Multiparametric magnetic resonance imaging (mpMRI) can be used in conjunction with the standard transrectal ultrasound (TRUS)-guided biopsy to improve detection of clinically significant prostate cancers.
PI-RADS v2 is an international system of scoring mpMRI lesions on a scale from 1 to 5. Lesions judged to be 1 or 2 do not require biopsy, whereas lesions scored at 4 or 5 require biopsy. A score of 3 indicates a lesion that may require biopsy, depending on clinical factors.
Recent technologies permit the fusion of mpMRI to real-time transrectal ultrasound (TRUS). This enables the performance of image-guided biopsy of the prostate outside the MRI suite under TRUS and enables far more biopsies to be obtained than would be feasible if the biopsy were limited to MRI-in-gantry biopsies, which are technically complex.
Typically, the new fusion devices register the mpMRI to the TRUS image, allowing the operator to use the real-time aspects of TRUS to perform the biopsy while using the superior diagnostic features of mpMRI to target the lesion. Such biopsies have allowed more accurate localization of prostate tumors and better clinical management of patients with clinically significant prostate cancer.
Multiparametric MRI of the prostate and targeted biopsy can reduce the overdiagnosis of indolent disease that otherwise could lead to overtreatment while at the same time increase the detection of clinically significant prostate cancers, many of which may have been missed by random biopsy.
TRUS biopsy alone tends to oversample the posterior aspect of the prostate, whereas regions such as the anterior, distal apical, midline, and subcapsular prostate are frequently undersampled. These challenging areas may harbor clinically significant disease and can alter clinical management.
Although mpMRI-TRUS fusion biopsy is the most widely used method of combining mpMRI with prostate biopsy, several other image-guided biopsy techniques are also in use. One approach is the in-gantry MRI-guided prostate biopsy, which involves obtaining biopsy samples under direct MRI guidance in the MRI gantry. The main advantage of this approach is that it enables accurate lesion sampling because a confirmatory MRI showing the biopsy needle inside the target lesion can be easily obtained. However, compared with the fusion method, the procedure is long, resource-intensive, and uncomfortable, requiring the patient to lie prone in the MRI for protracted periods.
Another approach is to simply use the MRI to visually guide the TRUS biopsy on a standard ultrasound machine without software-based image fusion. This has been termed “cognitive fusion,” as it requires the operator to mentally fuse the MRI to the ultrasound. This method can be quite satisfactory in the right hands. However, not all operators are as facile with cognitive fusion, and it is difficult to teach. Thus, various methods permit the results of the mpMRI to guide subsequent prostate biopsy; however, the most efficient is probably the mpMRI-TRUS fusion method.
The biopsy approach can also be transperineal, which is accepted as safer when possible sepsis-related complications of the transrectal approach are considered. Aside from how the needle is placed in the prostate, whether perineal or transrectal, MRI plays a major role in identifying the target lesion(s) and guiding the needle path.
In the fusion platform, the first procedure of fusion biopsy is a multiparametric MRI, which is evaluated by a genitourinary radiologist specializing in prostate MRI. The evaluation has three components: 1) The T2-weighted image identifies tumors by their inherent difference from other areas of tissue; 2) Diffusion-weighted imaging (DWI) displays the rate of water diffusion in tissue, leading to precise delineation of a clinically significant tumor; and 3) A dynamic contrast-enhanced view (DCE) gives intravenous contrast and allows the radiologist to observe the rapidity with which the tissue is enhanced, helping to distinguish the tumor from normal tissue.
The second procedure of fusion biopsy is the biopsy itself. After the MRI, the radiologist outlines areas of suspicion. Those images are then transmitted to the computer in the procedure room and fused to the ultrasound image so that the anatomy of the prostate on the MRI matches what is seen on the ultrasound. Thus, the MRI guides the ultrasound evaluation and biopsy, allowing for more precise targeting of areas under investigation.
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