Multiplanar MRI assists rectal cancer detection and staging

Tumor detection prior to invasion through the muscularis propria and before lymph node metastases appear offers the best prognosis and permits more limited surgery.2 Anterior or abdominoperineal resection is the standard treatment for rectal tumors, although a greater proportion of resections are now sphincter-saving or involve local excision.3,4 Despite surgical advances, local recurrence rates postresection of a primary rectal carcinoma range from 30% to 50%.5,6 Factors that predict local recurrence include tumor morphology, tumor and nodal stage (see table), surgical technique, and involvement of the lateral resection margin at the time of surgery.7,8 Two advances in therapy are reducing the frequency of local recurrence and improving survival: total mesorectal excision and preoperative neoadjuvant chemoradiation therapy, which can downstage locally advanced lesions and decrease locoregional recurrence.6,7 These advances have increased the importance of accurate preoperative assessment of a tumor's location and size and the extent of local infiltration in determining the appropriate therapeutic strategy.1,9,10 Imaging must provide reliable anatomical and spatial information to aid surgical planning. Accurate assessment of the depth of mural and extramural spread also influences selection of patients for preoperative adjuvant therapies. Precise evaluation of locally recurrent carcinoma is essential to treatment planning as well. MR has been used to assess preoperative and recurrent rectal carcinoma. Multiplanar MRI and high tissue contrast enable noninvasive 3D definition of tumor extension and reveal depth of local wall invasion, extramural infiltration, and regional lymph node metastases. MR OPTIONS Initial attempts to stage rectal carcinoma with conventional body coil MR imaging proved disappointing, showing no increase in accuracy over CT.4 The problems were mainly due to poor spatial resolution and difficulties resolving rectal wall layers, which in turn hampered evaluations of the depth of wall invasion.8,9 Because wall invasion is an essential part of tumor staging and treatment planning, MR staging has not been adopted as a routine tool. Development of phased-array flexible surface coils and rapid thin-section scanning techniques, with and without intravenous paramagnetic contrast, has improved the accuracy of MR staging. Endorectal surface coils and double-contrast techniques have further expanded MR's applications in evaluating rectal and perirectal neoplasm. Phased-array pelvic coils and fast spin-echo sequences offer improved rectal wall spatial resolution and fewer motion artifacts than body coil imaging.5,9 Detailed visualization of the entire mesorectum and the surrounding pelvic structures permits rectal tumor staging with a high degree of accuracy. Standard protocols should involve multiplanar high-resolution T1and T2-weighted MR imaging. Fat saturation sequences may be added to eliminate signal from perirectal fat; they may also increase lesion conspicuity, particularly in advanced lesions invading through the muscularis propria. Some authors do not find these sequences necessary, however.1,11 Fat saturation sequences can additionally help define advanced or recurrent tumor masses and detect potential complications such as fistulae. Oblique axial T2-weighted MR imaging through the rectal tumor and adjacent perirectal tissues, using a small field-of-view, can obtain true axial assessment of the tumor and reduce depth overestimations. The required scan time of approximately 20 minutes is acceptable even in busy departments that perform a large number of examinations. Some authors advocate adding dynamic MR sequences following administration of intravenous gadolinium, although this increases scan time.4,5 The mucosa and muscularis mucosa display early