Men with suspected or diagnosed prostate cancer who have had total hip replacements are challenging to image because of artifacts and distortions, even if they have MRI-compatible metal prostheses. A technique developed in the late 1990’s for motion artifact reduction in MRI has been successfully demonstrated as an improvement over conventional echo-planar imaging (EPI). It produces better image quality and decreases both artifact and distortion, according to a study in the May issue of the European Journal of Radiology.
Multiparametric MRI with functional sequences is a validated method for detecting prostate lesions. Diffusion-weighted imaging (DWI) has been shown to discriminate between low-, medium-, and high-risk prostate cancer, and has an inverse correlation to tumor Gleason grade. DWI is recommended to be the key sequence for assessment of the peripheral zone of the prostate. However, EPI is susceptible to interference by hip metalwork, which can produce significant image distortion.
Radiology researchers at Addenbrooke’s Hospital and the University of Cambridge in the United Kingdom conducted a retrospective study comparing standard echo planar (EP-DWI) sequences with sequences acquired by the Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction (PROPELLER) technique. This technique was developed by James G. Pipe, MD, of the Department of Radiology at Wayne State University in Detroit, MI, in the late 1990’s, and originally used for motion correction for head imaging and free-breathing cardiac imaging.
The method collects data in concentric rectangular strips rotated about the k-space origin. Introducing the technique in a 1999 Magnetic Resonance in Medicine article, Dr. Pipe explained that the central region of k-space is sampled for every strip.1 This allows correction of spatial inconsistencies in position, rotation, and phase between strips. Data can be rejected based on a correlation measure indicating through-plane motion, and further decreases motion artifacts through an averaging effect for low spatial frequencies.
Lead author Marcin Czarniecki, MD, and colleagues explained that by combining a fast-spin echo technique with radical sampling of k-space, a blade-like acquisition is produced. These PROPELLER sequences may decrease susceptibility-induced subsequent image distortion with the use of a spin-echo pulse prior to each read out to refocus T2 while removing T2 distorted signal from the nuclear magnetic resonance signal. Additionally, the oversampling of the central portion of k-space enables bulk motion artifacts to be corrected int he reconstruction process.
Twenty-one patients who had undergone a 1.5T prostate MRI with a phased-array body coil that included EP-DWI and PROPELLER-DWI were included in the retrospective study. Indications for the prostate MRI were diagnosis (29%), staging (14%), and active surveillance (57%). All patients had at least one total hip replacement. Three patients had bilateral hip replacements.
Two radiologists experienced in prostate MRI reviewed four sequences (T2-weighted, EP-DWI, PROPELLER-DWI with fat saturation, and PROPELLER-DWI without fat saturation) in separate sessions two weeks apart. They scored images on a 5-point scale for diagnostic image quality, and used 4-point scales to score distortion and presence of artifacts in diffusion sequences. Interreader variability differed in most categories, but was not statistically significant.
The authors reported that the PROPELLER-DWI with fat saturation significantly improved image quality and reduced distortion from metallic implants when compared to standard EP-DWI. Reduction in artifacts was not significantly improved. However, images acquired from PROPELLER-DWI without fat saturation in five patients did significantly reduce artifacts.
The term “PROPELLER” generically introduced by Dr. Pipe has been acquired by a MRI modality manufacturer. However, the authors believe that the same results in image quality improvement can be obtained with Cartesian diffusion-weighted turbo spin echo methods for implementing a fast-spin echo based diffusion-weighted sequence from multiple MRI scanner manufacturers. They encourage that use of Dr. Pipe’s technique be evaluated on other MRI scanners and that use of the Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction (PROPELLER) technique be considered for general prostate DWI of men with total hip replacements.
Reducing distortion and artifacts in prostate cancer imaging from hip implants. Appl Radiol.