5.2 Imaging Planes and Routine MR Sequences
Careful assessment of the region of clinical concern should precede any imaging to ensure its complete cov- erage with the most appropriate coil and to avoid waste of time due to repositioning of the patient after the first imaging sequence. The placement of a lipid marker at the area of interest may be helpful in this regard. Unless no lesion is detected on the initial sequences, it is usual- ly not necessary to examine the contralateral side for comparison when an extremity is being evaluated.
Multiplanar imaging is an important factor in tumor staging, as it is extremely helpful in determining the anatomical extent of the lesion and its relationship to adjacent structures. In planning the appropriate surgi- cal procedure, it is of the utmost importance to deter- mine whether a lesion is within a well-delineated anatomical compartment (e.g., intrafascial or intra-ar- ticular) or is diffusely infiltrating adventitial planes and spaces. Accurate staging information may also deter- mine the necessity for preoperative treatment [36].
Imaging usually starts with a sequence in the most appropriate longitudinal plane. Anteriorly or posterior- ly located lesions are best imaged in a sagittal plane. For medial or lateral localizations, coronal imaging is pre- ferred. Care should be taken to respect the anatomical orthogonal planes since, with excessive rotation of a limb, inappropriate positioning of longitudinal scan planes results in images which are difficult to interpret and probably useless for surgical planning. Since this sequence should depict the lesion, together with eventu- ally surrounding edema, with the highest conspicuity and over its entire cephalocaudal extent, fat-suppressed, fast spin-echo (FSE) T2-weighted or short-tau inversion recovery (STIR) imaging with a large field of view (FOV) is recommended. Inclusion of the nearest joint serving as a reference in at least one of the longitudinal imaging planes is well appreciated by all surgeons, since especially deeply situated masses can be hard to localize based on clinical examination alone.
This first sequence in the longitudinal plane is usual- ly followed by imaging in the axial plane. Most anatom- ical and functional compartments of the extremities are 5.1 Introduction
Due to its unequaled soft tissue contrast and multipla- nar imaging capability, magnetic resonance imaging (MRI) is the modality of choice to image soft tissue tu- mors. An impressive arsenal of sequence types has be- come available, especially with regard to fast MRI, fat- suppression techniques and contrast-enhanced studies.
In this section, we make a distinction between what is essential in daily practice, what should be avoided, and what are useful additional techniques, taking into ac- count the equipment available and the demands of the clinician. We address the following topics: appropriate choice of imaging planes and routine sequences, why and how to perform contrast-enhanced studies, and the choice of sequences, depending on the possible de- mands of tissue characterization and the assessment of tumor extent in the adjacent soft tissue or bone. Topics such as posttreatment imaging and dynamic contrast- enhanced imaging, which are highlighted in other chap- ters in this book, are briefly mentioned.
The strategy in designing the optimal MR examination will always depend on the location,the desired coverage of the anatomical region to be examined, the suspected ab- normality, the available hardware (field strength, local coil), time constraints, and local preferences.
Magnetic Resonance Imaging
P. Brys, H. Bosmans
5
5.1 Introduction . . . 61
5.2 Imaging Planes and Routine MR Sequences . . . . 61
5.3 Contrast-Enhanced MRI . . . 62
5.3.1 Static Enhanced MRI . . . 62
5.3.2 Dynamic Enhanced MRI . . . 65
5.4 Characterization . . . 66
5.5 Soft Tissue Extent . . . 68
5.6 Neurovascular Involvement . . . 69
5.7 Bone Invasion . . . 69
5.8 Imaging After Preoperative Chemotherapy or Radiation Therapy . . . 69
References . . . 70 Contents
oriented longitudinally. This requires imaging in the ax- ial plane for adequate evaluation of the tumor extent and its relation to vessels and nerves [2, 26]. Not only the entire tumor but also the peritumoral edema at its proximal and distal poles should be covered by these ax- ial sequences. As a rule, the most proximal and distal slices should show no pathology in the tissues. Usually T1- and T2-weighted acquisitions are obtained in the axial plane at exactly the same location, thus allowing an image-by-image comparison. Contrast-enhanced images have to be acquired at least in the axial and the most useful longitudinal plane and at the same posi- tions as the precontrast images. The choice of an addi- tional imaging plane depends on the location of the le- sion and the clinical questions to be answered (Fig. 5.1).
Oblique planes may also be useful. Typical examples are oblique sagittal images for optimal depiction of a le- sion’s relation to the scapula or the iliac wing.
The use of spin-echo MR sequences is recommended.
It is the most reproducible technique, the one with which we are most familiar for tumor evaluation, and the most often referenced in tumor-imaging literature [29].
The main disadvantage of classic, double-echo T2- weighted sequences remains the relatively long acquisi- tion times [29]. For its increased lesion conspicuity and shorter acquisition times, fat-suppressed, FSE T2- weighted MRI is frequently preferred. Fat-suppressed, FSE T2-weighted images also show a higher signal-to- noise ratio compared with STIR imaging [12]. However, since fat appears bright on all FSE sequences [3, 16],
with subsequent decrease in conspicuity of tumors or edema in bone marrow or juxtaposed to fat [29], FSE T2-weighted sequences without fat suppression should not be used in tumor imaging. After i.v. administration of gadolinium (Gd), STIR type sequences should not be used, since not only fat but also enhancing tissue will be shown with a reduced signal intensity.
5.3 Contrast-Enhanced MRI
Contrast-enhanced MR studies lead to a prolonged ex- amination time and high costs. In routine muscu- loskeletal MRI, routine i.v. Gd administration is not a re- quirement and should be reserved for cases in which the results would influence patient care [18].
Characterization and delineation of, e.g., lipomas and vascular malformations are easily performed on unenhanced sequences. However, contrast administra- tion is certainly helpful in MR characterization and the clinical management of most of the soft tissue tumors.
The superior diagnostic performance of contrast en- hancement, compared with unenhanced MRI, improv- ing the diagnosis of benign lesions but also the detec- tion of malignant ones, has been well established [43].
Contrast-enhanced imaging helps to narrow down dif- ferential diagnosis, facilitating clinical management.
Lesions in which the observer is highly confident of a benign diagnosis at MRI may not require histological biopsy [43]. Although contrast-enhanced MRI is not able to reliably differentiate between benign and malig- nant soft tissue tumors, it helps to increase the suspi- cion of malignant lesions, in which inappropriate exci- sional biopsies should be avoided because of the risk of tumor-positive, surgical section margins.
5.3.1 Static Enhanced MRI
Compared with unenhanced T1-weighted imaging (WI), enhanced T1-WI improves the delineation of a le- sion in terms of tumor-to-muscle contrast but without improvement of this contrast compared with T2-WI [10, 46]. On the other hand, enhanced T1-WI decreases or even obscures the tumor-to-fat contrast (Figs. 5.2b, c, 5.3, 5.4), which can be counteracted by the use of fat- suppressed T1-WI (Fig. 5.5c). Since fat-suppressed T1- weighted images after i.v. administration of gadolinium show areas of contrast enhancement with a greater con- spicuity than T1-WI without fat suppression, subse- quently resulting in images that are easier to interpret, the use of this sequence has become very popular.
However, one should be aware of the risk of misinter- pretation of fat-suppressed, enhanced T1-weighted im- ages, since a high signal intensity of a lesion can be the consequence of two variables: real Gd enhancement or
Fig. 5.1. Coronal spin-echo (SE) T2-weighted image (TE 110 ms).
In this patient, known to have neurofibromatosis, a painful mass was palpated halfway down the posterior part of the left thigh.
Here lower-resolution body-coil imaging was used to search for additional neurofibromata besides the one suspected along the course of the left sciatic nerve, which were demonstrated proxi- mally and distally along the right sciatic nerve (arrows)
Fig. 5.2. a Axial, unenhanced SE T1-weighted image. b Axial, SE T2-weighted image. c Axial, Gd-enhanced SE T1-weighted image.
Patient with a leiomyosarcoma. For local tumor staging, the unen- hanced T1-weighted image provides the most useful information, showing high tumor-to-fat contrast at the extraosseous side, as well as a sharp delineation between tumor-invaded and normal bone marrow. The conventional T2-weighted SE image (with TE 80 ms) shows a poor tumor-to-fat contrast at the extraosseous side and a moderate contrast between tumor and normal bone mar- row. As for tumor staging, the worst result is obtained with the Gd- enhanced, SE T1-weighted image. Both this and the T2-weighted acquisition would benefit from fat suppression
Fig. 5.3Ia–c.Dedifferentiated liposarcoma in the posterior com- partment of the thigh. a Coronal, unenhanced T1-weighted image.
A mass lesion is shown in the hamstrings muscles. However, tu- mor-to-muscle contrast is very low, resulting in impossibility of analysis of the soft tissue extent. b Coronal, Gd-enhanced T1- weighted image. In this patient, additional coronal imaging demonstrates marked extent of tumor or peritumoral edema dis- tally in the semitendinosus muscle, which is information of major value when the field of preoperative radiotherapy has to be delin- eated or the width of surgical resection has to be determined.
Since the medial half of the lesion shows extensive necrosis, a biopsy should be obtained from the viable later part. c Coronal, T2- weighted image (heavily T2-weighted with TE of 120 ms), showing peritumoral brightening (edema and/or tumor spread) in the ad- jacent semitendinosus muscle
a b
c
a b
c
apparent Gd uptake due to the scaling effect caused by the fat-suppression technique. Suppression of the high signal intensity of fat induces a rescaling, a redistribu- tion of gray levels, so that minor differences in signal in- tensity between tissues on non-fat-suppressed T1-WI are magnified (Figs. 5.7a, b, 5.8). The same rescaling ef- fect of fat suppression is responsible for an apparently obvious Gd enhancement of tissue that only shows min- imal enhancement on non-fat-suppressed T1-weighted images [14, 15]. As a consequence, reliable interpreta- tion of fat-suppressed T1-weighted images after i.v. ad- ministration of gadolinium is only possible if also un- enhanced fat-suppressed T1-WI and enhanced non-fat- suppressed T1-WI sequences are obtained. This results in a longer examination time. However, the use of fat- suppressed, enhanced T1-WI certainly is not a routine requirement [15].
One example in which enhancement conspicuity does not increase with addition of a chemical shift- based fat-suppression technique is when, on unen- hanced T1-weighted images, the lesion is hyperintense due to the presence of methemoglobin. Since there is little or no contrast between Gd-enhancing areas and hemorrhage, differentiation between a subacute hematoma and a hemorrhagic tumor can be quite diffi- cult. Here enhancement conspicuity is only obtained from subtraction images on which hyperintensity is on- ly consistent with enhancement (Fig. 5.6). Unenhanced images are, by means of a postprocessing tool, subtract- ed from the enhanced ones. To obtain useful subtrac- tion images, the patient should not change position dur- ing the examination and i.v. access should be acquired before the start of the examination.
Enhanced T1-WI not only improves the delineation but also the evaluation of the internal structure of a tumor (Figs. 5.3a, b, 5.4b, c, 5.6, 5.7). It helps to diffe- rentiate well-perfused, viable tumor from tumor necro- sis, cysts or cystic parts of a tumor from myxoid ones, and intratumoral hemorrhage from hematoma.
This is essential for deciding whether to perform a biopsy, and planning of the biopsy site, the field of preoperative radiotherapy, or the area of surgical resec- tion.
Since every additional procedure increases the risk of inadvertent tumor cell contamination, the selection of the biopsy site should be well considered. A biopsy con- taining vascularized viable tumor will be of greater val- ue than a nondiagnostic specimen with hemorrhage, edema, or necrotic tissue. Well-vascularized viable tu- mor will be of greater value for determination of the tu- mor type and grade. If only static enhanced MR images are used, the area showing the most intense enhance- ment should be selected as biopsy site [44] (Figs. 5.3b, 5.4c).
Fig. 5.4Ia–c. Patient with a myxoid liposarcoma. a Axial STIR se- quence shows a sharply margined mass lesion with homogeneous high-signal intensity interposed between the vastus lateralis mus- cle and the distal femur. b Axial, unenhanced T1-weighted image.
A very low tumor-to-muscle contrast is shown, with the lesion’s signal intensity slightly lower than muscle. In combination with the high signal intensity on STIR sequence, this lesion could be mistaken for a cyst based on unenhanced sequences alone. Also note the very high tumor-to-fat contrast, typical for unenhanced T1-weighted images. c Axial, Gd-enhanced T1-weighted image shows a definite and very heterogeneous enhancement, inconsis- tent with a cystic origin of the lesion.Although there is a decreased tumor-to-fat contrast, delineation of the tumor from the adjacent fat still is perfectly possible. In addition, due to a clear increase in tumor-to-muscle contrast, this sequence is very suitable for surgi- cal planning. Besides the adequate delineation of the tumor, this enhanced T1-weighted sequence improves the evaluation of the internal structure of the tumor. It helps to differentiate viable tu- mor from a cyst, with a totally different surgical approach, and helps to select an appropriate biopsy site. Because it mainly con- sists of less well-vascularized and myxoid or necrotic tissue, a biopsy in the posterior part of the lesion should be avoided
a
b
c
Since static enhanced MRI is performed in the equi- librium state when the Gd concentration in the intersti- tium equals that of plasma, it is not able to distinguish among the various simultaneously enhancing tissues [33]. Examples are differentiation between tumor ex- tension through a pseudocapsule and peritumoral ede- ma [31] (Fig. 5.3b, c), between highly vascularized and less well vascularized viable tumor tissue [33], between viable tumor and inflammation or granulation tissue af- ter chemotherapy [11], and between tumor recurrence and an inflammatory pseudotumor. Static MR is also unable to provide accurate quantitative measurement of tumor response to chemotherapy [17, 21, 30, 37]. Since the spatial resolution and hence the anatomical detail of dynamic enhanced MR images is suboptimal, even an
“advanced” dynamic enhanced MR study should be completed by the more “classic” static enhanced MR se- quences.
5.3.2 Dynamic Enhanced MRI
Dynamic contrast-enhanced MRI provides physiologi- cal information such as tissue perfusion and vascular- ization, capillary permeability, and the volume of the in- terstitial space, which is not available on static contrast- enhanced MRI [45, 47].
The analysis of the tumor structure by dynamic con- trast-enhanced studies improves differentiation be- tween highly vascularized, less well vascularized, and necrotic tumor areas, which is important in the selec- tion of the most highly vascularized, highest-grade part of the tumor for the biopsy site, in differentiation of tu- mor from peritumoral edema, in the assessment of good and poor responders to preoperative chemo- and radiation therapy, and in the assessment of possible tu- mor recurrence [33, 44]. It further narrows down the differential diagnosis and helps to increase the suspi- cion of malignant lesions in addition to unenhanced and static enhanced MRI [43]. Dynamic MRI should be done when conventional MRI results in indeterminate findings [33]. The role of dynamic MR studies is dis- cussed more extensively in Chap. 6.
a
b
c
Fig. 5.5Ia–c.Patient with a pretibial, subcutaneous soft-tissue mass in the left lower leg, most probably of inflammatory origin. a Axial, unenhanced T1-weighted images shows high lesion-to-fat contrast but a poor lesion-to-muscle contrast. b Axial, Gd-en- hanced T1-weighted image. Increasing signal intensity due to T1- shortening results in a poor lesion-to-fat contrast, but, on the oth- er hand, a much better lesion-to-muscle contrast than on the un- enhanced image. c Axial, fat-suppressed, Gd-enhanced T1-weigh- ted image. Because of fat suppression, maximal lesion-to-fat and lesion-to-muscle contrast is obtained in one sequence. Moreover, additional information of the lesion’s extension along the fascial plane is clearly provided
5.4 Characterization
Tissue characterization is based on several imaging para- meters [4]. Some of them are signal-intensity related: sig- nal homogeneity, changing pattern of homogeneity, and the presence of hemorrhage and peritumoral edema.
This information is obtained by comparison of the signal characteristics on T1- and T2-weighted sequences.
Extra information can be derived from the addition of a fat-suppression technique to a T1- as well as a T2- weighted sequence. Chemical shift-based fat-suppres- sion should be used in these cases.
Fat-suppressed T2-WI is used to increase not only the conspicuity of a lesion and its surrounding edema/reac- tive zone but also of nonlipomatous components in lipomatous tumors, the latter helping in distinguishing lipoma from well-differentiated liposarcoma [13]. Fat- suppressed T1-WI is well known for its capacity to dif- ferentiate between fatty tissue and melanin or methe- moglobin, which all show a high signal intensity on con- ventional T1-weighted images. A chemical shift-based, fat-suppressed T1-weighted sequence decreases the sig- nal intensity of fatty tissue, while melanin and methe- moglobin remain hyperintense on this sequence [25, 35]
(Figs. 5.6b, 5.7b, 5.8b)
The additional value of this spin-echo T1-weighted sequence with fat-suppression in characterization has
been established. The sequence facilitates characteriza- tion and differentiation of fat and melanin/methemo- globin and fibrous or hemosiderotic parts versus highly cellular parts in tumors. It also enhances confidence in the characterization of neurogenic tumors and heman- giomas. In addition, because of the magnification of small signal-intensity differences, nonhomogeneity, an important parameter in characterization and staging, is also better evaluated [14].
Static Gd-enhanced MRI has only a limited value in the characterization of soft tissue tumors and the differ- entiation of benign from malignant lesions. The appli- cation of dynamic Gd-enhanced MR studies yields in- formation about the malignant potential of a tumor, but with a certain degree of overlap between benign and malignant tumors [10, 45]. In dynamic enhanced MRI, the enhancement pattern only reflects tissue vascularity and perfusion. Especially in fast-enhancing lesions, this overlap is too high to be of practical value in most cases [44]. On the other hand, slowly enhancing malignant tumors are rare but do exist [43].
Gradient echo (GRE) techniques are not routinely used for tissue characterization. However, in some select- ed applications they can be very useful. In the absence of calcifications or gas on radiographs and computed to- mography (CT), a marked signal loss on GRE sequences is almost pathognomonic for hemosiderin. (Fig. 5.9)
Fig. 5.6Ia–d. Patient with a necrotic, hemorrhagic high-grade pleiomorphic sarcoma of the posterior thigh compartment.
aSagittal, unenhanced T1-weighted image. The large mass lesion is showing an inhomogeneous high signal intensity, possibly of lipomatous origin. b Sagittal, fat-suppressed, unenhanced T1- weighted image. Persistence of the high signal intensity despite fat suppression is inconsistent with the hypothesis of a fatty tumor, and suggests the presence of methemoglobin in a large hematoma or hemorrhagic tumor. c Sagittal, fat-suppressed, Gd-enhanced T1-weighted image. Differentiation between hematoma and hem-
orrhagic tumor needs Gd administration. Unfortunately, because of the presence of intralesional methemoglobin, the conspicuity of Gd enhancement does not benefit from the fat-suppression tech- nique. Based on this sequence, it is virtually impossible to differen- tiate enhancement from methemoglobin. d Sagittal subtraction image (b subtracted from c). Subtraction of pre- from post-Gd, fat- suppressed, T1-weighted images permits isolation of the areas of Gd enhancement, showing a thin rim-enhancement and only some small foci of mural enhancement in the upper-posterior part of the lesion
a b c d
Some centers have investigated the possible role of dif- fusion-weighted MRI in tumor characterization. Appar- ent diffusion coefficient (ADC) values of benign soft tis- sue tumors and sarcomas overlap and cannot be used to differentiate between the bulk of benign and malignant tumors [6, 42]. True diffusion coefficients (perfusion-cor- rected diffusion-WI) are reported to be significantly low- er in malignant than in benign soft tissue masses; howev- er, there is still a substantial overlap between both [42].
Fig. 5.7Ia–c. Soccer player with a subacute hematoma after direct contact trauma of his left calf. a Axial, unenhanced T1-weighted image. In the lateral part of the soleus muscle, a lesion with a slightly higher signal intensity than muscle is shown. b Axial, fat- suppressed, unenhanced T1-weighted image. Due to suppression of the high signal intensity of fat, a rescaling effect is induced, with subsequent magnification of the difference in signal intensity be- tween muscle and lesion. c Axial, Gd-enhanced, T1-weighted image. After Gadolinium contrast administration, only a thin en- hancing rim is shown, consistent with the presence of an intra- muscular hematoma
a
b
c
Fig. 5.8Ia–c.Patient with a high-grade sarcoma of the anterior thigh compartment. a Axial, unenhanced, T1-weighted image. This unen- hanced T1-weighted sequence typically shows a poor tumor-to-mus- cle contrast, the lesion showing a signal intensity only slightly higher than muscle. Note a hemorrhagic area with fluid-fluid level in the lat- eral part of the lesion, with a high signal-intensity upper level and a dependent part with a signal intensity slightly hyperintense to the tu- mor. b Axial, fat-suppressed, unenhanced T1-weighted image. This sequence does not contribute to characterization or delineation of the lesion but illustrates the rescaling effect caused by the fat-sup- pression technique: The upper level, which was the second most hy- perintense area after the fatty tissue on the non-fat-suppressed im- age, now becomes strongly hyperintense. The slight differences in signal intensity between tumor and muscle and between dependent part and tumor now get clearly magnified. c Axial, fat-suppressed, Gd-enhanced T1-weighted image. The addition of Gd enhancement induces a new rescaling effect: As the tumor strongly enhances with its signal intensity higher than the upper level, the signal intensity from the latter gets rescaled to a lower gray level. The rescaling of the dependent part and the slight normal enhancement of muscle tissue results in both areas being isointense to each other
a
b
c
5.5 Soft Tissue Extent
The most important variable for predicting local recur- rence is the quality of surgery [22]. Accurate depiction of the local extent of disease is important, since this is an indispensable factor in determining the extent of surgical resection and the need for preoperative chemo- therapy or radiation therapy.
In delineation of a soft tissue tumor, distinction must be made between tumor-to-muscle and tumor-to-fat contrast.
On T1-weighted images, with exception of fat-con- taining tumors, soft tissue tumors are generally isoin- tense to muscle, resulting in low tumor-to-muscle con- trast, but have a high contrast with the hyperintense fat (Figs. 5.2a, 5.3a, 5.4b, 5.5a, 5.8a). As a consequence, T1- weighted images are essential in the delineation of a tu- mor from intermuscular fat planes, fat surrounding neurovascular structures, subcutaneous fat, and fatty bone marrow. A fat plane between a tumor and an adja- cent anatomical structure is sufficient to exclude inva- sion or encasement [28].
T2-weighted or STIR imaging allows differentiation of hyperintense tumors and their surrounding edema
from the hypointense surrounding muscles (Figs. 5.3c, 5.4a). The reactive edema around a tumor, as visualized on T2-weighted sequences, often contains satellite tu- mor micronodules, is considered as an integral part of the lesion, and therefore is removed en bloc with the tu- mor [23, 36] (Fig. 5.3c). If the reactive zone extends be- yond the compartment, a tumor is considered extra- compartmental even if the tumor itself is confined to the compartment [8, 9]. Fat-suppressed T2-weighted se- quences and STIR sequences increase the conspicuity of this peritumoral edema and hence enable the detection of a greater volume of tissue at risk for malignancy, al- though this may result in overestimation [24]. This may have important implications for local staging and plan- ning of surgery and radiation therapy [34]. MRI should always precede biopsy, as blood and edema that follow a biopsy can be difficult to separate from tumor or the peritumoral reactive zone, with or without Gd adminis- tration. An appropriate decision about whether to per- form limb-salvaging surgery based on a postbiopsy MR examination may be impossible.
Reliable differentiation between tumor and peritu- moral edema cannot be made by means of T2-weighted nor by static enhanced T1-WI. Dynamic enhanced MR studies can contribute to the differentiation of tumor from edema, because edema always shows a much more gradual increase in signal intensity than the tumor tis- sue. Improved differentiation of tumor from edema can change the preoperative strategy (e.g., help the surgeon in deciding whether to perform amputation or a limb- salvage procedure) [19]. However, he precise role of dy- namic enhanced MRI in this topic has not yet been es- tablished.
On T1-weighted sequence images, tumor-to-muscle contrast increases markedly after i.v. administration of gadolinium diethyltriamine pentaacetic acid (Gd-DT- PA; Figs. 5.3a, b, 5.4b, c, 5.5a, b, 5.8a–c). Although there is no improvement of this tumor-to-muscle contrast when compared with T2-WI [10, 46], the static enhanced T1- weighted sequence without fat-suppression is a very useful one. As an “almost-all-in-one-sequence,” it has the anatomical detail, fat planes inclusive, typical for T1- weighted images, a tumor-to-muscle contrast equal to that of T2-weighted images, and it provides useful infor- mation on tumor content due to the Gd-administration (Fig. 5.4c). Most surgeons use these images for planning their interventions.
A disadvantage of enhanced T1-WI is a decreased tu- mor-to-fat contrast (Figs. 5.2c, 5.5b). The use of a chem- ical-shift type of fat-suppression technique decreases the signal intensity of fat, with consequently an excel- lent tumor-to-fat contrast (Fig. 5.5c). However, this fat suppression results in obscured fat planes, which is a disadvantage in planning surgery. Although very popu- lar among radiologists because of the very high con-
Fig. 5.9. Patient with pigmented villonodular synovitis (PVNS) of the ankle joint. Plain films showed joint distension without intra- articular calcifications. With this DESS gradient-echo technique, the presence of hemosiderin results in susceptibility artifacts and profound signal loss of the hypertrophic synovium, being almost pathognomonic for PVNS. This sagittal image shows extension in the anterior and posterior recess of the joint and along the tendon sheath of the flexor hallucis longus muscle
ment of medullary invasion, with T1-WI being more specific [7]. Since, on unenhanced T1-weighted images, the contrast between tumor/edema and hematopoietic marrow is poor, one should rely on FSE T2-weighted se- quences to assess tumor invasion in this type of bone marrow.
5.8 Imaging After Preoperative Chemotherapy or Radiation Therapy
The aim of monitoring of preoperative chemotherapy is to predict the percentage of tumor necrosis in order to differentiate responders from nonresponders [44].
It should be performed in chemotherapy-sensitive tumors because of impact on modification of neoadju- vant treatment protocols, selection of postoperative chemotherapy regimens, patient selection for perfor- mance and timing of limb-salvage surgery, and plan- ning of radiation therapy [32, 39].
Unenhanced MR images cannot be used to evaluate tumor necrosis after chemotherapy, mainly because sig- nal intensities of viable tumor, tumor necrosis, edema, and hemorrhage overlap on T2-WI. Static enhanced MRI also has been unable to provide an accurate quan- titative measure of tumor response after chemotherapy [17, 21, 30, 37]. After preoperative radiotherapy, static enhanced MR could not reliably differentiate between viable tumor, tumor necrosis, inflammation, and granu- lation tissue [5]. Reliable differentiation between viable tumor and necrosis is possible with dynamic enhanced MRI, which is now the method of choice to monitor pre- operative therapy [32, 38, 40, 41, 47].
Diffusion-weighted MRI has been used successfully to assess tumor necrosis in rats with osteosarcoma [20].
In soft tissue sarcomas, an increase in ADC values after radiotherapy has been demonstrated [6]. Although fur- ther studies are needed to establish the role of diffu- sion-weighted imaging (DWI) in the evaluation of re- sponse to chemotherapy, this technique is promising, as DWI is performed with fast sequences and does not re- quire contrast media injection, so it can be repeated fre- quently during therapy [6]. This technique may be ap- plicable in the future to monitor tumor viability during treatment. The MRI strategy developed for the assess- ment of postoperative tumor recurrence is discussed in Chap. 28.
spicuity of enhancement, this FSE enhanced T1-weight- ed sequence should never be acquired without its non- fat-suppressed equivalent and is only recommended when tumor abuts or infiltrates adjacent fat. When time constraints are a consideration, enhancement conspicu- ity can also be obtained by the use of image subtraction (Fig. 5.6).
5.6 Neurovascular Involvement
In a series of 133 soft tissue sarcomas, the frequency of encasement was reported as 4.5% for major vessels and 6.8% for major nerves [27]. A neurovascular bundle is encased when surrounded with tumor for at least half of its circumference, with associated obliteration of its fat plane [28].
On MR images, the appearance of blood vessels can be highly variable (low signal intensity flow void, high signal intensity entry slice phenomenon, target-like pat- terns of signal intensity). Their appearance depends on several parameters, such as the pulse sequence used, the position of the slice, and the flow velocity. Knowledge of the appearance of flow effects on MR images is needed for the accurate evaluation of blood vessels.
Neurovascular involvement is assessed best by axial images. Tumor-to-vessel contrast is demonstrated bet- ter on regular T1- and T2-weighted spin-echo images than on STIR-based acquisitions, whereas both ultrafast contrast-enhanced and plain “time-of-flight” tech- niques can be used to image blood vessels [1].
5.7 Bone Invasion
Osseous invasion by soft tissue sarcomas is uncommon, with a frequency of 9% reported in a series of 133 sar- comas [27]. Bone invasion is defined as extension of a tumor into the bone cortex. Periosteal contact alone is not considered sufficient to diagnose bone involvement [28].
T1-weighted and fat-suppressed T2-weighted se-
quences are equally highly accurate in the assessment of
cortical invasion, showing cortical signal intensity
changes and/or cortical destruction [7]. In sites of pre-
dominantly fatty bone marrow, T1-WI and fat-sup-
pressed T2-WI are equally highly sensitive in the assess-
Things to remember:
1. MRI should always precede biopsy, since tumor staging based on a postbiopsy MR examination may be impossible.
2. Adequate evaluation of tumor extent requires multiplanar imaging, invariably including the ax- ial plane, and should depict all peritumoral ede- ma.
3. Don’t use FSE T2-weighted sequences without fat suppression in tumor imaging.
4. Routine i.v. Gd-contrast administration is not a requirement but is helpful in MR characterization and the clinical management, including the choice of biopsy site, of most of the soft tissue tumors.
5. The use of fat-suppressed, enhanced T1-WI is not a routine requirement and is only recommended when tumor abuts or infiltrates adjacent fat. Gd- enhancement conspicuity can also be obtained from subtraction images.
6. Do not use fat-suppressed T1-WI without their non-fat-suppressed equivalent, since they obscure surgically important fat planes, and the risk of misinterpretation of Gd enhancement.
7. Static enhanced MRI is unable to provide accurate quantitative measurement of tumor response to chemotherapy. This requires dynamic MR studies.
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