundus and the most caudal part of the uterus, which is the cervix (Fig. 11.1). Histologically, the uterine corpus has three tissue layers: the serosa, which is a covering of peritoneum draped over the uterus;

the myometrium, which consists of smooth muscle, and the endometrium. The inner third of the myometrium is composed of smooth-muscle bundles, which are densely packed and orientated mostly along the long axis of the uterus. The outer myometrium contains more loosely packed and randomly orientated smooth- muscle fibers. The MR imaging anatomy of the uterine body and fundus is well-demonstrated by sagittal T2-W

SE/TSE sequences. A high signal-intensity stripe represents normal endometrium and secretions within the cavity. The width of the endometrial stripe varies with the menstrual cycle, and the average thickness has been reported to be 3–6 mm in the follicular phase and 5–13 mm during the secretory phase. Below the endometrial stripe, there is band of low signal referred to as the junctional zone. Beyond this is an outer layer of myometrium, which returns intermediate signal intensity on T2-W images. There is some controversy about the histological basis for the low signal intensity of the junctional zone. The hypothesis that it represents the densely packed muscle bundles of the inner layer of the myometrium is attractive; however, some in vitro studies have demonstrated that the thickness of the inner layer of myometrium does not correspond exactly to the junctional zone on either MRI or ultrasound. Some authors have attributed the low signal from the junctional zone to a lower water content, while others have drawn attention to an increase in the percentage of nuclear area within the cells of the junctional zone compared with that of the outer myometrium.

Fig. 11.1A,B. Normal uterine anatomy (T2-W TSE). A The uterus is anteverted. The uterine endometrial stripe is of high signal intensity. The low-signal junctional zone is only a few millimeters thick, and beyond this, the outer myometrium returns slightly higher signal. In the cervical canal, a central stripe of very high signal is surrounded by the cervical mucosa, which returns slightly lower signal. Beyond this, the fibrous stroma returns very low

signal, and the outermost layer of the cervix is of intermediate signal, representing muscle in continuity with the outer myometri- um. B An image has been obtained in a plane perpendicular to the long axis of the uterus. Because of the degree of anteversion of the uterus, this is close to a true coronal image. It demonstrates the high signal returned by the endometrium and luminal secretions, surrounded by the junctional zone and outer myometrium

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The cervix is separated from the uterine corpus by the internal os, which corresponds to a slight constric- tion, marked by the entrance of the uterine vessels. The cervical canal is lined by the columnar epithelium of the endocervix. Small folds can sometimes be seen (plicae palmatae). Surrounding the cervical endothelium is a dense, fibrous stroma. The outermost layer of the cervix is composed of muscle, which becomes increasingly thin in the lower cervix toward the external os and is marked histologically by the squamocolumnar mucosal junction. On T2-W images, the secretions in the canal form a zone of very high signal. The cervical mucosa itself returns slightly lower signal, and the plicae palmatae may be seen. The fibrous stroma is of very low signal, and the muscular outer cervix is of intermediate signal. This muscular layer is continuous with the outer myometrium of the uterine corpus. The MR appearance of the cervix varies little with the menstrual cycle.

The parametrium and suspensory ligaments of the uterus may serve as pathways for local spread of disease. The parametrium lies between layers of the broad ligament, which is a folded double sheet of peritoneum that reflects from the ventral and dorsal surface of the uterus and extends to the pelvic side wall. The lower border of the broad ligament is thickened by a conden- sation of connective tissue and muscle, forming the cardinal ligaments. The paired uterosacral ligaments are fused anteriorly with the cardinal ligaments and extend posteriorly to the sacrum. The uterovesical ligaments extend from the cervix to the base of the urinary bladder. These are the main suspensory ligaments of the uterus. The round ligaments run from the posterolater- al aspect of the uterine fundus through the inguinal canal to the labia majora. The ligaments are of low signal intensity on T1-WI and of variable intensity on T2- WI. The parametrium contains multiple venous plexus- es and some loosely packed connective tissue, which is of intermediate signal intensity on T1-W sequences and isointense with fat on T2-W sequences.

11.3.1.2

Congenital Anomalies

The fallopian tubes, uterus, and upper two-thirds of the vagina are derived from the paired Müllerian ducts.

Agenesis or hypoplasia may affect any part of the female genital tract. A variety of partial and complete duplications may also result from embryological aber- rations. Simple anomalies can usually be identified on

transvaginal ultrasound. MR imaging should be reserved for patients with technically difficult or inde- terminate ultrasound examinations, which may occur in patients with vaginal malformations and multiple complex anomalies. The coronal plane using T2-W sequences is frequently the most informative sequence.

11.3.1.3

Benign Pathology

Endometrial polyps and hyperplasia can usually be detected using transvaginal ultrasound, at which time endometrial sampling may be undertaken. MRI cur- rently has no established role in the initial investigation of endometrial pathology, although thickening of the endometrial stripe can be clearly demonstrated. In cases where there is difficulty in assessing the endometrium, for example, in cervical stenosis, MRI may provide useful information. Thickening of the endometrial stripe is of pathological significance, particularly in postmenopausal women, but the normal ranges are not clearly defined. It has been suggested that the postmenopausal endometrial thickness should not exceed 3 mm in women not receiving hormone-replacement therapy and should not exceed 6 mm in women on hormone- replacement treatment. On T2-W images, endometrial polyps return a slightly lower signal than normal endometrium. When they are large, endometrial polyps may be markedly heterogeneous with areas of high and low signal. They show variable degrees of enhancement on T1-W sequences following the administration of gadolinium. Typically, they enhance less than endometrium but more than adjacent myometrium.

Leiomyoma is the most common type of uterine tumor and is estimated to be present in 20%–30% of premenopausal women over the age of 35 years.

Following menopause, they may regress, as they are estrogen dependent. Most leiomyomas exhibit some form of degeneration pathologically, particularly if they are large. Degeneration may be hyaline, myxomatous, cystic, fatty, or hemorrhagic. In addition, they may cal- cify, and these diverse degenerative features account for the variable signal changes seen on MR imaging. T2-W sequences provide optimal contrast between leiomyomas and adjacent myometrium or endometrium. T1-W sequences may be useful in depicting hemorrhagic degeneration and may be helpful in demonstrating clear fat planes between the uterus and adnexal structures in cases where difficulty is encountered in dis-

(11)

criminating a uterine leiomyoma from an ovarian mass.

Leiomyomas typically appear as well-marginated masses of low signal intensity relative to myometrium on T2- W sequences (Fig. 11.2).Very small lesions are frequently identified, and MRI is more sensitive than transvaginal ultrasound. The detail available on MR imaging may help demonstrate the myometrial origin of a submuco- sal leiomyoma protruding into the endometrial cavity, thus assisting in discrimination from an endometrial polyp.

The cellular subtype of leiomyoma and those with significant degeneration are the most likely tumors to cause confusion, as they may return high signal on T2-W sequences. The appearance of leiomyomas following the administration of gadolinium is variable.

The majority enhance to a lesser degree than the surrounding myometrium on both early and delayed contrast-enhanced images. However, early intense enhancement may be seen with the cellular subtype.

Bizarre signal change in large leiomyomas should raise the possibility of sarcomatous degeneration, which is rare and cannot be reliably diagnosed by MRI alone.

The greatest utility of MR imaging in the diagnosis of uterine leiomyoma is in unequivocally demonstrating the myometrial origin of a lesion, where other investigations such as transvaginal ultrasound are indetermi- nate.

Uterine adenomyosis is a common condition caused by heterotopic endometrial gland and stroma in the myometrium. This ectopic tissue appears to be inde- pendent of hormonal stimuli, and the clinical presenta- tion usually involves irregular or excessive bleeding, pelvic pain, and sometimes uterine enlargement.

Adenomyosis may be focal, diffuse, or microscopic and is frequently found incidentally following hysterectomy for other indications.

Because the presenting symptoms are nonspecific, imaging is of value if the diagnosis is to be made preop- eratively. In this clinical setting, MRI has some advantages over transvaginal ultrasound, primarily its repro- ducibility and relative lack of operator dependency.

T2-W sequences are ideal for diagnosing adenomyosis.

The heterotopic endometrium generates adjacent myometrial hyperplasia, and this is represented as diffuse or focal thickening of the junctional zone (Fig. 11.3). On T1-W sequences, small hyperintense foci may be seen, which are thought to represent hemorrhage.

Fig. 11.2. Uterine fibroid (T2-W TSE, sagittal plane). The endome- trial stripe is wide in this premenstrual patient. The junctional zone is within normal limits. There are low-signal lesions in the outer myometrium, none of which exceed 1 cm in maximum dimension. These are uterine fibroids

Fig. 11.3. Adenomyosis (T2-W TSE). The endometrial stripe is within normal limits, but the junctional zone is diffusely thickened. The outer myometrium returns normal signal

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Gadolinium enhancement does not assist in the diagnosis. Various values for the maximum thickness of the junctional zone have been proposed, but the consensus view is that it should be no thicker than 12 mm. A value in excess of this is highly predictive of the presence of adenomyosis. If the junctional zone is less than 8 mm, adenomyosis is very unlikely. Between 8 mm and 12 mm, the diagnosis relies on other features, such as the presence of localized hemorrhagic areas, poor defi- nition of the junctional zone, and focal thickening of the junctional zone. The main differential diagnosis, clinically and radiologically, is from leiomyoma. MRI, despite some overlap of features, is the most reliable method of preoperative diagnosis. This is an important point, since uterine leiomyoma may be treated conser- vatively, whereas the treatment for clinically debilitat- ing adenomyosis is hysterectomy.

Benign conditions of the cervix include nabothian cysts, cervical stenosis, and cervical incompetence.

Nabothian cysts result from distension of the endocervical glands, and these very common lesions return high signal on T2-WI and are usually asymptomatic.

Cervical stenosis may be congenital, inflammatory, iatrogenic, or neoplastic. MR imaging can identify the location of cervical stenosis and demonstrate neo- plasms. It can also demonstrate the degree of distension of the proximal uterus by retained secretions.

11.3.1.4

Malignant Disease

Endometrial carcinoma is a common malignancy of the female genital tract in the developed world. Its peak incidence occurs between the ages of 55 and 65 years.

Most patients present with postmenopausal bleeding or irregular bleeding, usually early in the course of the disease. Patients are referred for dilatation and curettage if no obvious cause is found on clinical grounds. This allows for prompt diagnosis and treatment.

Approximately 85% of endometrial carcinomas are adenocarcinomas, although papillary serous and clear- cell carcinomas, which carry a worse prognosis, may also be found. Endometrial carcinoma may spread locally. Lymphatic spread may be directly to para-aortic nodes. Most clinicians use the staging system of the Federation Internationale de Gynaecologie et d’Obste- trique (FIGO) (Table 11.11).

Tumor grade, stage of disease, and depth of myometrial invasion are the most important prognostic fac-

tors. MR imaging is well-suited to staging endometrial carcinoma, and T2-W sequences in the sagittal and transverse planes are extremely helpful for assessing the depth of myometrial invasion. The MR appearance of noninvasive endometrial carcinoma (stage IA) is nonspecific, and MR imaging, therefore, has no role as a screening technique. Histological sampling is required for the diagnosis, and discrimination from hyperplasia is not possible by MRI. The signal intensity of endometrial carcinoma is variable. It may be isointense with normal endometrium or slightly hypointense on T2-W sequences. Alternatively, a heterogeneous mass of mixed high and low signal intensity may be seen.

Thickening of the endometrial stripe in postmenopausal women is a suspicious sign.

In patients with myometrial invasion (stage IB and IC), segmental or complete disruption of the junctional zone by a mass of intermediate signal intensity on T2- W sequence should be seen (Fig. 11.4). Disruption of the junctional zone should be seen on two imaging planes. There is overlap with the MR findings in adenomyosis, but once a histological diagnosis of carcinoma has been made, the MR signs can be interpreted with reasonable confidence. The percentage of myometrial invasion is estimated, separating patients into stage IB (less than 50% wall invasion) or stage IC (greater than 50% wall invasion). Superficial extension of endometrial carcinoma into the cervical mucosa (stage Table 11.11. Federation Internationale de Gynaecologie et d’Obstetrique (FIGO) staging of endometrial carcinoma Stage 0 Carcinoma in situ

Stage I Tumor confined to corpus IA Tumor limited to endometrium IB Invasion <50 % of myometrium IC Invasion >50 % of myometrium

Stage II Tumor invades cervix but does not extend beyond uterus

IIA Invasion of endocervix IIB Cervical stromal invasion

Stage III Tumor extends beyond uterus but not outside true pelvis

IIIA Invasion of serosa, adnexa, or positive peritoneal cytology

IIIB Invasion of vagina

IIIC Pelvic and/or para-aortic lymphadenopathy Stage IV Tumor extends outside of true pelvis

or invades bladder or rectal mucosa IVA Invasion of bladder or rectal mucosa IVB Distant metastases (includes intra-abdominal

or inguinal lymphadenopathy)

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IIA) can be demonstrated on T2-WI by widening of the endocervical canal and internal os. If the low signal- intensity fibrous stroma of the cervix is invaded, stage IIB disease is diagnosed. MR imaging has been demonstrated to be an accurate technique for staging of early endometrial carcinoma. Few data are available in the literature regarding stage III and stage IV endometrial carcinoma, but MRI is certainly capable of demonstrating bulky tumors with parametrial invasion, invasion of the vagina, and regional lymphadenopathy. It is less reliable in demonstrating peritoneal spread. In practice, many centers do not employ MR staging of endometrial carcinoma since surgeons proceed to hysterectomy in early-stage disease, and surgical/histological staging is, therefore, available.

Cervical carcinoma is the third most common malignancy of the female genital tract in the developed world, but it is also extremely common in Africa.

Screening by cytology picks up cervical intraepithelial neoplasia, which is considered to be a precursor of cervical carcinoma. The disease may, therefore, be picked

up when asymptomatic. It is estimated that 80%–90% of cervical carcinomas are squamous-cell carcinomas, but adenocarcinoma is undoubtedly becoming more common and carries a worse prognosis. In women under 35 years of age, most cervical carcinomas arise from the squamocolumnar junction, which lies on the vaginal surface of the cervix. These tumors grow in a polypoid fashion (Fig. 11.5). In older women, most tumors occur within the endocervical canal, resulting in a barrel- shaped cervix. Tumors located within the cervical canal are more difficult to evaluate clinically and have a high incidence of parametrial invasion (Fig. 11.6). Cervical carcinoma is usually staged clinically, using the FIGO system (Table 11.12), despite its well-known limitations.

T2-W sequences provide optimal contrast between tumors and the normal cervical structures. Sagittal and transverse imaging planes will normally evaluate local tumor extension accurately. Coronal sections may be useful in providing additional information regarding the parametrium and the lateral vaginal fornices.

Carcinoma in situ or microinvasive tumors (stage IA) Fig. 11.4A,B. Endometrial carcinoma (T2-W TSE). A Sagittal

sequence demonstrates some mixed signal within the endometrial stripe. The junctional zone is not clearly identified. Anteriorly, a uterine fibroid is noted. B Paraxial plane (perpendicular to long axis of uterus). The endometrial signal is mixed. The junctional

zone and anatomy of the outer myometrium are disrupted by endometrial carcinoma spreading to the left of midline and anteriorly. Low-signal areas within the myometrium are uterine fibroids. Hysterectomy demonstrated no parametrial spread of the tumor

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are not normally identified by MR imaging. However, MRI will identify tumors that invade the fibrous stroma, despite relatively normal-appearing epithelium.

Macroinvasive cervical carcinoma (stage IB) is defined as an invasive component greater than 5 mm in depth and 7 mm in horizontal spread; these appear on T2-WI as masses of intermediate signal intensity that deform the canal or disrupt the very low signal-intensity fibrous band. The lateral margins of the cervix, which are formed by muscle incontinuity with the outer myometrium, should remain smooth in stage-IB disease. On T1-WI, cervical tumors are usually isointense with normal cervix, and the zonal anatomy is difficult to appreciate. Small tumors are, therefore, not visible on T1-W sequences unless gadolinium is given. Cervical carcinoma will demonstrate increased enhancement relative to cervical stroma, which is most marked on images acquired within 60 s of administration, using a dynam- ic sequencing technique.

Cervical carcinoma is classified as stage IIA when it invades the upper two-thirds of the vagina. On T2-W images, disruption of the vaginal wall or diffuse thickening with high signal intensity are signs of tumor invasion. One of the crucial clinical decisions takes place in the diagnosis of stage-IIB disease, in which parametrial invasion is present. Under most circumstanc- Fig. 11.5. Carcinoma of cervix (T2-W TSE, sagittal plane). There is

a mass protruding from the anterior lip of the cervix. The canal appears intact, but the fibrous stroma has been disrupted. A mixed-signal mass is projecting into the vagina. The appearance is of carcinoma of the cervix. The uterus is slightly enlarged and the junctional zone indistinct, as this patient presented postpartum

Fig. 11.6. Bulky, locally advanced carcinoma of the cervix (T2-W TSE, axial plane). This axial image through the cervix shows an outline of the cervical anatomy (arrow). The low-signal ring of the fibrous stroma is irregular and disrupted. Beyond this, there is extensive parametrial invasion, almost to the pelvic side wall on the left. The posterior wall of the bladder is involved by the tumor

Table 11.12. Federation Internationale de Gynaecologie et d’Obstetrique (FIGO) staging of cervical carcinoma

Stage 0 Carcinoma in situ

Stage I Tumor confined to cervix (extension of corpus should be disregarded)

IA Microinvasion

IB Clinically invasive. Invasive component >5 mm in depth and >7 mm in horizontal spread Stage II Tumor extends beyond cervix, but not to pelvic

side wall or lower third of vagina IIA Vaginal invasion (no parametrial invasion) IIB Parametrial invasion

Stage III Tumor extends to lower third of vagina or pelvic side wall; ureter obstruction IIIA Invasion of lower third of vagina

(no pelvic side wall extension) peritoneal cytology

IIIB Pelvic side wall extension or ureteral obstruction Stage IV Tumor extends outside true pelvis or invades

bladder or rectal mucosa

IVA Invasion of bladder or rectal mucosa IVB Distant metastases

The presence of metastatic lymph nodes is not included in the FIGO classification

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es, surgery is not considered for these patients, and radiotherapy is the treatment of choice. It has been shown that a completely intact ring of cervical stroma accurately excludes parametrial extension. However, full-thickness disruption of the fibrous stroma with abnormal signal intensity in the parametrium has a significant false-positive rate in stage I tumors, and it seems that overstaging results from peritumoral inflammatory change.

Locally advanced (stage III or stage IV) disease is usually diagnosed clinically and can easily be con- firmed with MR imaging. There is relatively little histologically correlated data in the literature, as these tumors are rarely removed. Large masses frequently invade the pelvic sidewall, bladder, and rectum. When large, cervical carcinomas return mixed signal on T2- WI and low signal on T1-WI. The T2-W image is useful in demonstrating tumor invading muscles, such as levator ani, obturator internus or piriformis, as the muscles are usually of lower intensity than the invading tumor.

T1-W sequences maximize contrast if there is sufficient fat in the pelvis. Fat-suppression techniques may also be helpful in clarifying the anatomy.

Following radiotherapy for cervical carcinoma, diagnosis of suspected recurrent disease is a frequent and difficult clinical problem. By 12 months after comple- tion of the radiation treatment, the uterus and cervix should return low signal. However, during the initial 6–12 months after treatment, developing radiation changes may return high signal, due to inflammation and increased vascularity. Enhancement with gadolinium is not normally seen later than 12 months after radiation treatment. However, the rate of development of the typical hypointense signal from radiation fibrosis is extremely variable from patient to patient, and an abnormal signal may persist for years. Recurrent tumor should, therefore, be diagnosed on the basis of a demonstrable mass, rather than on signal change alone (Fig. 11.7).

11.3.2

Parametrium and Ovaries

MR imaging is capable of diagnosing adnexal masses as cystic or solid, and can characterize the components as fluid, fatty, or hemorrhagic. Up-to-date machinery using pelvic phased-array coils can usually demonstrate the ovaries and parametrial structures on T2-W SE sequences. The axial and coronal planes are most useful. T1-W SE sequences following gadolinium may be helpful, as may FS images. Transvaginal ultrasound remains the investigation most commonly performed first for investigation of the ovaries. However, in select- ed cases, MR imaging can yield additional information.

In women of reproductive age, the normal ovaries may demonstrate zonal anatomy on T2-W images. The medulla has a slightly higher signal intensity than the cortex. Cysts are frequently seen, returning high signal on T2-W sequences. On T1-W sequences, follicular cysts have thin walls, which enhance variably. Thick enhancing rims are typical of the corpus luteum, which is also prone to hemorrhage. Bizarre and complex cystic structures with irregular thickening and enhancement should raise a suspicion of malignant disease (Fig. 11.8). Solid tumors are also frequently malignant, although benign teratoma masses may demonstrate areas of signal intensity consistent with fat.

Pelvic inflammatory disease is fundamentally a clinical diagnosis. Tubo-ovarian abscess, a well-recognized complication of pelvic inflammatory disease, can be Fig. 11.7. Recurrent cervical carcinoma (T2-W TSE, axial plane).

This patient presented with pelvic pain following hysterectomy and radiotherapy for carcinoma of the cervix. On the left side of the pelvis, there is a mass returning predominantly low signal peripherally with an area of high signal centrally. The central area represents necrotic and hemorrhagic debris. The outer layer was biopsy proven to represent recurrent carcinoma. The diagnosis may be made on the morphology of the mass, which is clearly invasive, rather than the signal characteristics per se. The mass is penetrating fat planes laterally and disrupting the cortical bone of the acetabulum