RadioGraphics 2007;27:75-77
Invited Commentary
Akira Kawashima, MD, PhD and
Bernard F. King, Jr, MD
Department of Radiology, Mayo Clinic, Rochester, Minnesota
We are delighted to have the opportunity to comment on the comprehensive review of functional imaging of the prostate by Dr Choi and his colleagues in this issue of RadioGraphics (1). The authors have highlighted the current utility and potential clinical relevance of functional magnetic resonance (MR) imaging of the prostate with their discussion of various techniques, including dynamic contrast material–enhanced MR imaging, diffusion-weighted imaging, and three-dimensional proton MR spectroscopic imaging. The authors also presented nice examples of protocols and representative case images.
The high incidence and increasing awareness of prostate cancer, along with the ongoing development of new and improved treatment methods, has generated a considerable need for imaging techniques that allow accurate detection and staging of tumors prior to treatment, guidance of radiation therapy delivery, and monitoring of the response to treatment. MR imaging, despite its limitations, is currently the preferred modality for local pretreatment staging. The use of integrated endorectal and pelvic phased-array coils has led to improved visualization of the contours and zonal anatomy of the prostate gland and the periprostatic tissues. In addition, high-resolution axial and coronal T2-weighted imaging can depict gross extension to the seminal vesicles and neurovascular bundles. However, MR imaging primarily plays a confirmatory role in the initial staging of prostate cancer. Because the modality does not accurately depict microscopic disease, it is not reliably sensitive for the detection of extra-prostatic extension. Therefore, MR imaging is usually reserved for use in patients with an intermediate or high risk of prostate cancer extension. Furthermore, a finding of abnormal signal intensity in the peripheral zone of the prostate on T2-weighted images is not specific to cancer and may be due to benign causes. For these reasons, the quality of an MR imaging evaluation of the prostate depends largely on interpretative experience and skills.
The addition of other types of MR imaging sequences should help further increase the level of confidence in detection and characterization of prostate cancer by providing functional and molecular information. Techniques for functional MR imaging of the prostate at 1.5 T are increasingly used to supplement conventional MR imaging in diagnostic research and clinical studies, but their use is still uncommon in routine clinical practice. The accurate localization and characterization of prostate cancer remain a major problem in the clinical setting, and whether functional MR techniques should be implemented for additional clinical imaging of the prostate remains an open question. It may be difficult to perform all three functional imaging studies within a reasonable appointment time. Local resource availability and expertise also influence which technique may be added to conventional MR imaging for prostate cancer. If an endorectal coil is not used, MR imaging and three-dimensional MR spectroscopic imaging cannot be performed, but dynamic contrast-enhanced imaging, diffusion-weighted imaging, or both could be added instead.
Dynamic contrast-enhanced MR imaging can provide information about tumor angiogenesis. Most radiologists who specialize in whole-body MR imaging are familiar with the direct visual assessment of dynamic contrast enhancement after a contrast material bolus injection. Dr Choi and his colleagues previously examined patients with prostate cancer by using dynamic contrast-enhanced MR imaging performed with only a surface coil and with a temporal resolution of 30 seconds for each dynamic sequence. Interpretation of the resultant parametric maps of the wash-in rate helped improve the detection of peripheral-zone prostate cancer, compared with the interpretation of T2-weighted images alone (2). Contrary to this approach, which provides high spatial resolution and low temporal resolution, dynamic contrast-enhanced MR imaging with a fast imaging sequence provides high temporal resolution and low spatial resolution, thereby enabling depiction of the early enhancement phase (3). With the application of multicompartmental modeling, time-intensity curves of contrast enhancement can be plotted, pharmacokinetic parameters can be calculated, and parametric images can be coregistered with T2-weighted images. However, this approach requires specialized software for quantitative measurement of the functional parameters. In a recent study of fast dynamic contrast-enhanced MR imaging with a temporal resolution of 2 seconds, dynamic parametric imaging of contrast enhancement resulted in an incremental improvement in tumor staging by less-experienced readers (4). Discrimination between cancer and noncancerous abnormalities in tissue in the transitional zone is still difficult with dynamic contrast-enhanced MR imaging, as the authors have pointed out.
The addition of diffusion-weighted imaging is attractive because the acquisition time is relatively short and postprocessing at an MR system console or workstation is relatively straightforward. However, to the best of our knowledge, there has been no report in which diffusion-weighted imaging findings were correlated with histopathologic findings in whole-mount prostate specimens in a large study. In addition, the optimal b value has not yet been determined. Finally, spatial resolution at diffusion-weighted imaging is lower than that at T2-weighted imaging, and the use of a gas-inflated endorectal coil may accentuate magnetic susceptibility effects on echo-planar diffusion-weighted images and degrade image quality (5).
One of the most compelling recent advances is the use of MR spectroscopic imaging of the prostate to supplement endorectal MR imaging. The characteristic spectral profile and integral ratio of metabolites in prostate cancer—high choline and creatine and low citrate concentrations—can be depicted in individual voxels overlaid on corresponding axial T2-weighted images. Investigators at several institutions have demonstrated that the combined use of MR imaging and MR spectroscopic imaging can improve cancer localization and volume assessment beyond the levels achieved with endorectal MR imaging alone. However, in a recent study, the combined use of MR imaging and MR spectroscopic imaging for tumor staging did not result in any improvement in differentiation between stage T2 and T3 tumors beyond that achieved with MR imaging alone (6). A multicenter clinical trial led by the American College of Radiology Imaging Network is currently under way to validate the accuracy of MR imaging and MR spectroscopic imaging for localization of prostate cancer before radical pros-tatectomy. The steps involved in MR spectroscopic imaging, including the accurate placement of lipid saturation bands, spectral data acquisition, postprocessing, and interpretation, are labor-intensive, and there is a significant learning curve. Further progress in the development of adequate water and lipid suppression methods, as well as robust and fully automated postprocessing and spectral analysis methods, is needed to make MR spectroscopic imaging more easily applicable in routine clinical practice. Furthermore, until examinations with MR spectroscopy are reimbursable by Medicare, the clinical use of this technique will continue to be limited.
MR technology continues to evolve. Other recent advances include increased magnetic field strengths, from 1.5 T to 3.0 T and higher, and the further development of multichannel receiver coils (7). Limitations associated with current functional MR imaging techniques will likely be resolved by these and other technical advances in the future. More clinical studies are needed to correlate pathologic findings with features observed at functional imaging. Standardized protocols and diagnostic criteria for functional imaging of the prostate should be established and implemented in clinical practice. The development of standardized reporting methods and soft-copy display models also should be encouraged, to facilitate the combination of anatomic and functional findings into an integral report. Finally, new functional and molecular imaging techniques, such as MR elastography and optical imaging, are on the verge of being used for clinical examinations and no doubt eventually will be added to the list of functional imaging techniques available for evaluation of the prostate (8,9).
 |
References
|
|---|
- Choi YJ, Kim JK, Kim N, Kim KW, Choi EK, Cho K. Functional MR imaging of prostate cancer. RadioGraphics 2007;27:63–77.[Abstract/Free Full Text]
- Kim JK, Hong SS, Choi YJ, et al. Wash-in rate on the basis of dynamic contrast-enhanced MRI: usefulness for prostate cancer detection and localization. J Magn Reson Imaging 2005;22:639–646.[CrossRef][Medline]
- Padhani AR, Gapinski CJ, Macvicar DA, et al. Dynamic contrast enhanced MRI of prostate cancer: correlation with morphology and tumour stage, histological grade and PSA. Clin Radiol 2000;55:99–109.[CrossRef][Medline]
- Futterer JJ, Engelbrecht MR, Huisman HJ, et al. Staging prostate cancer with dynamic contrast-enhanced endorectal MR imaging prior to radical prostatectomy: experienced versus less experienced readers. Radiology 2005;237:541–549.[Abstract/Free Full Text]
- Kozlowski P, Chang SD, Jones EC, Berean KW, Chen H, Goldenberg SL. Combined diffusion-weighted and dynamic contrast-enhanced MRI for prostate cancer diagnosis—correlation with biopsy and histopathology. J Magn Reson Imaging 2006;24:108–113.[CrossRef][Medline]
- Wetter A, Engl TA, Nadjmabadi D, et al. Combined MRI and MR spectroscopy of the prostate before radical prostatectomy. AJR Am J Roentgenol 2006;187:724–730.[Abstract/Free Full Text]
- Rouviere O, Hartman RP, Lyonnet D. Prostate MR imaging at high-field strength: evolution or revolution? Eur Radiol 2006;16:276–284.[CrossRef][Medline]
- Manduca A, Oliphant TE, Dresner MA, et al. Magnetic resonance elastography: non-invasive mapping of tissue elasticity. Med Image Anal 2001;5:237–254.[CrossRef][Medline]
- Funovics M, Montet X, Reynolds F, Weissleder R, Josephson L. Nanoparticles for the optical imaging of tumor E-selectin. Neoplasia 2005;7:904–911.[CrossRef][Medline]
Top
References
