MRI Prostate

MRI of the prostate is a multiparametric study that combines high resolution T2-weighted imaging, diffusion weighted imaging and dynamic contrast enhanced imaging. It is used to detect and localise clinically significant prostate cancer, to stage disease, and to direct biopsy.

The study uses a 3 tesla scanner (1.5 tesla is acceptable) with a pelvic phased array coil. An endorectal coil is optional. Slices are 3 mm thick. The examination takes 25 to 40 minutes.

The core sequences are small-field-of-view T2-weighted images in three planes, diffusion weighted imaging with b values of up to 1400 to 2000 and an apparent diffusion coefficient map, and dynamic contrast enhanced T1 images after gadolinium (0.1 mmol each kg).

Large-field-of-view T2 and diffusion images of the pelvis assess the nodes and bone.

Prostate cancer is the commonest cancer in men in many regions, with about 1.4 million new cases a year worldwide. Systematic transrectal ultrasound directed biopsy misses up to 30% of significant cancers and finds many insignificant ones.

MRI before biopsy has a negative predictive value of about 90 to 95% for clinically significant cancer. Targeted biopsy of MRI-visible lesions finds more significant cancers and fewer insignificant ones than systematic biopsy alone.

Indications.

• Raised prostate specific antigen, abnormal digital rectal examination, or a rising level, before a first biopsy.
• Persistently raised prostate specific antigen after a negative biopsy.
• Staging of biopsy proven cancer: extracapsular extension, seminal vesicle invasion, nodes and bone.
• Active surveillance: confirmation at entry and follow up.
• Planning of surgery, radiotherapy and focal therapy.
• Suspected recurrence after prostatectomy or radiotherapy.
• Benign prostatic enlargement with atypical features, prostatitis and abscess.
• Congenital anomalies, cysts and infertility (ejaculatory duct obstruction).

Not for screening of well men outside a structured programme, or for acute prostatitis (wait until the infection settles).

Contraindications and precautions.

• The usual MRI safety screen. Hip prostheses degrade diffusion images and can make the study non-diagnostic.
• Gadolinium: renal precautions. The contrast enhanced series may be omitted when diffusion and T2 are adequate.
• Antispasmodic (hyoscine butylbromide 20 mg or glucagon): the precautions for pelvic MRI apply.
• Biopsy before MRI causes haemorrhage that can mimic or hide tumour. A delay of 4 to 6 weeks after biopsy is preferred.
• Rectal gas and stool cause artefact. An enema before scanning or a short low residue diet helps.
• Endorectal coil: avoid after recent rectal surgery and in anal stenosis.

Preparation and technique.

The patient fasts for 4 hours. A small enema may be given 1 to 2 hours before the scan if rectal gas is expected. The bladder is not overfilled.

An antispasmodic is given just before scanning. T2 images in axial, sagittal and coronal planes cover the prostate and seminal vesicles. Diffusion images are taken in the axial plane, with a calculated high b value image when direct acquisition is not possible.

Dynamic contrast images are taken every 5 to 10 seconds for 2 to 3 minutes. The report names each lesion by its location in a sector map (zone, level, side), its size, and its likelihood of significant cancer on a five point scale.

Normal appearances.

The prostate is a walnut sized gland with a volume of about 20 to 30 mL in young men, increasing with age. The peripheral zone is bright on T2 and makes up the posterior and lateral gland. It is the origin of about 70% of cancers.

The transition zone surrounds the urethra, and is heterogeneous on T2 and enlarged in benign hyperplasia. The central zone is dark on T2. The anterior fibromuscular stroma is dark on all sequences.

The capsule is a thin dark line. The neurovascular bundles lie posterolaterally. Seminal vesicles are bright on T2 with a convoluted pattern, and join the prostate at the base.

Normal peripheral zone has high apparent diffusion coefficient and no early focal enhancement.

Interpretation.

• Peripheral zone cancer: a focal low T2 signal area with a restricted diffusion pattern (high on high b value image, low on the coefficient map) and early focal enhancement. A lesion of 1.5 cm or more with extraprostatic extension is very likely cancer. Diffusion is the dominant sequence in the peripheral zone.
• Transition zone cancer: an ill defined homogeneous "erased charcoal" low T2 signal area, lenticular shape, and restricted diffusion. T2 is the dominant sequence in the transition zone, and diffusion is secondary.
• Likelihood scale: one means clinically significant cancer is highly unlikely, two unlikely, three equivocal, four likely, five highly likely. Targeted biopsy is usual for four and five and considered for three.
• Benign prostatic hyperplasia: nodular, well circumscribed transition zone nodules with a capsule and mixed T2 signal. Stromal nodules are low in T2 and can mimic cancer.
• Prostatitis: diffuse or wedge shaped low T2 and low coefficient signal in the peripheral zone, which settles on follow up. Abscess has a rim enhancing collection.
• Post-biopsy haemorrhage: high T1 signal (methaemoglobin) that confirms blood rather than tumour.
• Staging: extraprostatic extension is shown by capsular bulge or irregularity, tumour contact length over 1 cm, neurovascular bundle asymmetry, loss of the rectoprostatic angle, and breach of the capsule. Seminal vesicle invasion appears as low T2 signal in the vesicle with restricted diffusion.
• Nodes and bone: pelvic nodes over 8 to 10 mm short axis or round with restricted diffusion, and bone metastases (low T1 marrow replacement with restricted diffusion).
• Recurrence after prostatectomy: an enhancing nodule at the vesicourethral anastomosis, with early enhancement and restricted diffusion. After radiotherapy, the gland is shrunken and low in T2, and recurrence is a focal area of restricted diffusion and early enhancement.
• Other: cysts of the utricle and ejaculatory ducts, calcification, and non-prostatic pelvic findings (bladder tumour, rectal cancer, nodes).

Critical values and findings needing immediate action.

• Extraprostatic extension, seminal vesicle invasion or nodal disease: multidisciplinary review before treatment planning.
• Likelihood four or five lesion: targeted biopsy.
• Bone metastases with cord compression symptoms: urgent spinal imaging and oncology review.
• Prostatic abscess with sepsis: antibiotics and drainage.
• Urinary retention or hydronephrosis from bladder outlet obstruction: catheterisation and urology review.

Limitations and artefacts.

• Image quality depends on the scanner, coil, rectal gas, motion and hip metal. A poor study should be repeated rather than reported.
• Small and low grade cancers (under 5 mm) and diffusely infiltrative tumours may be invisible. A negative MRI does not exclude cancer.
• Transition zone cancer overlaps with stromal hyperplasia. Equivocal lesions need follow up or biopsy.
• Post-biopsy blood, prostatitis, post-treatment change and prostate calcification can mimic or hide tumour.
• Extraprostatic extension has moderate sensitivity (about 50 to 60%) and high specificity. Microscopic extension is not seen.
• Nodal assessment by size is inaccurate. Small metastatic nodes are missed.
• Prior hormonal therapy shrinks tumour and lowers its conspicuity.
• Reporting varies with experience. Use of a structured report improves consistency.

Special groups.

• Men on active surveillance: MRI at entry and at 12 to 18 month intervals, with repeat biopsy for progression.
• Men with a family history or known genetic risk: earlier consideration of MRI.
• Men after prior negative biopsy with rising prostate specific antigen: MRI-targeted biopsy of any lesion.
• Men with hip prostheses: diffusion may fail. Consider CT or PET.
• Older men with high risk disease: add staging of bone and nodes.
• Regions: late presentation with advanced or metastatic disease is frequent, and the incidence in men of African descent is higher with more aggressive disease. MRI is rarely available, so prostate specific antigen, digital examination, ultrasound directed biopsy and clinical staging with bone scan are used.

Follow up tests.

• MRI-targeted biopsy (cognitive, software fusion or in bore) with systematic cores.
• Prostate specific antigen with density (level divided by gland volume), free to total ratio, and the other blood or urine tests that refine risk.
• Prostate specific membrane antigen PET-CT for staging and recurrence.
• Bone scan, CT or whole body MRI for metastases.
• Urinalysis and culture for infection.
• Uroflowmetry and post-void residual volume for obstruction.
• Referral to urology, radiation oncology or oncology.