Wednesday, May 30, 2012

Imaging in Prostate Carcinoma 3- Magnetic Resonance Imaging

MRI can demonstrate the internal anatomy of the prostate and help clinicians to identify areas of altered signal intensity, which represent focal pathology in the gland. This modality can be used to provide the most complete evaluation of patients with prostate cancer because it can be used to assess primary disease in the prostate, as well as any involvement of the local lymph nodes. Although MRI is used primarily for staging, the availability of interventional MRI units means that MRI is likely to have a future role in the diagnosis of prostate cancer. Images of prostate cancer on MRI are provided below.

Coronal, T2-weighted magnetic resonance imaging (MRI) study of the prostate gland obtained by using an external coil. Low signal intensity (arrow) is seen on the left side of the prostate at the site of a biopsy-proven prostate cancer.

Endorectal, axial, T2-weighted magnetic resonance imaging (MRI) scan in a patient with a prostate-specific antigen level of 8ng/mL and right-sided prostate cancer. Low signal intensity is demonstrated in the right peripheral zone (arrow).

Patient with biopsy-proven prostate cancer. Axial, T1-weighted magnetic resonance imaging (MRI) scan of the pelvis shows an enlarged left obturator node (arrow).

On T1-weighted images, the prostate appears homogeneous with medium signal intensity; neither the zonal anatomy nor intraprostatic pathology is displayed. However, zonal anatomy and intraprostatic pathology are depicted on T2-weighted images, in which the cancer appears as an area of low signal intensity in the hyperintense PZ. The specificity of this appearance is low.
As with TRUS, MRI cannot accurately depict cancer in the TZ. In addition, cancer assessment with MRI may be complicated by post biopsy hemorrhage; therefore, MRI should not be performed until at least 3 weeks after biopsy.
The current role of MRI is the assessment of local extracapsular extension and invasion of the seminal vesicle. Signs of extracapsular spread include the following: irregular bulging of the prostatic outline (see the image below), breach of the capsule with extracapsular spread, asymmetry of the neurovascular bundles, and loss of the rectoprostatic angle.
Endorectal magnetic resonance imaging (MRI) scan in a patient with extensive prostate carcinoma. Image shows a bulge in the capsular outline on the right side (arrow). This is a stage T3 tumor.

Contiguous areas of low signal intensity extending into the seminal vesicles from the base of the prostate are evidence of invasion of the seminal vesicle. On T2-weighted images, reduced signal intensity in the seminal vesicles may be seen after radiation therapy or prostatic biopsy.
The optimal MRI technique for the staging of prostate cancer has not been established. Endorectal MRI appears more accurate than body-coil MRI in the local staging of the primary tumor, and 3Tesla (3T) MR scanners give improved image quality on T2-weighted sequences compared with 1.5T systems.

Diffusion-weighted and contrast-enhanced imaging

Dynamic endorectal MRI with gadolinium enhancement may provide optimal visualization of cancer in the prostate. MR spectroscopy performed with citrate and choline can provide specific information regarding prostatic metabolism; these data may be useful in assessing the biologic potential of the primary tumor and the extracapsular extension of the tumor. New approaches with diffusion-weighted imaging (DWI) sequences and dynamic contrast-enhanced (DCE) MRI sequences are currently under evaluation on 3T and 1.5T scanners in research centers.
DWI and DCE MRI are technically feasible in the prostate. Investigators have studied the potential of DWI with endorectal or phased array coils for the identification and staging of cancer within the prostate. Prostate cancer tissue has a higher cellular density than normal prostate PZ tissue, and this decreases the value of the apparent diffusion coefficient (ADC) on diffusion sequences when compared with normal prostate tissue.
DWI in the prostate suffers from poor spatial resolution compared with T2-weighted images but may be useful as a supplementary technique in drawing attention to areas of suspicion at 1.5T and 3T.
Contrast-enhanced MRI may be used as a complementary technique to T2 imaging; some studies have suggested that it is superior to T2-weighted MR for prostate cancer localization. Technical issues related to the use of contrast-enhanced MRI remain to be clarified and standardized.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF), also known as nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after they were given a gadolinium-based contrast agent to enhance MRI or MR angiography (MRA) scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

Nodal staging

Nodal staging relies on assessment of lymph node size, and neither CT scanning nor MRI can demonstrate cancer within lymph nodes that are not enlarged. However, a technique to detect clinically occult lymph node metastases using MR lymphography with a highly lymphotropic MR contrast agent was reported.[13]Intravenous lymphotropic paramagnetic nanoparticles of iron oxide, ferumoxtran-10 (Combidex; Advanced Magnetics, Cambridge, MA), were administered, and patients were examined using MRI 24 hours after contrast administration. Small lymph node metastases were identified with higher sensitivity than with conventional MRI.

3-D versus 2-D imaging

Rosenkrantz et al compared 3-dimensional (3-D), T2-weighted imaging sequences with conventional multiplanar, 2-D, turbo spin-echo, T2-weighted sequences for prostate cancer detection, staging, and image quality in 38 men with prostate cancer and found that 3-D, T2-weighted SPACE (sampling perfection with application optimized contrasts sequence with different flip angle evolutions) MRI provided a time saving of about 8 minutes, had similar image quality and accuracy in diagnosing tumors and extracapsular extension, and had better tumor conspicuity.
Images that were obtained with 2-D, turbo spin-echo sequences had a higher signal-to-noise ratio (SNR) for normal peripheral zones, but the SPACE images had greater tumor-to-peripheral zone contrast.

Degree of confidence

Extracapsular extension of a prostatic cancer is usually diagnosed with some certainty. A more difficult assessment is the interpretation of subtle bulges of the capsular outline. A significant number of prostatic cancers may be understaged, even when endorectal MRI is used.
Previous studies have shown that DRE and imaging techniques cause the understaging of cancer localized within the prostate. The most accurate imaging technique for staging prostate cancer appears to be endorectal MRI, but even this may cause significant understaging in approximately 30% of prostate cancers.



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