Prostate cancer detection with 3-T MRI: Comparison of diffusion-weighted and T2-weighted imaging
Introduction
The incidence and mortality rate of prostate cancer have been rising sharply during the last two decades. The latest estimates show that prostate cancer has already become the third most common cancer in men globally, with a half million new cases each year [1], [2], [3], [4].
Although early detection of prostate cancer has been greatly aided by prostate-specific antigen (PSA) testing, the identification of cancer lesions by various imaging methods still seems to be unreliable. The widely used method of T2-weighted (T2W) magnetic resonance (MR) imaging has a low accuracy because malignancies are usually masked or mimicked by the signal patterns of benign prostatic hyperplasia (BPH) in the transition zone (TZ) or prostatitis in the peripheral zone (PZ) [5], [6], [7]. In addition, the sensitivity of transrectal ultrasonography (TRUS)-guided prostate biopsy with 6–12 cores is not high enough, and a second biopsy is usually requested [8], [9], [10], [11]. Thus, improving the performance of prostate MR examinations for lesion detection and biopsy guidance with new sequences and protocols appears to be necessary.
Several studies have revealed that diffusion-weighted (DW) imaging is viable for differentiating lesions of the liver and kidneys [12], [13], [14]. There have also been reports of a statistical difference between the mean apparent diffusion coefficients (ADC) of normal and cancerous prostate tissue at 1.5 Tesla (15 T) [15], [16], so DW imaging is potentially viable for prostate cancer detection as well.
On 1.5 T systems however, the signal-to-noise ratio (SNR) is intrinsically low, and DW image quality is poor. Thus, the clinical use of prostate DW imaging has been limited. At 3 Tesla (3 T), the SNR is higher than that at 1.5 T. Combined with parallel imaging, the scan time can be greatly shortened, and artifacts caused by bowel movement and gas can be reduced. Thus, high SNR, less-distorted DW images, and ADC maps of the prostate for clinical purposes have become feasible on 3 T MR systems.
The aim of our study was to evaluate the clinical value of DW imaging; especially ADC maps, acquired at 3 T for prostate cancer detection, and to compare its performance with the widely used method of T2W imaging.
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Patients
Between July 2003 and December 2005, 40 patients with elevated PSA levels and suspected prostate cancer underwent prostate MR examination. Three patients were excluded due to severe magnetic susceptibility artifacts and distortion on DW or T2W images, caused by an obviously dilated rectum. As a result, 37 patients were finally included in the study. The mean age was 63.7 years (range: 43–82 years), and the mean serum PSA value at the time of MR examination was 22.4 ng/mL (range: 4.07–136 ng/mL).
Results
Thirty-four patients had prostate adenocarcinoma with differentiation gradings ranging from well to poor. For three patients, malignancy was ruled out by a second biopsy and clinical follow-ups. Within the total 296 regions evaluated, histopathology verified that 144 were tumor positive and 152 were negative.
Sensitivity, specificity, positive predictive value, negative predictive value and accuracy of prostate cancer detection were respectively 84%, 81%, 81%, 84% and 82% for DW imaging, and
Discussion
MR imaging with a phased-array coil or an endorectal coil has shown good sensitivity for prostate cancer of the peripheral zone, which usually appears as a hypointense area on T2W images. However, a focal decrease in T2W signal intensity could also have a cause other than cancer, such as prostatitis or granulomatous disease. Moreover, distinguishing between cancer lesions and stromal hyperplasia in the TZ area by T2W imaging alone is usually rather challenging [5], [6], [7], [17], [18].
MR
Conclusions
In this initial study, the performance of DW imaging at 3 T in detecting prostate cancer was better than that of T2W imaging. DW imaging should be a robust and reliable method to examine the whole prostate within an acceptable scan time in clinical settings.
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