OASIS 1.2T: MULTIPARAMETRIC MRI OF PROSTATE CANCER

Transcription

1 OASIS 1.2T: MULTIPARAMETRIC MRI OF PROSTATE CANCER By Dr. John Feller, MD, Radiologist Desert Medical Imaging, Palm Springs, CA

2 MRI is clinically accepted as the best imaging modality for displaying anatomical details of the prostate, and has been a valuable diagnostic tool for aiding the detection and localization of prostate cancer. Prostate MR is often incorporated into the clinical workup as a staging tool after a diagnosis is made through trans-rectal ultrasound (TRUS) biopsy to exclude extension or metastasis of cancer outside the prostate gland. Despite being recognized as the gold standard for detecting prostate cancer, TRUS biopsy misses 30-35% of prostate cancers with abnormally high PSA levels or a positive digital rectal exam - a high false negative rate that complicates the decision path for the Urologist. In addition, in respect to prostate cancer it does diagnose, TRUS biopsy under-grades Gleason scores 35-45% of the time. Recently, MRI has offered a promising new implementation for prostate cancer diagnosis and intervention through the use of multiparametric techniques as an alternative to TRUS or patients with negative TRUS-guided biopsies. Diffusion-weighted imaging (DWI) allowing for better assessment of prostate cancer aggressiveness through inverse correlation with low, intermediate, and high Gleason scores. Dynamic contrast-enhanced (DCE) imaging shows promise for better characterizing prostate cancer. Combined contrast methods to improve detection and localization of prostate cancer for targeting lesions for either biopsy or minimally invasive surgical procedures. Body coils, OASIS can achieve acceptable performance characteristics of prostate cancer detection and localization compared to 1.5T and 3.0T field strengths, allowing more imaging options for the Radiologist and the patient. John Feller, MD Board certified Radiologist with Desert Medical Imaging and leading global advocate for the advancement of Prostate MR Patient History A 73-year-old male presented with elevated serum PSA levels (11) and symptoms including the urgency to urinate. The patient had a history of benign prostate hyperplasia (BPH), or enlargement of the prostate, but no obstructive urinary flow symptoms. The patient had also received two negative TRUS biopsies. MRI was performed to evaluate for prostate cancer and the potential for MR guided biopsy of the prostate gland. Multiparametric imaging of the prostate using the OASIS 1.2T open high-field platform allows a thorough diagnostic workup to fully assess structural and functional detail. The open design adds not only the commonly understood benefit of patient comfort and accessibility, but also the flexibility for patient and interventional device positioning to enhance the access and results of the biopsy. Leveraging current

3 MR Imaging Technique A number of different pulse sequences are currently used in our clinical practice for diagnostic prostate cancer MR exams using a RAPID Body 6ch receiver coil. Table 1 details our sequences and general parameters for T1- weighted imaging (T1WI), T2-weighted imaging (T2WI), diffusion weighted imaging (DWI), and dynamic contrast enhanced imaging (DCE), each playing a role in the assessment of the glandular zones of the prostate (central, peripheral, transition, anterior fibro-muscular) and adjacent tissue structures (seminal vesicle, urinary bladder, osseous structures). T1WI due to the shortening of T1 caused by blood. When hypointensities appear on T2W images that are suggestive of cancer, T1W images should be evaluated to rule out hemorrhage if there is a history of prior prostate biopsy. Using a classic fast-spin echo (FSE) with a small FOV (< 30), the patient scan showed very little contrast between tissues, indicating little to no hemorrhaging effects from prior TRUS biopsies. T1 Weighted Imaging Unlike T2WI, T1WI is generally not helpful for contrasting diseased from healthy tissue. The primary reason for acquiring T1WI is for the determination of the presence and location of hemorrhage. T1WI can contrast blood products occurring as a consequence of biopsy sampling, where areas of hemorrhage appear as hyperintense regions on Figure 1 - T1 FSE axial acquisition depicts no evidence of blood products / hemorrhage Table 1 - Details of Our Prostate Cancer Staging MR Examination as Implemented on a 1.2T Scanner (Hitachi Medical Systems America) Acquisition Plane TR (msec) TE (msec) Thick (mm) FOV (cm) Matrix Other Settings Scan Time FSE T1 Axial x208 4:12 Axial x704 RADAR 5:37 FSE T2 Coronal x704 RADAR 5:14 Sagittal x704 RADAR 5:37 Axial x96 B 100 1:36 DWI Axial x96 B 800 2:06 Axial x96 B :06 3D DCE T1 Axial x108 TR Repetition time, TE echo time, FOV field of view Post gad performing 40 acquistions 0:10 (7:57 total)

4 T2 Weighted Imaging T2WI is captured using FSE in the three orthogonal planes (axial, sagittal, coronal) typically resulting in excellent T2 contrast for the depiction of zonal anatomy, including the peripheral zone (PZ), central zone (CZ), and transition zone (TZ). Imaging is done using repetition time (TR) of 5.7 seconds, effective echo times of 96 msec, small 20cm FOV, 4mm slice thickness, and in-plane matrices of 256 X 704 deploying a radial acquisition motion compensation technique (RADAR). Though radial acquisition techniques add acquisition time, motion compensation is important to implement to reduce potential artifacts from involuntary bowel and bladder wall motion. With our DWI, we sample each slice within the 28cm FOV with a b-value of 100, 800, and 1000 s/mm 2. The b-value of 100 is used to show the combination of DWI and T2WI information, with higher b-values of 800 and 1000 showing DWI effects alone, each in relatively short acquisition times of 2:06. Combining b-value trace images with the baseline (b-value 0) allows for calculation of the ADC image, known also as the ADC map. The ADC map is free of all T1, T2, and receiver coil sensitivities, and quantitative measurements of tissue water diffusion may be made from individual voxels, or regions-of-interest (ROIs). For our patient, T2WI showed inhomogeneous signal intensity through the TZ associated with nodular enlargement most commonly due to BPH (Figure 2a-2b). Ill-defined T2 shortening is evident in the TZ, indicating a tumor suspicious region far anteriorly in the midline at the base and mid gland levels that extends to the left into the right of midline measuring 1.3cm X 1.8cm X 2.6cm. The left PZ posteriorly to the apex also shows T2 shortening effects. However, in reviewing the sagittal acquisition (Figure 2d) which is the most useful for evaluating seminal vesicle invasion, there was no evidence of tumor progression, most likely indicating fibrosis. a b Figure 2 - T2 FSE with RADAR motion compensation were acquired reflecting prostate gland measurements of 5.0cm X 5.4cm x 6.1cm captured in axial (a) and sagittal (b) images, consistent with a highly enlarged prostatic gland volume of 86cc. Diffusion Weighted Imaging One of the more important recent advances in prostate cancer imaging is the use of apparent diffusion coefficient (ADC) values derived from diffusion weighted imaging (DWI) for characterizing prostate tissue. With the onset of neoplasia, the diffusion capacity of water molecules is diminished. DWI measures the water diffusion within tissue, helping improve the contrast between cancer and normal parenchyma. When combined with T2WI, DWI dramatically improves specificity in prostate cancer detection. c d Figures 2(c) and 2(d) indicate tumor suspicious region measuring 1.3cm x 1.8cm x 2.6cm in the far anterior TZ.

5 In this patient, you can see evidence of restricted diffusion in the high b-value (1000) DWI trace and ADC map (Figure 3) shown in the same area as the T1WI and T2WI acquisitions. The DWI trace with b-value of 1000 clearly reflects a hyperintensity in the suspected region, while the resulting ADC map show lower ADCs (hypointense or darker) than the surrounding healthy prostate tissue. For our patient, the resulting DCE image acquisitions allow us to generate signal versus time kinetic curves from selected ROIs that adequately characterize the signal changes accompanying the distribution of the Gd contrast agent over the entire prostate gland. The temporal resolution of typical DCE acquisitions is fast enough to capture the rapid increase of signal, or wash-in phase, occurring within the first 60 seconds of contrast administration as noted by the 450% enhancement peak (Figure 4). The temporal resolution is also sufficient to allow for a quantitative assessment of the rate that this signal enhances from the initial slope of the signal versus time curves. The 3D RSSG imaging data set is then transmitted to a dedicated computer-aided detection (CAD) and 3D workstation where additional 3D images were reconstructed. a b Figure 3 (a) High b-value 1000 DWI trace depicts a hyperintense dominant tumor suspicious region while the corresponding ADC map (b) demonstrates restricted diffusion areas indicated by the hypointense lesion (dark) in the same region. Dynamic Contrast Enhanced Imaging The goal of dynamic contrast enhanced (DCE) imaging is to capture the passage of contrast material into and out of the prostate using T1WI with high temporal resolution (between 5 10 sec). DCE imaging acquires data on tissue perfusion characteristics and tumor wash-in / wash-out contrast to look for increased angiogenesis in suspected tumors. On OASIS, we accomplish this using a dynamic T1 weighted 3D RSSG protocol using very short TR periods of 4.3msec, flip angle of 15, and slice thickness of 4.0mm, which acquires 120 X 108 X 256 3D matrix data at 10 second intervals with parallel imaging factors of 1.5. We generally perform about 40 such acquisitions consecutively, with gadolinium (Gd) contrast automatically infused following 1 precontrast baseline acquisition. The overall acquisition time took approximately 7:57. b a Figure 4 DCE acquisitions show (a) 450% peak enhancement with a rapid wash-out kinetic curve pattern (b) correlating to region of interest color overlay in axial image.

6 Diagnostic MR Findings Using T2WI alone, findings effectively indicated nodular enlargement in the CZ most commonly due to BPH and a tumor suspicious region. However, by leveraging a broader Multiparametric Prostate MR protocol, we were able to combine findings from DWI and DCE techniques that significantly improve prostate cancer sensitivity and the negative predictive value (NPV) compared to T2WI alone. Our diagnosis concluded that while there was no evidence of extracapsular extension, seminal vesicle invasion, or periprostatic lymphadenopathy, the TZ demonstrated characteristics suggestive of prostate cancer including ill-defined T2 shortening, evidence of restricted diffusion, and a rapid washout pattern of dynamic gadolinium contrast enhancement indicating increased vascular permeability. As a result, we recommended MR targeted biopsy of the prostate gland. MR Guided Prostate Biopsy Technique The patient returned for the follow-up biopsy 8 weeks later to be conducted on the OASIS 1.2T MR system with a Body Flex XL receiver coil. MR guided biopsy was performed utilizing Invivo Prostate DynaCAD and DynaTRIM hardware and software. The patient was placed on the MRI table in the prone position and conscious sedation was performed at the start of the procedure. The needle sleeve biopsy guide was placed in the rectum with the DynaTRIM localization device attached to the biopsy guide. Preliminary sequences were obtained to document optimal positioning. For needle placement, T2 axial and sagittal sequences were acquired summarized in Table 2. Figure 5 OASIS with RAPID Body Coil and Invivo DynaTRIM patient setup. Body Flex coils for larger patients are available as used in this case. Utilizing Invivo prostate biopsy software, the lesion was localized in the TZ far anteriorly at the mid gland level. The DynaTRIM was calibrated and the needle track pathway was then determined obtaining coordinates for the DynaTRIM. Once the coordinates were obtained, the biopsy sleeve was adjusted in the sagittal, axial, and coronal planes. A coronal oblique confirmation scan was performed to confirm adequate needle track position through the lesion. Using the needle graphic confirmation dataset, an 18-gauge core biopsy needle was advanced into the lesion and a core biopsy specimen was obtained. A confirmation scan before removal of the needle was performed to ensure optimal needle position. The needle was removed and the core was placed in the specimen container. Table 2 - Details of Our Prostate Cancer MR Guided Biopsy as Implemented on a 1.2T Scanner (Hitachi Medical Systems America) Acquisition Plane TR (msec) TE (msec) Thick (mm) FOV (cm) Matrix Other Settings Scan Time FSE T2 Axial x256 RADAR 1:42 Sagittal x256 RADAR 2:56

7 a Figure 6 (a) Axial T2WI (b) Sagittal T2WI sequences acquired for optimal needle positioning. b Desert Medical Imaging (DMI) operates four imaging centers in California located in Indian Wells, Indio, Palm Springs, and Yucca Valley. All four of our facilities provide magnetic resonance imaging (MRI), spiral computed tomography (CT) imaging, and ultrasound. With vendor relationships that allow DMI to partake in research and development of various products, DMI has become a leader in prostate diagnosis and treatment, performing the first Focal Laser Ablation of prostate cancer in an outpatient center in the world. Dr. John F. Feller is a Board Certified Diagnostic Radiologist MR Guided Biopsy Findings The pathology report confirmed the tumor suspicious region is an adenocarcinoma of the prostate with a Gleason score of 3+4=7. As a result of the scanning and diagnosis, the Urologist referred the patient to a local cancer center to undergo brachytherapy radiation therapy. Discussion with a subspecialty in Orthopedic/Sports Medicine Imaging, Body MRI, and Level II Cardiac CT Certification. Dr. Feller served as an U.S. Air Force officer and Chief of MRI at David Grant USAF Medical Center and 15 years as an Assistant Clinical Professor in the Department of Radiology at Stanford University. Currently he is an Assistant Clinical Professor in the Department of Radiology at Loma Linda University and Founding Partner of DMI. In this case, we highlighted how open high-field MRI platforms like the OASIS 1.2T can achieve good performance characteristics for the effective detection and localization of prostate cancer. Multiparametric Prostate MRI combined with MRI targeted biopsy of any tumor suspicious region substantially improves the detection of clinically significant prostate cancer with a high NPV and the overall accuracy of the Gleason Scoring. The ability to conduct diagnostic and interventional prostate MR without the confinement or limitations of a closed-bore MR system will allow more imaging options with improved interventional workflow for the Radiologist, and a more comfortable experience for the patient.

8 Hitachi Medical Systems America, Inc Summit Commerce Park Twinsburg, Ohio USA Tel: Fax: Hitachi Medical Corporation Akihabara UDX Soto-Kanda, Chiyoda-ku Tokyo, Japan /DM#95708v1 Printed in U.S.A. Hitachi reserves the right to change specifications described herein without prior notice. This document provides general technical descriptions of both optional and standard features.