Person:

Soman, Salil

Functional imaging provides hemodynamic and metabolic information and is increasingly being incorporated into clinical diagnostic and research studies. Typically functional images have reduced signal-to-noise ratio and spatial resolution compared to other non-functional cross sectional images obtained as part of a routine clinical protocol. We hypothesized that enhancing visualization and interpretation of functional images with anatomic information could provide preferable quality and superior diagnostic value. In this work, we implemented five methods (frequency addition, frequency multiplication, wavelet transform, non-subsampled contourlet transform and intensity-hue-saturation) and a newly proposed ShArpening by Local Similarity with Anatomic images (SALSA) method to enhance the visualization of functional images, while preserving the original functional contrast and quantitative signal intensity characteristics over larger spatial scales. Arterial spin labeling blood flow MR images of the brain were visualization enhanced using anatomic images with multiple contrasts. The algorithms were validated on a numerical phantom and their performance on images of brain tumor patients were assessed by quantitative metrics and neuroradiologist subjective ratings. The frequency multiplication method had the lowest residual error for preserving the original functional image contrast at larger spatial scales (55%–98% of the other methods with simulated data and 64%–86% with experimental data). It was also significantly more highly graded by the radiologists (p<0.005 for clear brain anatomy around the tumor). Compared to other methods, the SALSA provided 11%–133% higher similarity with ground truth images in the simulation and showed just slightly lower neuroradiologist grading score. Most of these monochrome methods do not require any prior knowledge about the functional and anatomic image characteristics, except the acquired resolution. Hence, automatic implementation on clinical images should be readily feasible.

Purpose To improve pseudo continuous arterial spin labeling (PCASL) robustness to off-resonance and pulsatile blood flow velocity.

Methods The Bloch equations were solved to evaluate the effect of labeling parameters in a pulsatile flow model for a range of off-resonance. Experimental confirmation was achieved in volunteers using linear phase increase between labeling pulses to approximate off-resonance errors. The location of the labeling plane was first assessed on four volunteers, then a range of parameters, including balanced and unbalanced gradients, were explored in five more volunteers at an optimal labeling plane location.

Results Simulations demonstrated that high velocities are vulnerable to off-resonance, that unbalanced PCASL outperforms balanced PCASL, that increased B1 and low average gradient improve the labeling efficiency for high velocity flow, and a low ratio of selective to average gradient improves off-resonance robustness. A good agreement between theory and experiment was observed.

Conclusion The robustness of PCASL can be increased by selecting an unbalanced scheme with a low average gradient (0.5mT/m), a low ratio (7x) of selective to average gradients and the highest feasible B1 (1.8uT). Placing the labeling plane above the carotid bifurcation and below the V3 segment, usually between the second and third vertebrae, produces robust results.