Quantitative evaluation of metal artifact reduction techniques in MRI

Several techniques for metal artifact reduction in MRI were studied in order to determine their effectiveness. The noise and blur introduced by the techniques were also investigated. To this end, non-metallic replicas of two metal implants (stainless steel and titanium/chromium-cobalt femoral prostheses) were fabricated from wax, and MR images were obtained of each component immersed in water. The difference between the images of each metal prosthesis and its wax replica was measured in terms of energy. The difference energy attributed to noise and blur were isolated, resulting in a measure of the metal artifact. This new "gold standard" method was successfully demonstrated to provide a quantitative means of measuring metal artifact. Several pulse sequences were evaluated in terms of metal artifact reduction capability, as well as signal-to-noise ratio and blur. The analysis revealed that increasing the image bandwidth from ±16 kHz to ±64 kHz reduced metal artifact by an average of 60%, while employing View Angle Tilting (VAT) was only slightly more efficient, reducing metal artifact by an average of 63%. The metal artifact reduction sequence (MARS), which combines the increased image bandwidth with VAT as well as an increased slice-select bandwidth, resulted in the least amount of image distortion, reducing the artifact by an average of 79%. The signal-to-noise ratio was lower for MARS, but blurring was found to be significantly improved.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Myelin water imaging : development at 3.0T, application to the study of multiple sclerosis, and comparison to diffusion tensor imaging

T2 relaxation imaging can be used to measure signal from water trapped between myelin bilayers; the ratio of myelin water signal to total water is termed the myelin water fraction (MWF). First, results from multi-component T2 relaxation and diffusion tensor imaging (DTI) were compared for 19 multiple sclerosis (MS) subjects at 1.5 T to better understand how each measure is affected by pathology. In particular, it was determined that the detection of a long-T2 signal within an MS lesion may be indicative of a different underlying pathology than is present in lesions without long-T2 signal. Next, the single-slice T2 relaxation measurement was implemented, refined, and validated at 3.0 T. Scan parameters were varied for phantoms and in-vivo brain, and changes in multi-exponential fit residuals and T2 distribution-derived parameters such as MWF were monitored to determine which scan parameters minimized artifacts. Measurements were compared between 1.5 T and 3.0 T for 10 healthy volunteers. MWF maps were qualitatively similar between field strengths. MWFs were significantly higher at 3.0 T than at 1.5 T, but with a strong correlation between measurements at the different field strengths. Due to long acquisition times, multi-component T2 relaxation has thus far been clinically infeasible. The next study aimed to validate a new 3D multi-component T2 relaxation imaging technique against the 2D single-slice technique most commonly used. Ten healthy volunteers were scanned with both the 2D single-slice and 3D techniques. MWF maps were qualitatively similar between scans. MWF values were highly correlated between the acquisition methods. Although MWF values were generally lower using the 3D technique, they were only significantly so for peripheral brain structures, likely due to increased sensitivity of slab-selective refocusing pulses used for the 3D approach. The 3D T2 relaxation sequence was then applied to the study of MS to take advantage of the increased brain coverage. Thirteen MS subjects and 11 controls underwent T2 relaxation and DTI examinations to produce histograms of MWF and several DTI-derived metrics. MS MWF histograms differed considerably from those of controls, and differences in MS MWF histograms did not mirror differences in DTI histograms relative to matched controls.Science, Faculty ofPhysics and Astronomy, Department ofGraduat