12 Dynamic Magnetic Resonance Imaging and Nuclear Imaging

rest, during squeeze, and during evacuation by a large-field-of-view gamma camera. The anal canal axis is defined by a line drawn through the middle of the balloon image of the anal canal.

The rectal axis is defined by a line drawn paral- lel to the most inferior portion of the rectal image, just cephalad to the anal canal. The loca- tion of the anorectal junction is measured as the perpendicular distance from a line drawn between the pubococcygeal line to the most caudal aspect of the rectal image. In healthy persons, rectal evacuation is associated with widening of the anorectal angle and perineal descent (Fig. 12.2). In patients with symptoms of a functional defecation disorder, scintigraphy may reveal either reduced motion (i.e., anorectal angle change or descent), or increased perineal descent (i.e., descending perineum syndrome). ^6,7 To quantify rectal emptying, the rectum is filled with a 7.5% colloidal dispersion of alu- minium magnesium silicate labeled with 1 mCi of ^99m Tc with patients in the left lateral decubitus position. ^3,8 Images are obtained before evacua- tion (3-minute acquisition), during evacuation (2-second dynamic images over 4 minutes), and after evacuation (3-minute acquisition) on a commode. The percentage and rate of total ^99m Tc evacuation is calculated; less than 37% evacua- tion (i.e., 2 standard deviations below mean) is considered abnormal ³ (Fig. 12.3). The median evacuation time for seven asymptomatic sub- jects was 40 seconds.

Utility

By documenting abnormal (i.e., reduced or increased) anorectal motion during evacuation, Defecation requires intraabdominal pressure

coordinated with relaxation of the anal sphinc- ters and pelvic floor muscles, particularly the puborectalis muscles. Pelvic floor motion can be imaged in real-time by barium fluoroscopy, radioisotope scintigraphy, or magnetic reso- nance imaging (MRI). Barium evacuation proc- tography was described in separate reports by Burhenne ¹ in 1964 and Phillips and Edwards ² in 1965. O’Connell et al ³ developed scintigraphy to quantify rectal evacuation after ileal pouch anal anastomosis with lesser radiation exposure compared to barium proctography. Dynamic MRI visualizes anorectal and pelvic floor motion during rectal evacuation without radiation exposure. ⁴

Scintigraphic Defecography

Anorectal motion during evacuation is meas- ured with a radioisotope-filled balloon within the rectum and anal canal. ⁵ Rectal evacuation is measured by filling the rectum with a radioisotope.

Procedure

A low-compliance, 16-cm long, latex balloon is inserted through the anal canal into the rectum (Fig. 12.1). The balloon is connected to a reser- voir filled with 500 cc of water labeled with technetium-99m ( ^99m Tc). Radioactive markers are taped to the skin over the pubis anteriorly and over the tip of the coccyx posteriorly. With the patient in the left lateral decubitus position, static images are acquired over 15 seconds at

12

Dynamic Magnetic Resonance Imaging and Nuclear Imaging

Adil E. Bharucha, Joel G. Fletcher, and John H. Pemberton

111

scintigraphy is useful for confirming a clinically suspected functional disorder of defecation, with far less radiation than is delivered by standard fluoroscopic technique. ^5,6 Anorectal scintigraphy has not been directly compared to barium evacuation proctography. However, in contrast to barium proctography, scintigraphic proctography cannot visualize anatomic dis- turbances during rectal evacuation such as a rectocele, enterocele, rectal intussusception, or uterine prolapse.

Magnetic Resonance Proctography

Magnetic resonance imaging (MRI) is a rela- tively new modality for imaging anal sphincter anatomy and pelvic floor motion during defeca- tion and squeeze without radiation exposure. ⁹ The anal sphincters can also be visualized, preferably using an endoanal MRI coil. ¹⁰

Figure 12.1. Diagram of the scintigraphic balloon device in place. The proximal portion of the balloon rests in the rectum, the central portion traverses the anal canal, and the distal portion projects beyond the anal verge. Radioactive markers are taped to the skin over the pubis anteri- orly and over the tip of the coccyx posteriorly. (From Barkel et al,

copyright Lippincott Williams & Wilkins, 1988.)

Figure 12.2. Scintigraphic evaluation

of anorectal motion during evacuation

and squeeze. Observe that the part of the

balloon shaded black is outside the anal

canal.

Rapid image acquisition is necessary to visu- alize pelvic floor motion in real time because patients can maintain maximum rectal excur- sion during evacuation or puborectalis contrac- tion when they squeeze for 15 to 30 seconds at most (Figs. 12.4 and 12.5). In contrast to scintig- raphy or fluoroscopy, conventional, closed- configuration MR systems permit imaging in the supine position only. However, there is little difference in the detection of clinically relevant findings between supine MR and seated MR using open-configuration magnets. The excep- tion is in detecting rectal intussusceptions for which seated MRI was superior. ¹³

Similar to barium evacuation proctography, anorectal motion is quantified by measuring anorectal angles and anorectal junction during evacuation and squeeze maneuvers. Because the bony landmarks (e.g., pubis, sacrococcygeal junction) necessary to measure anorectal descent are more distinctly visualized during MRI, it is easier to make these measurements from MR compared to routine fluoroscopic images. A rectocoele is defined as an anterior bulge beyond the expected and extrapolated line

Procedure

No preparation is necessary. After adding about 150 mL of ultrasound (US) gel to the rectum, anorectal and pelvic floor motion are visualized in real time while patients squeeze (i.e., contract) the pelvic floor muscles and expel rectal con- tents. Using a modified T2-weighted single-shot fast spin echo (SSFSE) imaging sequence or T2- weighted fast imaging with steady-state preces- sion (FISP) MRI sequence, pelvic floor motion can be imaged at 1.2- to 2-second intervals. ¹¹ Images can be reconstructed in real time, or shortly after acquisition, so that patients can be instructed or encouraged during maneuvers.

After the evacuation sequence, patients are removed from the magnet and asked to empty the bladder and remaining rectal contents.

Patients are then repositioned within the magnet to obtain additional dynamic sagittal images during the Valsalva maneuver, in order to maximize the detection of cystoceles and ente- roceles. ¹² Finally, contiguous coronal images are acquired at rest and during the Valsalva maneuver. ¹⁰

Evacuation Curve

Evacuation R01 Pre Evacuation R01

72430 counts

Post Evacuation R01 24184 counts

1000 800 600 400 200

Courts

0

0 20 40 60 80 100 120

Duration (sec)

Pre Evacuation

Evacuation target Post Evacuation

Percent Evacuated = 66

Figure 12.3. Scintigraphic evaluation of normal rectal evacuation.

of the anterior rectal wall or a posterior bulge beyond the posterior wall. ¹⁴ A peritoneocele is characterized by protrusion of the peritoneal fat or fluid crossing the junction of the upper third and distal two thirds of the vagina, with separa- tion of the rectovaginal septum. When this pouch contains small intestinal loops, it is described as an enterocoele (Fig. 12.6). Descent of the bladder base and either the cervix (when the uterus is present) or vaginal apex (for women who had a previous hysterectomy) are recorded relative to the pubococcygeal line. ^14,15

Utility

In asymptomatic women without a history of, or risk factors for, pelvic floor injury, anorectal

motion parameters were comparable to previous studies using barium defecography (Table 12.1). ¹⁶ These normal values are derived from a group of women (mean age, 43; range 23–69 years) who did not have risk factors for anorec- tal trauma or bowel symptoms by detailed questionnaire. Observe that the normal range for anorectal angle change during evacuation was wide, perhaps because some asymptomatic sub- jects may have pelvic floor dysfunction. Normal values for anorectal motion may be technique dependent. ¹⁵ The utility of dynamic MRI for diagnosing functional disorders of defecation has not been thoroughly studied. An early study suggested that MR revealed disordered anorec- tal motion in 13 patients with symptoms of disordered defecation and normal routine anorectal physiologic tests. ¹⁷ More detailed studies suggest that dynamic MRI may have a

a b c

Figure 12.4. Magnetic resonance fluoroscopic images of the pelvis at rest (a), during squeeze (b), and simulated defecation (c) in a 52-year-old asymptomatic patient after filling the rectum with ultrasound gel.At rest,the pelvic floor was well supported; the anorectal angle was relatively obtuse (126 degrees). Pelvic floor contraction during the squeeze maneuver was accompanied by normal upward and anterior motion of the anorectal junc- tion; the angle declined to 95 degrees. During rectal evacuation, the bladder base dropped by 2.5 cm below the pubococcygeal line; the 2.8-cm ante- rior rectocoele emptied completely and was probably not clinically signi ficant; perineal descent (5cm) was outside the normal range for evacuation proctography.

a b

Figure 12.5. Pelvic magnetic reso- nance fluoroscopic images at rest (a) and squeeze (b) in a 57-year-old woman who has fecal incontinence.

During squeeze, the puborectalis

indentation on the posterior rectal

wall was exaggerated compared to

rest, and the anorectal angle declined

from 143 degrees at rest to 90 degrees

during squeeze; however, the anal

canal remained patulous.

and a normal balloon expulsion test. ¹⁸ In addi- tion to characterizing disordered squeeze or evacuation, dynamic MRI can also characterize pelvic organ prolapse, including rectocoele size, which is associated with increased perineal descent. ¹⁹ Lastly, patients find it useful to review images of evacuation for understanding the nature of their disorder, and the need for pelvic floor retraining.

Studies utilizing a relatively slow image acquisition sequence (i.e., every 4 to 31 seconds), found a poor correlation between MRI and barium proctography. ^20,21 In contrast, when the acquisition time is 1.2 to 2.0 seconds, the corre- lation between dynamic MRI and colposysto- proctography for all three pelvic floor compartments (anterior, middle, and posterior) was excellent. ²²

Acknowledgments

This work was supported in part by United States Public Health Service–National Institutes of Health grants R01 HD38666 and R01 HD41129, and General Clinical Research Center grant M01 RR00585. Copyright 2004 Mayo Foundation.

role when the diagnosis of an evacuation disor- der cannot be confirmed by routine methods, for example, in patients with normal or increased perineal descent during a clinical examination,

Table 12.1. Normal values for anorectal motion by dynamic mRI 10

, 90

percentile

Parameter Mean values

Anorectal angle at rest 103 degrees 87, 124 Anorectal angle during 122 degrees 105, 138

evacuation

Anorectal angle during squeeze 72 degrees 52, 85 Anorectal angle change during 19 degrees −1, 40

evacuation

Anorectal angle change during −31 degrees −13, −53 squeeze

Location of anorectal junction 2.2 cm 0.6, 3.9 at rest

Perineal descent during 3.3 cm 1.6, 5.1 evacuation

Perineal ascent during squeeze 1.7 cm 1.0, 2.6 Negative values indicate a smaller angle during the maneuver compared to rest. Distances reflect perpendicular distance from the anorectal junction to the pubococcygeal line.

a b

c d

Figure 12.6. Endoanal T2-weighted

fast spin echo imaging of the anal

sphincters demonstrates marked

diffuse thinning of the internal anal

sphincter (arrowheads, a), along with

correlative endoanal ultrasound (arrow-

heads, b). Dynamic MR proctography

images at rest (c) demonstrates a patu-

lous anal canal at rest (arrow). Dynamic

MR proctography images during defe-

cation (d) show a large enterocele

(arrowheads), with mesenteric fat and

small bowel.

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