MR Imaging of the Prostate: 1.5T versus 3T
Section snippets
Current role of MR imaging and magnetic resonance spectroscopic imaging in the evaluation of prostate cancer
Once prostate cancer is diagnosed by biopsy, it is important to determine whether the cancer has spread beyond the prostate capsule or metastasized, because prognosis and treatment options differ between organ-confined and locally advanced disease (TMN stage II versus stage III and Jewett-Whitmore stage B versus stage C). Organ-confined disease or cancer with minimal ECE may be treated with radical prostatectomy, ablation therapies, brachytherapy, or watchful waiting. Once the cancer has spread
Evolving role of MR imaging and magnetic resonance spectroscopic imaging in the evaluation of prostate cancer
In response to evolving therapies for treatment of prostate cancer that are tailored for individual patients and tumors, much research is focused on using various MR imaging techniques not just for determination of ECE but for noninvasive assessment of tumor size, location, and grade [12], [13], [14]. For example, size of tumor affects radiation dose, and location of tumor is important for disease-targeted therapy. In one trial, localization of tumor by means of MR imaging altered radiotherapy
3T prostate imaging protocol
Before imaging, an endorectal coil (ERC) is inserted into the rectum (Medrad, Indianola, Pennsylvania) (Fig. 1). An adapter connects the ERC with an eight-element torso phased-array coil, and the two coils are used in conjunction.
At 1.5T, there is consensus that an ERC, usually in combination with a torso phased-array coil, is advantageous for prostate imaging compared with a torso coil alone [27], [28]. There has been some discussion about the necessity for an ERC for prostate imaging at 3T.
Coil positioning
Patients are imaged supine. Proper ERC placement involves positioning and taking steps to keep the coil from moving during the examination (see Fig. 1). When the coil moves or is in the wrong position, overall image quality is poor. The flat end of the coil is positioned flush against the posterior aspect of the prostate, centered on the prostate in the craniocaudad direction. Positioning is easily checked on the localizer images. If the coil needs to be repositioned, it should be completely
Imaging protocol
A basic prostate protocol contains axial T1W and multiplanar T2W images. MRSI, DWI, and DCE are added depending on the availability of technology and ability. The authors routinely perform DWI and DCE but do not currently have the software to perform spectroscopy at 3T.
T1-weighted imaging and T2-weighted imaging
High-resolution oblique axial and oblique coronal fast spin echo (FSE) T2W images are obtained through the prostate and seminal vesicles to evaluate for neoplasm and ECE. Some advocate also obtaining sagittal T2W images. Small field-of-view (FOV) axial FSE T1W images are acquired through the prostate to evaluate for postbiopsy hemorrhage, periprostatic nodes, and ECE. Some institutions alternatively obtain large FOV FSE T1W images through the entire pelvis to evaluate for adenopathy. Because
Magnetic resonance spectroscopic imaging
Increased magnetic field strength provides two powerful advantages for MRSI. First, the increased SNR allows smaller voxel size. MRSI voxel size at 1.5T is approximately 0.30 cm3, and it is 0.15 cm3 at 3T [40]. This reduces volume averaging effects with periprostatic fat, postbiopsy hemorrhage, seminal vesicles, BPH nodules, and periurethral tissues.
Second, separation and height of the metabolite peaks is increased at 3T. Normal prostate tissue is characterized by a high citrate-to-choline
Diffusion weighted imaging
DWI measures the Brownian motion of free water. Diffusion is decreased in areas of increased cellularity and in areas of cellular edema secondary to the loss of ATP-dependent sodium-potassium pumps. Both conditions are believed to be present within neoplasms. The authors' experience is that increased SNR at 3T improves the quality of DWI. The combination of increased field strength and an ERC allows for high-resolution DWI, which is an intrinsically low SNR technique.
Studies at 1.5T have shown
Dynamic contrast enhancement MR imaging
DCE MR imaging techniques differentiate tumor from normal tissue by evaluating the kinetics of enhancement and de-enhancement. Enhancement depends on blood supply (large feeding vessels and local microvessel density) and capillary permeability. Increased blood supply and capillary permeability allow for increased enhancement. De-enhancement, or washout, depends on capillary permeability and the size of the local extravascular/extracellular space (ie, the space outside the vessels and cells).
T2∗ mapping
A final technique, but one with limited support in the current literature, is T2∗ mapping. A multiecho gradient echo sequence is used to obtain images at multiple values of TE. Pixel-by-pixel signal intensities plotted as a function of TE are fit to an exponential decay to create a pixel map of T2∗. In one study of eight patients with biopsy-proven prostate cancer, the T2∗ maps identified 17 of 18 tumors compared with 13 of 18 tumors identified by T2W imaging. Two central gland tumors were only
Magnetic resonance lymphography
Imaging with ultrasmall iron oxide particles, such as Ferumoxtran-10, is also a T2∗ technique. Ferumoxtran-10 is injected intravenously 24 to 36 hours before imaging and accumulates in normal lymph nodes. Normal nodes appear dark on T2∗ images secondary to susceptibility from the iron particles. Nodes infiltrated with tumor do not accumulate the iron particles and remain bright on these images. Pathologic nodes as small as 5 mm can be detected with standard sequences at 1.5T [80]. Initial
Summary
Many new techniques in prostate MR imaging are under development and refinement. Most of these techniques should benefit from the increased SNR and other features at 3T. Conventional T2W imaging can be obtained at increased spatial resolution. DWI benefits from increased signal to noise. The ability to image at increased bandwith and partial excitations without sacrificing SNR allows DCE MR imaging to be performed at higher temporal resolution. Alternatively, DCE can be performed at higher
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Cited by (28)
Assessment of the accuracy of multi-parametric MRI with PI-RADS 2.0 scoring system in the discrimination of suspicious prostatic focal lesions
2016, Egyptian Journal of Radiology and Nuclear MedicineCitation Excerpt :Prostate cancer is the 3rd leading cause of death and is the most common genitourinary malignancy in men (1).
Advanced Magnetic Resonance Techniques: 3 T
2015, Radiologic Clinics of North AmericaProstate MRI: Who, when, and how? Report from a UK consensus meeting
2013, Clinical RadiologyCitation Excerpt :They are minimum standards, which with additional techniques (3 T machines and endorectal coils) may be exceeded. A field strength of 1.5 T is adequate, although optimized images at 3 T are superior.36 Most of the benefits of MRI can be achieved with a multichannel pelvic phased-array coil.
Multiparametric Magnetic Resonance Imaging of the Prostate
2013, Magnetic Resonance Imaging Clinics of North AmericaCitation Excerpt :The European Consensus on MRI for the Detection, Localization, and Characterization of Prostate Cancer decided that imaging should be performed at 3.0 T to reach an optimum level of detection but there was no consensus as to whether an endorectal coil should be part of the requirements for optimum imaging.17 Currently, the best signal-to-noise ratio and spatial resolution for the evaluation of the prostate can be obtained with a combination of 3.0 T and an endorectal coil.57–59 Current practices for prostate MR imaging differ between Europe and the United States; endorectal coils are used more frequently in the United States.17
Pelvic applications of diffusion magnetic resonance images
2011, Magnetic Resonance Imaging Clinics of North AmericaCitation Excerpt :Advances in MR imaging and computing technology have resulted in the ability to detect disease within the male pelvis with a high degree of accuracy. Specifically, the use of endorectal coils and pelvic phased-array coils coupled with 3-T magnetic fields has resulted in improved signal-to-noise as well as improved spatial, temporal, and spectral resolution in the evaluation of the prostate gland for malignancy.57–59 Further improvement in diagnostic accuracy in the detection of malignancy within the prostate gland can be achieved with the addition of DWI (Fig. 16).60