4.4 Pelvis Protocols

Pelvis Protocols

MRI of the pelvis is a highly specialized clinical modality utilized for the staging and assessment of gynecological malignancies (endometrial, cervical, and ovarian cancers), prostate cancer, rectal cancer, and pelvic inflammatory disease. Successful pelvic imaging requires a balance between high spatial resolution, tissue contrast resolution, and the management of involuntary bowel motion, respiratory motion, and cardiac/vascular flow artifacts.

Male Pelvis & Prostate

Multiparametric MRI (mpMRI) of the prostate is the clinical standard for the detection, localization, and staging of prostate cancer, using the Prostate Imaging-Reporting and Data System (PI-RADS v2.1) guidelines.

• Coil Selection: A multichannel phased-array pelvic coil (surface coil) is positioned over the patient's pelvis. On modern 3.0 Tesla scanners, this coil configuration provides sufficient Signal-to-Noise Ratio (SNR) for high-resolution imaging without the need for an endorectal coil (ERC). On older 1.5 Tesla scanners, an ERC may be utilized to achieve equivalent spatial resolution.
• Key mpMRI Sequences & Zonal Anatomy: The prostate is divided into anatomical zones, and different sequences are considered primary for diagnosis depending on the zone:
  • Peripheral Zone (PZ): Located posteriorly and laterally, containing 70% of glandular tissue. Normal PZ tissue is highly hyperintense (bright) on T2-weighted scans. Prostate cancer appears as a focal hypointense (dark) area. DWI/ADC is the primary (dominant) sequence for evaluating the PZ. Significant restricted diffusion (bright on high b-value DWI, dark on ADC map) is the key indicator of malignancy.
  • Transition Zone (TZ): Surrounds the urethra and is the site of Benign Prostatic Hyperplasia (BPH). T2-weighted imaging is the primary (dominant) sequence for evaluating the TZ. Tumors in the TZ appear as erased-charcoal, ill-defined hypointense areas, whereas BPH nodules appear as well-circumscribed "organized chaos" nodules with a hypointense rim.
  • Dynamic Contrast Enhancement (DCE): Acquired using rapid, serial 3D T1-weighted fat-suppressed gradient-echo sequences post-GBCA injection. Prostate cancer typically demonstrates early, rapid enhancement ("wash-in") and rapid washout (Type III curve) compared to normal tissue. DCE serves as a tie-breaker for PI-RADS category 3 lesions in the Peripheral Zone.
• PI-RADS Scoring System: PI-RADS scores range from 1 to 5, indicating the likelihood of clinically significant prostate cancer:
  • PI-RADS 1: Very low (clinically significant cancer is highly unlikely)
  • PI-RADS 2: Low (clinically significant cancer is unlikely)
  • PI-RADS 3: Equivocal (clinically significant cancer presence is equivocal)
  • PI-RADS 4: High (clinically significant cancer is likely)
  • PI-RADS 5: Very high (clinically significant cancer is highly likely)

Female Pelvis & Gynecological Oncology

Pelvic protocols in female patients are optimized to evaluate uterine, cervical, and ovarian pathology.

• Slice Planning and Orientation:
  • Sagittal T2-Weighted: The primary localizer and planning plane. It allows for the direct visualization of the longitudinal axis of the uterus, cervix, and vagina.
  • Endometrial Cancer Staging: Axial-oblique T2-weighted sequences are planned perpendicular to the long axis of the uterine cavity to assess the depth of myometrial invasion (Stage IA: < 50% invasion; Stage IB: >= 50% invasion).
  • Cervical Cancer Staging: Axial-oblique T2-weighted sequences are planned perpendicular to the cervical canal to assess for parametrial invasion (extension of tumor beyond the fibrous cervical stroma).
  • Dynamic Post-Contrast T1: Dynamic contrast-enhanced T1-weighted sequences are used to assess the depth of myometrial invasion in endometrial cancer. The normal myometrium enhances intensely, whereas the tumor enhances more slowly, improving detection of invasion depth.
• Ovarian Mass Characterization: Includes axial and coronal T2-weighted sequences, along with pre- and post-contrast T1-weighted fat-suppressed sequences. Utilizing subtraction imaging is critical to distinguish high T1 signal intensity of hemorrhage or fat (seen in endometriomas or dermoid cysts) from true contrast enhancement in solid tissue components.

Rectal Cancer Protocol

High-resolution rectal MRI is essential for local staging of rectal adenocarcinoma.

• Slice Planning: Standard axial slices are not sufficient due to the oblique orientation of the rectum. Coronal and axial oblique T2-weighted sequences must be planned perpendicular and parallel to the long axis of the rectum at the level of the tumor.
• Diagnostic Targets: The key goal is to assess tumor extension through the muscularis propria into the mesorectum. Technologists must achieve sub-millimeter in-plane resolution to evaluate the mesorectal fascia (MRF) and the circumferential resection margin (CRM). If the tumor is within 1 mm of the MRF, the CRM is threatened, indicating a high risk of local recurrence.

Bowel Peristalsis Management and Prep

Bowel peristalsis (involuntary muscular contractions of the small and large bowel) creates severe ghosting artifacts that propagate across the pelvis, obscuring the prostate, uterus, ovaries, and rectum.

• Anti-Spasmodic Agents: The administration of an anti-spasmodic agent, such as glucagon (typically 0.5–1.0 mg IV or IM) or hyoscine butylbromide (Buscopan), immediately prior to the scan is the most effective method to temporarily paralyze bowel peristalsis.
  • Contraindications: Glucagon is contraindicated in patients with pheochromocytoma, insulinoma, or glucagonoma. Buscopan is contraindicated in patients with narrow-angle glaucoma, urinary retention, or myasthenia gravis.
• Patient Prep: Patients should fast (NPO for 4–6 hours) to reduce bowel activity.
• Bladder Prep: For gynecological oncology protocols, a partially full bladder is preferred. This pushes loops of the small bowel superiorly out of the pelvis, away from the uterus and ovaries. For prostate MRI, the patient should completely empty their bladder prior to the scan to reduce bladder motion and minimize patient discomfort during the scan.
• Spatial Presaturation Bands: Placed superiorly and inferiorly to the imaging volume to suppress pulsatile flow artifacts from the iliac vessels.

Pelvic Lymph Node Assessment

In staging pelvic malignancies, evaluating regional lymph nodes is critical. A larger field of view (FOV) axial and coronal T2 and T1 sequence is typically included, extending from the aortic bifurcation down to the pubic symphysis. This ensures coverage of the common iliac, external iliac, internal iliac, and obturator lymph node chains. Diffusion-weighted imaging (DWI) with high b-values assists in locating small nodes by highlighting their high cellularity.

Summary of Pelvis Protocols