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Prostate Cancer: Body-Array versus Endorectal Coil MR Imaging at 3 T—Comparison of Image Quality, Localization, and Staging Performance¹

¹ From the Departments of Radiology (S.W.T.P.J.H., J.J.F., T.H., S.T., T.W.J.S., H.J.H., J.O.B.), Pathology (C.A.H.), Urology (B.C.K., L.A.L.M.K., J.A.W.), and Epidemiology and Biostatistics (L.A.L.M.K.), Radboud University Nijmegen Medical Centre, Geert Grooteplein zuid 10, NL 6500 HB, Nijmegen, the Netherlands. From the 2005 RSNA Annual Meeting. Received March 7, 2006; revision requested May 5; revision received June 6; accepted June 19; final version accepted November 9. Supported by Dutch Cancer Society grant KUN 2003-2925. Address correspondence to S.W.T.P.J.H. (e-mail: S.Heijmink{at}rad.umcn.nl).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Purpose: To prospectively compare image quality and accuracy of prostate cancer localization and staging with body-array coil (BAC) versus endorectal coil (ERC) T2-weighted magnetic resonance (MR) imaging at 3 T, with histopathologic findings as the reference standard.

Materials and Methods: After institutional review board approval and written informed consent, 46 men underwent 3-T T2-weighted MR imaging with a BAC (voxel size, 0.43 x 0.43 x 4.00 mm) and an ERC (voxel size, 0.26 x 0.26 x 2.50 mm) before radical prostatectomy. Four radiologists independently evaluated data sets obtained with the BAC and ERC separately. Ten image quality characteristics related to prostate cancer localization and staging were assigned scores. Prostate cancer presence was recorded with a five-point probability scale in each of 14 segments that included the whole prostate. Disease stage was classified as organ-confined or locally advanced with a five-point probability scale. Whole-mount-section histopathologic examination was the reference standard. Areas under the receiver operating characteristic curve (AUCs) and diagnostic performance parameters were determined. A difference with a P value of less than .05 was considered significant.

Results: Forty-six patients (mean age, 61 years) were included for analysis. Significantly more motion artifacts were present with ERC imaging (P < .001). All other image quality characteristics improved significantly (P < .001) with ERC imaging. With ERC imaging, the AUC for localization of prostate cancer was significantly increased from 0.62 to 0.68 (P < .001). ERC imaging significantly increased the AUCs for staging, and sensitivity for detection of locally advanced disease by experienced readers was increased from 7% (one of 15) to a range of 73% (11 of 15) to 80% (12 of 15) (P < .05), whereas a high specificity of 97% (30 of 31) to 100% (31 of 31) was maintained. Extracapsular extension as small as 0.5 mm at histopathologic examination could be accurately detected only with ERC imaging.

Conclusion: Image quality and localization improved significantly with ERC imaging compared with BAC imaging. For experienced radiologists, the staging performance was significantly better with ERC imaging.

© RSNA, 2007

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Magnetic resonance (MR) imaging can play a role in the diagnosis of prostate cancer. First, the characteristic pattern of prostate cancer as a low-signal-intensity lesion on T2-weighted MR images makes MR imaging suitable for detection and localization purposes (1,2). However, at standard clinical field strengths of 1.5 T, an endorectal coil (ERC) is necessary to obtain a sufficiently high signal-to-noise ratio with subsequent spatial resolution to allow reliable cancer delineation (3) in a clinically reasonable time frame. At higher field strengths such as 3 T (4), the signal-to-noise ratio increases (5), and the need for an ERC for detection or localization of prostate cancer at this field strength has yet to be determined. The use of a body-array coil (BAC) alone for signal reception (6) would save time and costs and would cause less discomfort for the patient. A clinical application that requires accurate localization of cancer is intensity-modulated radiation therapy (7,8). Accurate localization can further aid in preoperative decisions about the status of the neurovascular bundles, in the direction of ultrasound-guided biopsy in patients with negative findings at previous biopsy, or in the guidance of high-intensity focused ultrasound treatment.

Second, MR imaging can help in the determination of the extent of local disease, specifically whether the cancer is organ confined (stage T2) or locally advanced (stage T3). Recently, it was shown that MR imaging at 1.5 T with an ERC outperformed staging nomograms (9). Nevertheless, its use for prostate cancer staging is still a matter of debate (10). Previously, it was shown that staging by means of 1.5-T MR imaging had a joint maximum sensitivity and specificity of 71% (11). Researchers in a preliminary study in 32 patients in whom T2-weighted MR imaging with an ERC at 3 T was performed found a sensitivity for staging of 88% (seven of eight), with a high specificity of 96% (23 of 24), for experienced readers (12). However, to our knowledge, no studies have been published about prostate cancer staging at 3-T MR imaging with the use of only a BAC for signal reception nor about comparisons of the use of a BAC with that of an ERC at 3-T MR imaging. Therefore, it is important to determine whether an ERC is still necessary or a BAC could suffice for staging purposes at 3-T MR imaging.

Thus, the purpose of our study was to prospectively compare image quality and accuracy of prostate cancer localization and staging with a BAC versus those with an ERC at T2-weighted MR imaging at 3 T, with histopathologic findings as the reference standard.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Medrad (Pittsburgh, Pa) unconditionally supplied the prototypes of the balloon-mounted disposable 3-T surface ERCs and the interface device. The authors had complete control of the data and information submitted for publication.

Patient Characteristics
The study was approved by the institutional review board. Before inclusion, all patients gave their written informed consent. From June 2004 to January 2006, consecutive patients who met our study criteria and had biopsy-proved and clinically localized prostate cancer were scheduled to undergo MR imaging at 3 T with both a BAC and an ERC prior to radical prostatectomy. Exclusion criteria were contraindications for MR imaging (eg, pacemaker, metal cerebral clips) or ERC insertion (eg, prior anorectal surgery, inflammatory bowel disease), as well as severe claustrophobia and high anal sphincter tension that prevented ERC introduction. For all included patients, the time between the last biopsy procedure and MR imaging, as well as the time between MR imaging and surgery, was determined.

MR Imaging Acquisition
All images were obtained by using a 3-T whole-body imager (Magnetom Trio; Siemens Medical Solutions, Erlangen, Germany). In the first part of the examination, a commercially available eight-element standard BAC was placed around the pelvic area with the patient in the supine position. After localizing images were obtained, T2-weighted fast spin-echo image series in the transverse, sagittal, and coronal planes were obtained, with the transverse image series aligned perpendicularly to the anterior rectal wall. Radiofrequency power deposition from the spin-echo train was reduced by using hyperechoes (13). The optimized sequence parameters were as follows: repetition time msec/echo time (TE) msec, 3700/124; field of view, 220 x 100 mm; section thickness, 4 mm; matrix, 512 x 512; voxel size, 0.43 x 0.43 x 4.00 mm; number of signals acquired, two; and acquisition time, 4 minutes 57 seconds. A three-dimensional T1-weighted gradient-echo image series was obtained to exclude hemorrhage after biopsy.

In the second part of the examination, after removal of the BAC and performance of a digital rectal examination, a prototype ERC (Medrad) was inserted, with the patient in the left lateral decubitus position. The balloon was then inflated with either 40–60 mL of demineralized water or perfluoro polyether (Fomblin; Solvay Solexis, Milan, Italy). The patient was placed in the supine position, and bowel movement was suppressed with an intramuscular injection of 1 mg glucagon (Glucagen; Novo Nordisk, Bagsvaerd, Denmark). An ERC localization image series was obtained and, if necessary, the ERC position was readjusted. Subsequently, T2-weighted fast spin-echo images were obtained with the use of hyperechoes in the transverse, sagittal, and coronal planes, with the transverse image series aligned perpendicularly to the anterior rectal wall. Sequence parameters were as follows: 5000/153; field of view, 200 x 100 mm; section thickness, 2.5 mm; matrix, 768 x 384; voxel size, 0.26 x 0.26 x 2.50 mm; number of signals acquired, one; and acquisition time, 2 minutes 58 seconds. Last, a three-dimensional T1-weighted gradient-echo image series was obtained. No adverse events were reported, and all patients tolerated the examination well.

MR Imaging Interpretation
Prospectively, four radiologists, two experienced and two less experienced, independently read all imaging data sets from which patient identification was removed. The two experienced readers, radiologists A (J.J.F.) and B (S.W.T.P.J.H.), had 4 and 2 years of experience, respectively, in ERC prostate MR imaging interpretation, which corresponded to the previous reading of 400 and 150 ERC MR studies, respectively. The two less experienced readers, radiologists C (S.T.) and D (T.H.), both had 3 months of experience in preoperative ERC prostate MR image interpretation; this experience corresponded to the reading of approximately 20 MR studies each. Radiologist C, however, had 8 years of experience (which corresponded to the reading of 100 MR studies) with preoperative prostate cancer MR imaging with the use of a pelvic phased-array coil at 1.5 T. Contrary to the experience level of radiologist C, radiologist D participated in a short training course about the reading of ERC prostate MR images obtained at 3 T prior to assignment of scores. This course comprised the reading of MR images obtained in approximately 15 patients with prostate cancer, with direct histopathologic feedback. These patients were not included in the current study.

To prevent information bias of either MR study influencing the other, imaging sets obtained with the BAC and the ERC in the same patient were never read in the same session, and the order was randomized. The authors were in control of the number of sessions and time between sessions. The readers knew the patients had histopathologically confirmed prostate cancer at biopsy but were blinded to all other clinical data. Readers classified the image quality of each set as excellent, sufficient, or insufficient on the basis of the presence of any artifacts that affected image interpretation, as well as the signal-to-noise ratio. If artifacts or a low signal-to-noise ratio hampered interpretation of the images, sets were classified as insufficient. If no artifacts were present and the signal-to-noise ratio was high, sets were classified as excellent. All other imaging sets were classified as sufficient. If either imaging set was classified as insufficient by two or more readers, the patient was excluded from analysis.

First, the readers were asked to assign a score to 10 image quality characteristics derived from both the literature (5,14) and the authors' (J.J.F., J.O.B.) prior experience. Characteristics related to localization included the following: the discrimination between the peripheral zone and the central gland, visibility of the peripheral zone itself, visibility of the central gland itself, visibility of the lesion, and visualization of the internal architecture of the central gland. Staging-related characteristics included the following: the delineation of the prostatic capsule, visualization of the neurovascular bundle, and visualization of the rectoprostatic angle. A general characteristic was the impression of the overall image quality. These nine characteristics were assigned scores on a five-point scale as follows: score 1, poor; score 2, moderate; score 3, satisfactory; score 4, good; and score 5, excellent. The final characteristic, the presence of motion artifacts that affected image interpretation, was assigned a score on the basis of the following five-point scale: score 1, no artifacts were present; score 2, hardly any motion artifacts were present; score 3, image quality was satisfactory; score 4, image quality was moderately affected, and score 5, image quality was severely affected by motion artifacts. All assignment of scores was performed subjectively without specific criteria for each of the five points on the scale.

Second, for the purpose of cancer localization, the prostate was divided into apex, mid gland, and base regions. Both apex and base were subdivided into quadrants, and the mid gland was divided into sextants; thus, the whole prostate was mapped into 14 segments. All readers determined the presence of cancer in each segment with a five-point probability scale as follows: score 1, definitely absent; score 2, probably absent; score 3, possibly present; score 4, probably present; and score 5, definitely present. The criterion for assignment of a classification to a segment as cancerous in the peripheral zone was the appearance of an area of low signal intensity on T2-weighted fast spin-echo images (2,15). In the central gland, the criterion was the appearance of an area of homogeneously low signal intensity with ill-defined margins on T2-weighted fast spin-echo images (16,17). Areas of low signal intensity on T2-weighted fast spin-echo images and areas of high signal intensity on T1-weighted gradient-echo images were considered indicative of hemorrhage after biopsy (18) and were classified as not containing cancer.

Third, the readers assigned a score for the disease stage with a five-point probability scale as follows: score 1, definitely stage T2; score 2, probably stage T2; score 3, possibly stage T3; score 4, probably stage T3; and score 5, definitely stage T3. Criteria derived from the literature for extraprostatic extension included bulging or irregularity of the prostatic capsule, obliteration of the rectoprostatic angle or neurovascular bundle, an area of low signal intensity within the periprostatic fat, and marked seminal vesicle asymmetry or an area of low signal intensity in the seminal vesicles on T2-weighted fast spin-echo images (12,14,19,20). All radiologists were instructed to perform high-specificity staging (21,22) to prevent false-positive judgments. Readers assigned the localization of the perceived extraprostatic extension to one or more of the 14 segments.

Surgery with Histopathologic Examination as Reference Standard
All radical retropubic prostatectomies were performed by one of two oncologic urologists with 18 (J.A.W.) and 5 (B.C.K.) years of experience who were cognizant of the MR imaging results.

After excision, prostatectomy specimens were fixed overnight in 10% neutral buffered formaldehyde and coated with India ink. Four-millimeter-interval whole-mount sections were cut at a plane comparable to that of the transverse MR imaging plane. All sections were routinely embedded in paraffin. Tissue sections of 5 µm were prepared and stained with hematoxylin-eosin. The exact localization, volume, Gleason score, extent of each cancer focus, and radial distance (23) of each extracapsular penetration were determined by a genitourinary histopathologist (C.A.H.) with 13 years of experience who was blinded to the MR imaging results. All specimens were assigned to a stage according to the 2002 TNM classification (24).

Data Analysis
Cancer locations and extraprostatic extension predicted on MR images were compared with histopathologic findings by radiologists A and B after they had completed assignment of the scores and evaluation of the data. Landmarks used for the alignment of T2-weighted fast spin-echo MR images with whole-mount sections were the morphologic features of the peripheral zone, central gland, apex, and base of the prostate, as well as cysts, calcifications, the verumontanum, and the urethra (25). Only cancer foci of 0.5 cm³ or larger were considered suitable for matching with findings on MR images. Therefore, all foci smaller than 0.5 cm³ were excluded from analysis. Each of the 14 segments was classified as containing either cancer or healthy tissue. The authors were confident to be within an 8-mm accuracy (eg, two sections) (26).

Statistical Analysis
The 10 scores for image quality characteristics for MR images obtained with the BAC and the ERC were compared by applying the Wilcoxon rank sum test for paired samples to the mean scores of the four radiologists for each patient.

Receiver operating characteristic (ROC) curves were obtained. The localization performance was summarized by using the area under the ROC curve (AUC) (27). The AUCs were statistically compared by using nonparametric bootstrap resampling, with the patient as the unit of analysis (28,29). A bootstrap draw either contained a patient, including all regions of interest and the readers who were observing the patient, or did not. The number of bootstrap replicates was set to 999. The P values for multiple comparisons were adjusted by using Bonferroni correction. The plotted ROC curve was the mean over all bootstrap curves for a specific factor. In addition, separate analyses were performed for peripheral zone and central gland cancer localization. The AUCs of the overall staging performance were calculated. The sensitivity, specificity, positive predictive value, negative predictive value, and overall accuracy for cancer localization, overall staging, detection of extracapsular penetration, and detection of seminal vesicle invasion were calculated by using dichotomization of the readings. For high-sensitivity localization and thus prevention of false-negative findings, scores of 3 or higher were considered to indicate cancer, whereas scores of 1 and 2 were deemed to indicate healthy tissue. For high-specificity staging and thereby prevention of false-positive findings (21,22), scores of 4 and 5 were considered stage T3 disease, whereas scores of 1–3 were considered stage T2 disease. To compare the diagnostic performance parameters between MR imaging with the BAC and MR imaging with the ERC, the McNemar test for matched pairs was performed.

All P values reported were from two-sided tests. A P value of .05 or less was considered to indicate a statistically significant difference. Statistical analyses were performed by using R, version 2.1.0 (30), and software (Rockit, version 0.9.1B, Charles E. Metz, PhD, Department of Radiology, University of Chicago, Chicago, Ill; Prism, version 4.00, GraphPad Software, San Diego, Calif; SPSS, version 12.0.1, SPSS, Chicago, Ill).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Patients and Histopathologic Findings
Six patients were excluded from the study (Fig 1). In three patients, the ERC MR image sets were assigned scores for having insufficient quality caused by coupling artifacts of the ERC with the body transmit coil or caused by the presence of excessive motion artifacts, which precluded accurate image interpretation. In two patients, the ERC could not be inserted due to high anal sphincter tension. One patient had gross extracapsular extension on MR images and was subsequently treated with radiation therapy instead of surgery. Forty-six patients were included in the study (Table 1). For inflation of the balloon of the ERC, water and perfluoro polyether were used in 10 and 36 patients, respectively. The mean time between the last biopsy procedure and MR imaging was 112 days (range, 21–226 days), and the mean time between MR imaging and surgery was 14 days (range, 1–89 days).

View larger version (12K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Flow diagram of the study.

Image Quality Comparison
All image quality characteristics, except motion artifacts, were improved significantly (P < .001) with ERC MR imaging compared with BAC MR imaging (Table 2). Significantly more motion artifacts were present during ERC imaging compared with BAC imaging (P < .001) (Fig 2). No differences in image quality characteristics were found between images obtained with the ERC filled with water and those obtained with the ERC filled with perfluoro polyether.

View larger version (148K): [in this window] [in a new window] [Download PPT slide]   Figure 2a: Comparison of image quality of T2-weighted fast spin-echo MR images at 3 T with BAC (3700/124) and ERC (5000/153) in a 58-year-old man (prostate-specific antigen level, 13.9 ng/mL; final Gleason score, 3 + 4; stage, pT3b). (a) Transverse and (b) coronal BAC MR images and (c) transverse and (d) coronal ERC MR images. On all images, the cancer (arrows) was localized correctly. Note the improved visibility of the internal architecture of the central gland (*) and the increased presence of motion artifacts (arrowheads) with ERC imaging on c. A right-left phase-encoding direction was chosen to prevent the motion artifacts from propagating over the prostate. (e) Corresponding axial whole-mount-section histopathologic slice of the mid gland shows the tumor (T) outlined in blue.

View larger version (132K): [in this window] [in a new window] [Download PPT slide]   Figure 2b: Comparison of image quality of T2-weighted fast spin-echo MR images at 3 T with BAC (3700/124) and ERC (5000/153) in a 58-year-old man (prostate-specific antigen level, 13.9 ng/mL; final Gleason score, 3 + 4; stage, pT3b). (a) Transverse and (b) coronal BAC MR images and (c) transverse and (d) coronal ERC MR images. On all images, the cancer (arrows) was localized correctly. Note the improved visibility of the internal architecture of the central gland (*) and the increased presence of motion artifacts (arrowheads) with ERC imaging on c. A right-left phase-encoding direction was chosen to prevent the motion artifacts from propagating over the prostate. (e) Corresponding axial whole-mount-section histopathologic slice of the mid gland shows the tumor (T) outlined in blue.

View larger version (108K): [in this window] [in a new window] [Download PPT slide]   Figure 2c: Comparison of image quality of T2-weighted fast spin-echo MR images at 3 T with BAC (3700/124) and ERC (5000/153) in a 58-year-old man (prostate-specific antigen level, 13.9 ng/mL; final Gleason score, 3 + 4; stage, pT3b). (a) Transverse and (b) coronal BAC MR images and (c) transverse and (d) coronal ERC MR images. On all images, the cancer (arrows) was localized correctly. Note the improved visibility of the internal architecture of the central gland (*) and the increased presence of motion artifacts (arrowheads) with ERC imaging on c. A right-left phase-encoding direction was chosen to prevent the motion artifacts from propagating over the prostate. (e) Corresponding axial whole-mount-section histopathologic slice of the mid gland shows the tumor (T) outlined in blue.

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 2d: Comparison of image quality of T2-weighted fast spin-echo MR images at 3 T with BAC (3700/124) and ERC (5000/153) in a 58-year-old man (prostate-specific antigen level, 13.9 ng/mL; final Gleason score, 3 + 4; stage, pT3b). (a) Transverse and (b) coronal BAC MR images and (c) transverse and (d) coronal ERC MR images. On all images, the cancer (arrows) was localized correctly. Note the improved visibility of the internal architecture of the central gland (*) and the increased presence of motion artifacts (arrowheads) with ERC imaging on c. A right-left phase-encoding direction was chosen to prevent the motion artifacts from propagating over the prostate. (e) Corresponding axial whole-mount-section histopathologic slice of the mid gland shows the tumor (T) outlined in blue.

View larger version (85K): [in this window] [in a new window] [Download PPT slide]   Figure 2e: Comparison of image quality of T2-weighted fast spin-echo MR images at 3 T with BAC (3700/124) and ERC (5000/153) in a 58-year-old man (prostate-specific antigen level, 13.9 ng/mL; final Gleason score, 3 + 4; stage, pT3b). (a) Transverse and (b) coronal BAC MR images and (c) transverse and (d) coronal ERC MR images. On all images, the cancer (arrows) was localized correctly. Note the improved visibility of the internal architecture of the central gland (*) and the increased presence of motion artifacts (arrowheads) with ERC imaging on c. A right-left phase-encoding direction was chosen to prevent the motion artifacts from propagating over the prostate. (e) Corresponding axial whole-mount-section histopathologic slice of the mid gland shows the tumor (T) outlined in blue.

View larger version (27K): [in this window] [in a new window] [Download PPT slide]   Figure 3: ROC curves and AUCs of prostate cancer localization for all four radiologists combined. At 3 T, the AUC improved significantly from 0.62 with BAC MR imaging to 0.68 with ERC MR imaging (P < .001). The AUCs for the individual readers are presented in Table 3. T2BAC = T2-weighted BAC MR imaging, T2ERC = T2-weighted ERC MR imaging.

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: Example of a 1.8-cm3 prostate cancer focus located in the central gland that was localized correctly only with ERC MR imaging in a 64-year-old man (prostate-specific antigen level, 6.9 ng/mL; final Gleason score, 3 + 3; stage, pT2c). (a) All four radiologists missed the cancer focus with BAC MR imaging. (b) With ERC MR imaging, three of the four radiologists localized the focus (arrows) correctly. (c) Histopathologic examination revealed the ventral cancer focus (T) outlined in blue, as well as two small peripheral zone tumor foci (volumes, <0.5 cm3) outlined in blue.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: Example of a 1.8-cm3 prostate cancer focus located in the central gland that was localized correctly only with ERC MR imaging in a 64-year-old man (prostate-specific antigen level, 6.9 ng/mL; final Gleason score, 3 + 3; stage, pT2c). (a) All four radiologists missed the cancer focus with BAC MR imaging. (b) With ERC MR imaging, three of the four radiologists localized the focus (arrows) correctly. (c) Histopathologic examination revealed the ventral cancer focus (T) outlined in blue, as well as two small peripheral zone tumor foci (volumes, <0.5 cm3) outlined in blue.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 4c: Example of a 1.8-cm3 prostate cancer focus located in the central gland that was localized correctly only with ERC MR imaging in a 64-year-old man (prostate-specific antigen level, 6.9 ng/mL; final Gleason score, 3 + 3; stage, pT2c). (a) All four radiologists missed the cancer focus with BAC MR imaging. (b) With ERC MR imaging, three of the four radiologists localized the focus (arrows) correctly. (c) Histopathologic examination revealed the ventral cancer focus (T) outlined in blue, as well as two small peripheral zone tumor foci (volumes, <0.5 cm3) outlined in blue.

View larger version (23K): [in this window] [in a new window] [Download PPT slide]   Figure 5a: (a) ROC curves and AUCs of prostate cancer staging for the experienced radiologists A (J.J.F.) and B (S.W.T.P.J.H.) with 4 and 2 years of experience, respectively, in prostate MR imaging. The AUCs for both radiologists improved significantly (P < .05) with the ERC. (b) ROC curves and AUCs of prostate cancer staging for radiologists C (S.T.) and D (T.H.) with 3 months of experience. Radiologist C had 8 years of experience in MR imaging at 1.5 T with a pelvic phased-array coil. The AUC for radiologist D, who participated in a short training course about reading endorectal prostate MR images obtained at 3 T prior to assignment of scores, increased significantly (P < .05).

View larger version (21K): [in this window] [in a new window] [Download PPT slide]   Figure 5b: (a) ROC curves and AUCs of prostate cancer staging for the experienced radiologists A (J.J.F.) and B (S.W.T.P.J.H.) with 4 and 2 years of experience, respectively, in prostate MR imaging. The AUCs for both radiologists improved significantly (P < .05) with the ERC. (b) ROC curves and AUCs of prostate cancer staging for radiologists C (S.T.) and D (T.H.) with 3 months of experience. Radiologist C had 8 years of experience in MR imaging at 1.5 T with a pelvic phased-array coil. The AUC for radiologist D, who participated in a short training course about reading endorectal prostate MR images obtained at 3 T prior to assignment of scores, increased significantly (P < .05).

View larger version (146K): [in this window] [in a new window] [Download PPT slide]   Figure 6a: Example of a tumor with 2-mm radial distance of extracapsular extension (prostate-specific antigen level, 8.6 ng/mL; final Gleason score, 3 + 4; stage, pT3a) that was detected only with ERC MR imaging in a 61-year-old man. (a, b) Two consecutive transverse BAC T2-weighted fast spin-echo MR images (3700/124) show an area of low signal intensity in the left peripheral zone (arrow in a) that displayed no signs of locally advanced disease. (c, d) Corresponding consecutive transverse ERC T2-weighted fast spin-echo MR images (5000/153) show the area of low signal intensity with irregular bulging and obliteration of the rectoprostatic angle (arrows in c). (e) After wide excision at the left lateral side and resection of the neurovascular bundle on the left side, histopathologic examination confirmed the presence of tumor (T), outlined in blue, and the 2-mm radial extracapsular extension (ECE++ 2mm) (arrow) with surgical margins negative for tumor.

View larger version (145K): [in this window] [in a new window] [Download PPT slide]   Figure 6b: Example of a tumor with 2-mm radial distance of extracapsular extension (prostate-specific antigen level, 8.6 ng/mL; final Gleason score, 3 + 4; stage, pT3a) that was detected only with ERC MR imaging in a 61-year-old man. (a, b) Two consecutive transverse BAC T2-weighted fast spin-echo MR images (3700/124) show an area of low signal intensity in the left peripheral zone (arrow in a) that displayed no signs of locally advanced disease. (c, d) Corresponding consecutive transverse ERC T2-weighted fast spin-echo MR images (5000/153) show the area of low signal intensity with irregular bulging and obliteration of the rectoprostatic angle (arrows in c). (e) After wide excision at the left lateral side and resection of the neurovascular bundle on the left side, histopathologic examination confirmed the presence of tumor (T), outlined in blue, and the 2-mm radial extracapsular extension (ECE++ 2mm) (arrow) with surgical margins negative for tumor.

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 6c: Example of a tumor with 2-mm radial distance of extracapsular extension (prostate-specific antigen level, 8.6 ng/mL; final Gleason score, 3 + 4; stage, pT3a) that was detected only with ERC MR imaging in a 61-year-old man. (a, b) Two consecutive transverse BAC T2-weighted fast spin-echo MR images (3700/124) show an area of low signal intensity in the left peripheral zone (arrow in a) that displayed no signs of locally advanced disease. (c, d) Corresponding consecutive transverse ERC T2-weighted fast spin-echo MR images (5000/153) show the area of low signal intensity with irregular bulging and obliteration of the rectoprostatic angle (arrows in c). (e) After wide excision at the left lateral side and resection of the neurovascular bundle on the left side, histopathologic examination confirmed the presence of tumor (T), outlined in blue, and the 2-mm radial extracapsular extension (ECE++ 2mm) (arrow) with surgical margins negative for tumor.

View larger version (128K): [in this window] [in a new window] [Download PPT slide]   Figure 6d: Example of a tumor with 2-mm radial distance of extracapsular extension (prostate-specific antigen level, 8.6 ng/mL; final Gleason score, 3 + 4; stage, pT3a) that was detected only with ERC MR imaging in a 61-year-old man. (a, b) Two consecutive transverse BAC T2-weighted fast spin-echo MR images (3700/124) show an area of low signal intensity in the left peripheral zone (arrow in a) that displayed no signs of locally advanced disease. (c, d) Corresponding consecutive transverse ERC T2-weighted fast spin-echo MR images (5000/153) show the area of low signal intensity with irregular bulging and obliteration of the rectoprostatic angle (arrows in c). (e) After wide excision at the left lateral side and resection of the neurovascular bundle on the left side, histopathologic examination confirmed the presence of tumor (T), outlined in blue, and the 2-mm radial extracapsular extension (ECE++ 2mm) (arrow) with surgical margins negative for tumor.

View larger version (88K): [in this window] [in a new window] [Download PPT slide]   Figure 6e: Example of a tumor with 2-mm radial distance of extracapsular extension (prostate-specific antigen level, 8.6 ng/mL; final Gleason score, 3 + 4; stage, pT3a) that was detected only with ERC MR imaging in a 61-year-old man. (a, b) Two consecutive transverse BAC T2-weighted fast spin-echo MR images (3700/124) show an area of low signal intensity in the left peripheral zone (arrow in a) that displayed no signs of locally advanced disease. (c, d) Corresponding consecutive transverse ERC T2-weighted fast spin-echo MR images (5000/153) show the area of low signal intensity with irregular bulging and obliteration of the rectoprostatic angle (arrows in c). (e) After wide excision at the left lateral side and resection of the neurovascular bundle on the left side, histopathologic examination confirmed the presence of tumor (T), outlined in blue, and the 2-mm radial extracapsular extension (ECE++ 2mm) (arrow) with surgical margins negative for tumor.

The sensitivity and specificity for detection of seminal vesicle invasion (stage pT3b disease) for radiologist A with BAC MR imaging were 0% (zero of five) and 100% (41 of 41), respectively, whereas for radiologist B these values were 20% (one of five) and 90% (37 of 41), respectively. The ERC MR imaging results were the same for radiologists A and B: Sensitivity and specificity values were 40% (two of five) and 100% (41 of 41), respectively. For BAC imaging, radiologists C and D had a sensitivity of 0% (zero of five) and 20% (one of five), respectively, with a specificity of 100% (41 of 41) and 88% (36 of 41), respectively. For ERC MR imaging, radiologists C and D had sensitivity values of 20% (one of five) and 0% (zero of five), respectively, with a specificity of 100% (41 of 41).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Results of our study show that, compared with BAC MR imaging at 3 T, ERC MR imaging at 3 T significantly improved image quality and, combined with higher spatial resolution, significantly increased the localization and staging performance for the experienced radiologists and the less experienced radiologist who participated in a short training course about reading images of the prostate obtained with ERC MR imaging at 3 T prior to assignment of scores.

The significant improvement in image quality is supported by an initial study in healthy volunteers in which the researchers concluded that several details of the prostate seen with endorectal MR imaging at 3 T could not be seen with endorectal MR imaging at 1.5 T or 3 T without the ERC (31).

Localization performance results of our study are similar to those obtained at 1.5 T for MR imaging with an ERC by investigators who found AUCs for localization by using T2-weighted imaging that varied between 0.67 and 0.83 when the results were compared with findings at radical prostatectomy (32). This point illustrates that localization on T2-weighted MR images alone remains difficult without addition of functional MR imaging techniques. Dynamic contrast material–enhanced MR imaging and proton MR spectroscopic imaging have been reported to increase the localization performance of MR imaging at 1.5 T (33–35). Addition of these techniques at 3 T (5) may enhance the capability for localization of prostate cancer, also in areas of hemorrhage after biopsy, and for staging of the disease (26). In our study, we classified areas of low signal intensity on T2-weighted fast spin-echo images with corresponding areas of high signal intensity on T1-weighted gradient-echo images as negative because biopsy artifacts precluded interpretation. Thus, cancer foci could have been missed since hemorrhage after biopsy does not exclude the presence of cancer.

The significant increase in capsular delineation, visualization of the neurovascular bundle and rectoprostatic angle, and visibility of the lesion with endorectal MR imaging, caused by higher spatial resolution, improved staging performance. This finding explains the increase in sensitivity for detection of locally advanced disease for all readers. The significant increase in AUC for staging by the reader who participated in a short course with direct histopathologic feedback may indicate the need for such a course for radiologists with little experience with endorectal imaging at 3 T. Our results are congruous with earlier results by Mullerad et al (36) who reported a substantially improved staging performance for radiologists who specialized in urogenital radiology compared with general body radiologists. In our study, the radiologists who had 2 or more years of experience in prostate MR imaging obtained the highest AUCs and sensitivity values for staging. The differences in results between the experienced and less experienced readers were particularly large with the use of the ERC, thereby providing evidence for the particular expertise necessary to interpret endorectal MR images obtained at 3 T. This finding also is supported by the fact that reader C, who had experience with pelvic phased-array coil MR imaging at 1.5 T, performed better with the BAC than with the ERC at 3 T. It thus appears that in order to obtain accurate high-specificity staging with ERC MR imaging at 3 T, experience is an important factor.

It is difficult to compare our results with those of studies from the 1980s or early 1990s, because, currently, patients are identified earlier, and also, therefore, the number of patients with extraprostatic disease spread is lower and the spread is likely to be smaller as well. In our patient population, the average extraprostatic spread was 1.4 mm (range, 0.5–3.0 mm), and six of 13 patients had an extension of 0.5 mm or less. With BAC MR imaging, this small extension is extremely difficult to depict and determine with certainty. This difficulty in depiction, coupled with high-specificity reading, could explain the low sensitivity values for BAC imaging in the staging of patients. Comparison with previous studies is difficult because in none has the exact amount of the extraprostatic extent been described. Nevertheless, the spatial resolution of BAC imaging is too low for detection of the small extraprostatic extent that one can depict with ERC MR imaging.

Our staging results are in accordance with those obtained from an initial study of staging with 3-T ERC MR imaging, which show consistent high diagnostic performance with ERC imaging (12). These results exceed those obtained at 1.5 T. Particularly when the ERC is used at 3 T, minimal capsular penetration can be detected more accurately. In a large (n = 336) published study about ERC MR imaging at 1.5 T, only extensive locally advanced disease could be detected accurately: A sensitivity of only 40% (45 of 113) with a specificity of 95% (211 of 223) was obtained by two radiologists reading in consensus (37). Nakashima et al (38) found a sensitivity of 62% (18 of 29) in the detection of locally advanced disease. With ERC 1.5-T MR imaging, two independent readers with 5 and 2 years of experience achieved sensitivity values of 54% (13 of 24) and 17% (four of 24), respectively, for the detection of extracapsular penetration (19).

Although specificity values for determination of seminal vesicle invasion were high (88%–100%) for all readers with both BAC MR imaging and ERC MR imaging, the sensitivity values were low (0%–40%). Other researchers in studies with imaging at 1.5 T also found rather low sensitivity values (38). With the same cutoff point as used in our study, Sala et al (20) also found high (97%–99%) specificity with lower (50%–63%) sensitivity with ERC 1.5-T MR imaging in 354 patients with a prevalence of seminal vesicle invasion of 14% (51 of 354). The small number of patients with seminal vesicle invasion limited the interpretation of the results of our study, and the diagnostic performance for detection of seminal vesicle invasion at 3 T needs to be investigated in larger patient populations. By its low prevalence (five patients in our study), it appears that readers are less likely to detect seminal vesicle invasion. Continuous training with imaging sets obtained in patients who had seminal vesicle invasion may increase the performance.

A general limitation of ERC MR imaging is that it is technically more challenging and may not be performed in all patients. In our study, in five of the six excluded patients, the reason for the exclusion was related to the ERC.

A limitation of our study is the relatively low prevalence (15 of 46, 33%) of patients with locally advanced disease. Also, extracapsular penetration was small (mean penetration distance, 1.4 mm). Nevertheless, these factors reflect the current patient population in general urologic clinics because of the widespread use of the prostate-specific antigen level for screening (39).

Furthermore, by applying bowel movement suppression only before ERC MR imaging, we may have artificially increased ERC image quality. However, the introduction and presence of the ERC in the rectum itself causes bowel discomfort and reactionary bowel movement, as can be deduced from the higher motion artifact scores with ERC imaging. Thus, bowel suppression actually may have enhanced objective comparison between BAC MR imaging and ERC MR imaging.

In our study, the choice of the TE for both BAC MR imaging and ERC MR imaging was restricted by a combination of the specific absorption rate, matrix size, and echo train length. To fully use the high signal-to-noise ratio of the ERC, a large matrix size was applied. As shortening of the radiofrequency pulses was precluded by the specific absorption rate limit, the larger matrix size increased the inter-TE and thereby also the TE of the image series. Thus, our TEs were longer than the established mean T2 value of the entire prostate at 3 T (40). However, using TEs beyond the T2 of the prostate is common practice at 1.5 T (eg, TE of 96–132 msec vs T2 of 88 msec) (16,26), as the difference in signal intensity between healthy prostate tissue and cancerous prostate tissue is important. At longer TEs, this difference is more profound, provided the tissue with shortest T2 still has adequate signal-to-noise ratio. With novel radiofrequency pulse techniques, however, it is advised that TEs of approximately 100 msec are used when the signal is acquired.

Readers could not be blinded to whether they were reviewing BAC MR images or ERC MR images, since the presence or absence of the ERC and the prostate shape already would have indicated the type of image. This could possibly have led readers to systematically provide lower scores for BAC images. To minimize bias, BAC and ERC image sets were evaluated separately and randomly. Furthermore, readers were instructed to perform a certain type of reading (high-sensitivity localization and high-specificity staging). This specification of the type of reading could have influenced our results, particularly the sensitivity in staging. Possibly, this factor influenced BAC MR imaging more than it did ERC MR imaging. Nevertheless, in preoperative prostate cancer staging, it is common practice to perform high-specificity reading.

The limitation of a possible discrepancy and potential difficulty in comparing both BAC MR imaging and ERC MR imaging findings with histopathologic examination findings because of the changed alignment of the prostate after insertion of the ERC was minimized by using anatomic landmarks of the prostate (25) to correlate results of T2-weighted fast spin-echo imaging with histopathologic findings.

Endorectal MR imaging can play a role in accurate delineation of cancer foci in the planning of intensity-modulated radiation therapy and enable radiation boosting to areas within the prostate. Moreover, it can facilitate nerve-sparing decisions with determination of the proximity of the cancer to the neurovascular bundle (41). Likewise, since the clinical implications of small extracapsular penetration are being debated as to whether this penetration has an effect on patient survival, prior knowledge of its localization may aid urologists in obtaining negative surgical margins.

In conclusion, MR image quality of the prostate improved significantly with the use of an ERC. For prostate cancer localization, performance with ERC MR imaging was significantly better than was that with BAC MR imaging. Also, when we used the ERC, the staging performance improved significantly in the two experienced readers and the less experienced reader who participated in a short course about endorectal prostate MR reading at 3 T prior to assignment of scores. Therefore, also at 3 T, the ERC is necessary for accurate localization and staging of prostate cancer.

	ADVANCES IN KNOWLEDGE

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 

• Image quality of the prostate at 3-T MR imaging improves significantly (P < .001) with the use of an endorectal coil (ERC) compared with a body-array coil (BAC).
  
• ERC T2-weighted MR imaging at 3 T significantly (P < .001) increases the diagnostic performance for localization of prostate cancer.
  
• Compared with BAC T2-weighted MR imaging at 3 T, ERC MR imaging significantly (P < .05) improves the overall staging performance, as well as diagnostic sensitivity (73%–80%), of experienced radiologists with equally high specificity (97%–100%).
  

	ACKNOWLEDGMENTS

 
The authors thank Yvonne L. Hoogeveen, PhD (Radboud University Nijmegen Medical Center, Nijmegen, the Netherlands), for her assistance in preparing the manuscript, as well as George J. Misic (Medrad, Pittsburgh, Pa) and Dennis W. J. Klomp, BSc (Radboud University Nijmegen Medical Center, Nijmegen, the Netherlands), for their expertise and work on the prototype 3-T endorectal coil.

	FOOTNOTES

 

Abbreviations: AUC = area under the ROC curve • BAC = body-array coil • ERC = endorectal coil • ROC = receiver operating characteristic • TE = echo time

See Materials and Methods for pertinent disclosures.

Author contributions: Guarantors of integrity of entire study, S.W.T.P.J.H., T.W.J.S., H.J.H., J.A.W., J.O.B.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, S.W.T.P.J.H., J.J.F., T.W.J.S., H.J.H.; clinical studies, S.W.T.P.J.H., J.J.F., T.H., S.T., T.W.J.S., H.J.H., C.A.H., B.C.K., J.A.W., J.O.B.; statistical analysis, S.W.T.P.J.H., J.J.F., H.J.H., L.A.L.M.K., J.O.B.; and manuscript editing, all authors

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