This prospective single institution study was approved by the local institutional review board and was compliant with Health Insurance Portability and Accountability Act (HIPAA); informed consent was obtained from each patient. Fifty patients who underwent anatomical prostate MRI at 3T and subsequent TRUS- guided prostate biopsy with electromagnetic (E-M) needle tracking were included in the study population. The median age of the patients was 61 years (mean 61.6 ± 8.4 years), and the median serum PSA level was 5.8 ng/mL (mean 8.7 ± 14.6 ng/mL).
MRI studies were performed using a combination of an endorectal coil (BPX-30, Medrad, Pittsburgh, PA, USA) and a 6-channel phased array surface coil (Philips Healthcare, Best, the Netherlands) on a 3T magnet (Achieva, Philips Healthcare) without prior bowel preparation. After digital rectal examination, the endorectal coil was inserted using a semi-anaesthetic gel (Lidocaine, Alcorn, Lake Forest, IL, USA) while the patient was in the left lateral decubitus position. The balloon surrounding the coil was distended with perfluorocarbon (Fluorinert FC-770, 3M, St. Paul, MN, USA) to a volume of approximately 50 mL. T2-weighted (T2W) images in three planes (axial, coronal and sagittal) were obtained with the parameters summarized in .
T2-weighted magnetic resonance imaging parameters used in the current study
The median interval between MRI and TRUS-guided prostate biopsy procedure was 12 days (mean 30.2 days, range 3–133 days). A 2D axial TRUS sweep was performed from the base to the apex of the prostate to reconstruct a 3D volume of the prostate before each biopsy procedure. This volume was used as a reference for MRI–TRUS registration and motion compensation (). TRUS-guided biopsies were performed using a navigation system that was previously developed for targeted prostate biopsy [7
]. A disposable needle guide with two 5-degree-of-freedom electromagnetic sensors (Traxtal Inc., Waterloo, ON, Canada) was attached to an end-firing endorectal probe (C9-5 Philips Healthcare, Bothell, WA, USA), allowing the probe to be tracked throughout the procedure with 6 degrees of freedom. The real-time TRUS images were captured using a frame grabber. The tracking information and the synchronized ultrasound video stream were recorded with a dedicated workstation. A 12-core biopsy was performed for each patient and the needle track for each biopsy core was also documented. Biopsies were performed blinded to pre-procedural MRI data.
FIG. 1 Magnetic resonance imaging-transrectal ultrasonography (MRI–TRUS) fusion in a 67-year-old male with an elevated prostate-specific antigen of 14 ng/dL. (a) Axial T2 weighted magnetic resonance image demonstrates a right-sided low signal intensity (more ...)
The position of each biopsy specimen was annotated on the MRI by translating the three coordinates of the needle track from the TRUS to the MRI. The analysis first identifies the specimen location on TRUS and then transposes the coordinates of the specimen location from TRUS to MRI using image-based registration software (Philips Research North America, Briarcliff Manor, NY, USA) that allows for image fusion between MRI and TRUS (). The software is customized from the software for MRI/TRUS-guided targeted biopsy [8
] by replacing real-time ultrasound images and probe tracking with the recorded data ().
FIG. 2 The software used for magnetic resonance imaging analysis at 12-core sextant specimen locations. (a) Left 2 column windows show multi-planar reconstructed images perpendicular to the biopsy core (single blue dot) and the sagittal view aligned with the (more ...)
Reconstructed sagittal T2-weighted magnetic resonance image shows the map belonging 12-core sextant biopsy sites.
The site of each biopsy core was correlated with the T2W MRI findings at the anatomic location of each biopsy site. An analysis of the MRI sites was performed by two radiologists (B.T., P.L.C.) in consensus blinded to the 12-core biopsy results, using customized in-house software. The software enabled display of the multi-planar reconstructions of MR images based on the position and orientation of each specimen, allowing the radiologist to browse the T2W MRI images along the angle of each biopsy core and look for prostate cancer lesions on the MR image. Each biopsy core was modelled as a cylinder of 4 mm in diameter and 16 mm in length, which corresponded to the expected location of the biopsy core of the cutting needle. If a biopsy core intersected an MRI-visible tumour, it was classified as ‘positive’ for the sequence; otherwise it was classified as ‘negative’. On T2W MRI, the criterion for a ‘positive’ was a discrete well circumscribed, round-ellipsoid low-signal-intensity lesion within the prostate gland ().
MRI findings were correlated with the biopsy results. Tumour detection rate for T2W MRI was based on positive biopsy sites as well as Gleason score and this was compared with the histological biopsy result.