Ajit Shankaranarayanan

PROMO: Real-time prospective motion correction in MRI using image-based tracking.

Artifacts caused by patient motion during scanning remain a serious problem in most MRI applications. The prospective motion correction technique attempts to address this problem at its source by keeping the measurement coordinate system fixed with respect to the patient throughout the entire scan process. In this study, a new image‐based approach for prospective motion correction is described, which utilizes three orthogonal two‐dimensional spiral navigator acquisitions, along with a flexible image‐based tracking method based on the extended Kalman filter algorithm for online motion measurement. The spiral navigator/extended Kalman filter framework offers the advantages of image‐domain tracking within patient‐specific regions‐of‐interest and reduced sensitivity to off‐resonance‐induced corruption of rigid‐body motion estimates. The performance of the method was tested using offline computer simulations and online in vivo head motion experiments. In vivo validation results covering a broad range of staged head motions indicate a steady‐state error of less than 10% of the motion magnitude, even for large compound motions that included rotations over 15 deg. A preliminary in vivo application in three‐dimensional inversion recovery spoiled gradient echo (IR‐SPGR) and three‐dimensional fast spin echo (FSE) sequences demonstrates the effectiveness of the spiral navigator/extended Kalman filter framework for correcting three‐dimensional rigid‐body head motion artifacts prospectively in high‐resolution three‐dimensional MRI scans. Magn Reson Med, 2010.

Reliability and Precision of Pseudo-continuous Arterial Spin Labeling Perfusion MRI on 3.0 T and Comparison with 15O-water PET in Elderly Subjects at Risk for Alzheimer’s Disease

Arterial spin labeling (ASL) offers MRI measurement of cerebral blood flow (CBF) in vivo, and may offer clinical diagnostic utility in populations such as those with early Alzheimers disease (AD). In the current study, we investigated the reliability and precision of a pseudo‐continuous ASL (pcASL) sequence that was performed two or three times within one hour on eight young normal control subjects, and 14 elderly subjects including 11 with normal cognition, one with AD and two with Mild Cognitive Impairment (MCI). Six of these elderly subjects including one AD, two MCIs and three controls also received 15O‐water positron emission tomography (PET) scans 2 h before their pcASL MR scan. The instrumental reliability of pcASL was evaluated with the intraclass correlation coefficient (ICC). The ICCs were greater than 0.90 in pcASL global perfusion measurements for both the young and the elderly groups. The cross‐modality perfusion imaging comparison yielded very good global and regional agreement in global gray matter and the posterior cingulate cortex. Significant negative correlation was found between age and the gray/white matter perfusion ratio (r = –0.62, p < 0.002). The AD and MCI patients showed the lowest gray/white matter perfusion ratio among all the subjects. The data suggest that pcASL provides a reliable whole brain CBF measurement in young and elderly adults whose results converge with those obtained with the traditional 15O‐water PET perfusion imaging method. pcASL perfusion MRI offers an alternative method for non‐invasive in vivo examination of early pathophysiological changes in AD. Copyright

Arterial spin labelling reveals an abnormal cerebral perfusion pattern in Parkinson’s disease

There is a need for objective imaging markers of Parkinsons disease status and progression. Positron emission tomography and single photon emission computed tomography studies have suggested patterns of abnormal cerebral perfusion in Parkinsons disease as potential functional biomarkers. This study aimed to identify an arterial spin labelling magnetic resonance-derived perfusion network as an accessible, non-invasive alternative. We used pseudo-continuous arterial spin labelling to measure cerebral grey matter perfusion in 61 subjects with Parkinsons disease with a range of motor and cognitive impairment, including patients with dementia and 29 age- and sex-matched controls. Principal component analysis was used to derive a Parkinsons disease-related perfusion network via logistic regression. Region of interest analysis of absolute perfusion values revealed that the Parkinsons disease pattern was characterized by decreased perfusion in posterior parieto-occipital cortex, precuneus and cuneus, and middle frontal gyri compared with healthy controls. Perfusion was preserved in globus pallidus, putamen, anterior cingulate and post- and pre-central gyri. Both motor and cognitive statuses were significant factors related to network score. A network approach, supported by arterial spin labelling-derived absolute perfusion values may provide a readily accessible neuroimaging method to characterize and track progression of both motor and cognitive status in Parkinsons disease.

Prospective motion correction of high-resolution magnetic resonance imaging data in children

Motion artifacts pose significant problems for the acquisition and analysis of high-resolution magnetic resonance imaging data. These artifacts can be particularly severe when studying pediatric populations, where greater patient movement reduces the ability to clearly view and reliably measure anatomy. In this study, we tested the effectiveness of a new prospective motion correction technique, called PROMO, as applied to making neuroanatomical measures in typically developing school-age children. This method attempts to address the problem of motion at its source by keeping the measurement coordinate system fixed with respect to the subject throughout image acquisition. The technique also performs automatic rescanning of images that were acquired during intervals of particularly severe motion. Unlike many previous techniques, this approach adjusts for both in-plane and through-plane movement, greatly reducing image artifacts without the need for additional equipment. Results show that the use of PROMO notably enhances subjective image quality, reduces errors in Freesurfer cortical surface reconstructions, and significantly improves the subcortical volumetric segmentation of brain structures. Further applications of PROMO for clinical and cognitive neuroscience are discussed.

Reduced resolution transit delay prescan for quantitative continuous arterial spin labeling perfusion imaging.

Arterial spin labeling perfusion MRI can suffer from artifacts and quantification errors when the time delay between labeling and arrival of labeled blood in the tissue is uncertain. This transit delay is particularly uncertain in broad clinical populations, where reduced or collateral flow may occur. Measurement of transit delay by acquisition of the arterial spin labeling signal at many different time delays typically extends the imaging time and degrades the sensitivity of the resulting perfusion images. Acquisition of transit delay maps at the same spatial resolution as perfusion images may not be necessary, however, because transit delay maps tend to contain little high spatial resolution information. Here, we propose the use of a reduced spatial resolution arterial spin labeling prescan for the rapid measurement of transit delay. Approaches to using the derived transit delay information to optimize and quantify higher resolution continuous arterial spin labeling perfusion images are described. Results in normal volunteers demonstrate heterogeneity of transit delay across different brain regions that lead to quantification errors without the transit maps and demonstrate the feasibility of this approach to perfusion and transit delay quantification. Magn Reson Med, 2012.

Time-resolved Vessel-selective Digital Subtraction MR Angiography of the Cerebral Vasculature with Arterial Spin Labeling

PURPOSE To demonstrate an arterial spin-labeling (ASL) magnetic resonance (MR) angiographic technique that covers the entire cerebral vasculature and yields transparent-background, time-resolved hemodynamic, and vessel-specific information similar to that obtained with x-ray digital subtraction angiography (DSA) without the use of exogenous contrast agents. MATERIALS AND METHODS Prior institutional review board approval and written informed consent were obtained for this HIPAA-compliant study in which 12 healthy volunteers (five women, seven men; age range, 21-62 years; average age, 28 years) underwent imaging. An ASL technique in which variable labeling durations are used to acquire hemodynamic inflow information and a vessel-selective pulsed-continuous ASL technique were tested. Region-of-interest signal intensities in various vessel segments were averaged across subjects and used to quantitatively compare images. For comparison, a standard time of flight (TOF) acquisition was performed in the circle of Willis. RESULTS Inflow temporal resolution of 200 msec was demonstrated, revealing arterial transit times of 750, 950, and 1100 msec to consecutive segments of the middle cerebral artery from distal to the circle of Willis to deep regions of the midbrain. Selective labeling resulted in an average of eightfold suppression of contralateral vessels relative to the labeled vessel. Signal-to-noise ratios and contrast-to-noise ratios on maximum intensity projection images obtained with 88-second volumetric acquisitions (60 ± 15 [standard deviation] and 57 ± 15, respectively) and 11-second single-projection acquisitions (19 ± 5 and 17 ± 5, respectively) were comparable with standard TOF acquisitions, in which a 2.7-fold longer imaging duration for a 2.6-fold lower pixel area was used. Normal variations of the vasculature were identified with ASL angiography. CONCLUSION ASL angiography can be used to acquire hemodynamic vessel-specific information similar to that obtained with x-ray DSA.

Prospective motion correction improves diagnostic utility of pediatric MRI scans

A new technique for prospectively correcting head motion (called PROMO) during acquisition of high-resolution MRI scans has been developed to reduce motion artifacts. To evaluate the efficacy of PROMO, four T1-weighted image volumes (two with PROMO enabled, two uncorrected) were acquired for each of nine children. A radiologist, blind to whether PROMO was used, rated image quality and artifacts on all sagittal slices of every volume. These ratings were significantly better in scans collected with PROMO relative to those collected without PROMO (Mann-Whitney U test, P < 0.0001). The use of PROMO, especially in motion-prone patients, should improve the accuracy of measurements made for clinical care and research, and potentially reduce the need for sedation in children.

Volumetric measurement of perfusion and arterial transit delay using hadamard encoded continuous arterial spin labeling.

Creating images of the transit delay from the labeling location to image tissue can aid the optimization and quantification of arterial spin labeling perfusion measurements and may provide diagnostic information independent of perfusion. Unfortunately, measuring transit delay requires acquiring a series of images with different labeling timing that adds to the time cost and increases the noise of the arterial spin labeling study. Here, we implement and evaluate a proposed Hadamard encoding of labeling that speeds the imaging and improves the signal‐to‐noise ratio efficiency. Volumetric images in human volunteers confirmed the theoretical advantages of Hadamard encoding over sequential acquisition of images with multiple labeling timing. Perfusion images calculated from Hadamard encoded acquisition had reduced signal‐to‐noise ratio relative to a dedicated perfusion acquisition with either assumed or separately measured transit delays, however. Magn Reson Med 69:1014–1022, 2013.

Helical MR: continuously moving table axial imaging with radial acquisitions.

A technique for extended field of view MRI is presented. Similar to helical computed tomography, the method utilizes a continuously moving patient table, a 2D axial slice that remains fixed relative to the MRI magnet, and a radial k‐space trajectory. A fully refocused SSFP acquisition enables spatial resolution comparable to current clinical protocols in scan times that are sufficiently short to allow a reasonable breathhold duration. RF transmission and signal reception are performed using the RF body coil and the images are reconstructed in real time. Experimental results are presented that illustrate the techniques ability to resolve small structures in the table‐motion direction. Simulation experiments to study the steady‐state response of the fully refocused SSFP acquisition during continuous table motion are also presented. Finally, whole body images of healthy volunteers demonstrate the high image quality achieved using the helical MRI approach. Magn Reson Med 50:1053–1060, 2003.

A selective insular perfusion deficit contributes to compromised salience network connectivity in recovering alcoholic men.

BACKGROUND Alcoholism can disrupt neural synchrony between nodes of intrinsic functional networks that are maximally active when resting relative to engaging in a task, the default mode network (DMN) pattern. Untested, however, are whether the DMN in alcoholics can rebound normally from the relatively depressed task state to the active resting state and whether local perfusion deficits could disrupt network synchrony when switching from conditions of rest to task to rest, thereby indicating a physiological mechanism of neural network adaptation capability. METHODS Whole-brain, three-dimensional pulsed-continuous arterial spin labeling provided measurements of regional cerebral blood flow (CBF) in 12 alcoholics and 12 control subjects under three conditions: pretask rest, spatial working-memory task, and posttask rest. RESULTS With practice, alcoholics and control subjects achieved similar task accuracy and reaction times. Both groups exhibited a high-low-high pattern of perfusion levels in DMN regions during the rest-task-rest runs and the opposite pattern in posterior and cerebellar regions known to be associated with spatial working memory. Alcoholics showed selective differences from control subjects in the rest-task-rest CBF pattern in the anterior precuneus and CBF level in the insula, a hub of the salience network. Connectivity analysis identified activation synchrony from an insula seed to salience nodes (parietal, medial frontal, anterior cingulate cortices) in control subjects only. CONCLUSIONS We propose that attenuated insular CBF is a mechanism underlying compromised connectivity among salience network nodes. This local perfusion deficit in alcoholics has the potential to impair ability to switch from cognitive states of interoceptive cravings to cognitive control for curbing internal urges.