Subhendra N. Sarkar

Brain MR Imaging at Ultra-low Radiofrequency Power

PURPOSE To explore the lower limits for radiofrequency (RF) power-induced specific absorption rate (SAR) achievable at 1.5 T for brain magnetic resonance (MR) imaging without loss of tissue signal or contrast present in high-SAR clinical imaging in order to create a potentially viable MR method at ultra-low RF power to image tissues containing implanted devices. MATERIALS AND METHODS An institutional review board-approved HIPAA-compliant prospective MR study design was used, with written informed consent from all subjects prior to MR sessions. Seven healthy subjects were imaged prospectively at 1.5 T with ultra-low-SAR optimized three-dimensional (3D) fast spin-echo (FSE) and fluid-attenuated inversion-recovery (FLAIR) T2-weighted sequences and an ultra-low-SAR 3D spoiled gradient-recalled acquisition in the steady state T1-weighted sequence. Corresponding high-SAR two-dimensional (2D) clinical sequences were also performed. In addition to qualitative comparisons, absolute signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) for multicoil, parallel imaging acquisitions were generated by using a Monte Carlo method for quantitative comparison between ultra-low-SAR and high-SAR results. RESULTS There were minor to moderate differences in the absolute tissue SNR and CNR values and in qualitative appearance of brain images obtained by using ultra-low-SAR and high-SAR techniques. High-SAR 2D T2-weighted imaging produced slightly higher SNR, while ultra-low-SAR 3D technique not only produced higher SNR for T1-weighted and FLAIR images but also higher CNRs for all three sequences for most of the brain tissues. CONCLUSION The 3D techniques adopted here led to a decrease in the absorbed RF power by two orders of magnitude at 1.5 T, and still the image quality was preserved within clinically acceptable imaging times.

Utilizing Fast Spin Echo MRI to Reduce Image Artifacts and Improve Implant/Tissue Interface Detection in Refractory Parkinson’s Patients with Deep Brain Stimulators

Introduction. In medically refractory Parkinsons disease (PD) deep-brain stimulation (DBS) is an effective therapeutic tool. Postimplantation MRI is important in assessing tissue damage and DBS lead placement accuracy. We wanted to identify which MRI sequence can detect DBS leads with smallest artifactual signal void, allowing better tissue/electrode edge conspicuity. Methods. Using an IRB approved protocol 8 advanced PD patients were imaged within MR conditional safety guidelines at low RF power (SAR ≤ 0.1 W/kg) in coronal plane at 1.5T by various sequences. The image slices were subjectively evaluated for diagnostic quality and the lead contact diameters were compared to identify a sequence least affected by metallic leads. Results and Discussion. Spin echo and fast spin echo based low SAR sequences provided acceptable image quality with comparable image blooming (enlargement) of stimulator leads. The mean lead diameters were 2.2 ± 0.1 mm for 2D, 2.1 ± 0.1 mm for 3D, and 4.0 ± 0.2 mm for 3D MPRAGE sequence. Conclusion. Low RF power spin echo and fast spin echo based 2D and 3D FSE sequences provide acceptable image quality adjacent to DBS leads. The smallest artifactual blooming of stimulator leads is present on 3D FSE while the largest signal void appears in the 3D MPRAGE sequence.

Three-Dimensional Brain MRI for DBS Patients Within Ultra-Low Radiofrequency Power Limits

For patients with deep brain stimulators (DBS), local absorbed radiofrequency (RF) power is unknown and is much higher than what the system estimates. We developed a comprehensive, high‐quality brain magnetic resonance imaging (MRI) protocol for DBS patients utilizing three‐dimensional (3D) magnetic resonance sequences at very low RF power.

Optimized double inversion recovery for reduction of T1 weighting in fluid‐attenuated inversion recovery

Fluid‐attenuated inversion recovery (FLAIR) is a routinely used technique in clinical practice to detect long T2 lesions by suppressing the cerebrospinal fluid. Concerns remain, however, that the inversion pulse in FLAIR imparts T1 weighting that can decrease the detectability and mischaracterize some lesions. Hence, FLAIR is usually acquired in conjunction with a standard T2 to guard against these concerns. Recently, double inversion recovery (DIR) preparations have highlighted certain types of lesions by suppressing both cerebrospinal fluid and white matter but produce even stronger T1 contrast than FLAIR. This work shows that the inversion times in a DIR sequence can be optimized to minimize unwanted T1 weighting, enabling the acquisition of cerebrospinal fluid‐suppressed images with pure T2 weighting. This technique is referred to as T1‐nulled DIR. The theory to determine the optimized inversion times is discussed and the results are shown by simulations, normal volunteer studies, and multiple sclerosis patient studies. T1‐nulled DIR provides equivalent or superior contrast between gray and white matters as well as white matter and multiple sclerosis lesion at the same repetition time. Multiple sclerosis lesions appeared sharper on T1‐nulled DIR compared to FLAIR. T1‐nulled DIR has the potential to replace the combination of standard T2 and FLAIR acquisitions in many clinical protocols. Magn Reson Med, 2011.

Low-Power Inversion Recovery MRI Preserves Brain Tissue Contrast for Patients with Parkinson Disease with Deep Brain Stimulators

BACKGROUND AND PURPOSE: Fast spin-echo short τ inversion recovery sequences have been very useful for MR imaging–guided deep brain stimulation procedures in Parkinson disease. However, high-quality fast spin-echo imaging deposits significant heat, exceeding FDA-approved limits when patients already have undergone deep brain stimulation and need a second one or a routine brain MR imaging for neurologic indications. We have developed a STIR sequence with an ultra-low specific absorption rate that meets hardware limitations and produces adequate tissue contrast in cortical and subcortical brain tissues for deep brain stimulation recipients. MATERIALS AND METHODS: Thirteen patients with medically refractory Parkinson disease who qualified for deep brain stimulation were imaged at 1.5T with a fast spin-echo short τ inversion recovery sequence modified to meet conditional MR imaging hardware and specific absorption rate restrictions. Tissue contrast-to-noise ratios and implant localization were objectively and subjectively compared by 2 neuroradiologists, and image quality for surgical planning was assessed by a neurosurgeon for high and low specific absorption rate images. RESULTS: The mean contrast-to-noise ratio for cerebral tissues without including the contrast-to-noise ratio for ventricular fluid was 35 and 31 for high and low specific absorption rate images. Subjective ratings for low specific absorption rate tissue contrast in 77% of patients were identical to (and in a few cases higher than) those of high specific absorption rate contrast, while the neurosurgical coordinates for fusing the stereotactic atlas with low specific absorption rate MR imaging were equivalent to those of the high specific absorption rate for 69% of patients. CONCLUSIONS: Patients with Parkinson disease who have already had a deep brain stimulation face a risk of neural injury if routine, high specific absorption rate MR imaging is performed. Our modified fast spin-echo short τ inversion recovery sequence conforms to very conservative radiofrequency safety limits, while it maintains high tissue contrast for presurgical planning, postsurgical assessment, and radiologic evaluations with greater confidence for radiofrequency safety.

Improved quantification of brain perfusion using FAIR with active suppression of superior tagging (FAIR ASST)

To address two problems for perfusion studies in the middle or inferior brain regions: (1) to reduce venous artifacts due to the intrinsic superior labeling of FAIR; (2) to alleviate the discrepancy of the existence of both superior and inferior boluses, but with only the inferior bolus having a temporally defined bolus width with Q2TIPs or QUIPSS.

Subthalamic Nuclear Tissue Contrast in Inversion Recovery MRI Decreases with Age in Medically Refractory Parkinson's Disease

MRI appearance of subthalamic nucleus (STN) boundaries in Parkinsons patients is often unreliable and not well understood. An objective comparison between FSE T2 and inversion recovery (FSTIR) sequences for stereotactic placement of deep brain stimulators is presented to advance current understanding of STN tissue contrast for refractory Parkinsons disease (PD).

Quantifying Cerebellum Grey Matter and White Matter Perfusion Using Pulsed Arterial Spin Labeling

To facilitate quantification of cerebellum cerebral blood flow (CBF), studies were performed to systematically optimize arterial spin labeling (ASL) parameters for measuring cerebellum perfusion, segment cerebellum to obtain separate CBF values for grey matter (GM) and white matter (WM), and compare FAIR ASST to PICORE. Cerebellum GM and WM CBF were measured with optimized ASL parameters using FAIR ASST and PICORE in five subjects. Influence of volume averaging in voxels on cerebellar grey and white matter boundaries was minimized by high-probability threshold masks. Cerebellar CBF values determined by FAIR ASST were 43.8 ± 5.1 mL/100 g/min for GM and 27.6 ± 4.5 mL/100 g/min for WM. Quantitative perfusion studies indicated that CBF in cerebellum GM is 1.6 times greater than that in cerebellum WM. Compared to PICORE, FAIR ASST produced similar CBF estimations but less subtraction error and lower temporal, spatial, and intersubject variability. These are important advantages for detecting group and/or condition differences in CBF values.

A subjective and objective comparison of tissue contrast and imaging artifacts present in routine spin echoes and in iterative decomposition of asymmetric spin echoes for soft tissue neck MRI

OBJECTIVE FSE sequences play key roles in neck MRI despite the susceptibility issues in neck region. Iterative decomposition of asymmetric echoes (IDEAL, GE) is a promising method that separates fat and water images resulting in high SNR and improved fat suppression. We tested how neck tissue contrasts, image artifacts and fat separation as opposed to fat suppression in terms of image quality compare between routine and IDEAL FSE. METHODS IDEAL based and routine T1 and T2-weighted FSE sequences were applied for neck MRI at 1.5T and 3T. Overall image quality including fat suppression, tissue contrast, image artifacts and lesion conspicuity were subjectively assessed for 20 patients clinically indicated for neck MRI. Quantitative tissue contrast estimates from parotid area were compared between IDEAL and routine FSE for 7 patients. Four patients with oncocytoma were also reviewed to assess benefits of separately reconstructed fat specific image sets. RESULTS Subjective tissue contrast and overall image quality including image sharpness, fat suppression and image artifacts were superior for IDEAL sequences. For oncocytoma fat specific IDEAL images provided additional information. Objective CNR estimates from a central slice were equivalent for IDEAL and routine FSE at both field strengths. CONCLUSIONS We demonstrated that high SNR inherent in IDEAL FSE consistently translates into high tissue contrast with image quality advantages in neck anatomy where large susceptibility variation and physiological motions reduce image quality for conventional FSE T1 and T2. However, the objective contrast estimates for parotid gland at isocenter were statistically equivalent for IDEAL and conventional FSE perhaps because at or near isocenter routine FSE works well. Additionally, fat specific IDEAL image sets add to diagnostic specificity for fat deficient lesions.

Microvascular and large vein abnormalities in young patients after mild head trauma and associated fatigue: a brain SPECT evaluation and posture dependence modeling

OBJECTIVE MRI and CT scans are usually normal in mild traumatic brain injury (mTBI) although 15-20% of such patients suffer for months from fatigue, headache, anxiety, sleep and other disorders. mTBI is suspected to be a cerebrovascular injury, similar to moderate and severe TBI. Brain SPECT is more sensitive and shows perfusion abnormalities immediately after mTBI. This work explores the perfusion abnormalities for young patients suffering from fatigue several months after mTBI. PATIENTS AND METHODS Twelve mTBI patients (age:8-36 yr, 4 male) with no history of fatigue prior to trauma were prospectively studied following onset of fatigue 6-12 months after mTBI utilizing 99 m-Tc ECD brain SPECT with early and delayed radiotracer imaging. RESULTS The perfusion pattern in the mTBI + fatigue group included left hemispheric deficits in frontal lobes (early phase: 15.2 ± 4.2%, delayed phase: 9.9 ± 2.2%) and medial temporal lobes (early phase 11.2 ± 3.7%, delayed phase: 9.0 ± 2.3%). Seven patients additionally showed excess tracer accumulation in the parenchyma surrounding internal jugular bulb inferior to temporal lobe. This was modeled as due to increased cellular permeability from TBI induced oxidative stress affecting endothelial tight junctions and consequent tracer leakage across jugular bulbs. Prolonged posture changes from erect to supine position during imaging increase jugular cross-sectional area and venous wall pressure as has been observed in other disease processes and seem to be responsible for tracer leakage from jugular bulbs in our study. CONCLUSION This work supports an oxidative stress and BBB disruption model for mTBI. The frontal and temporal lobe perfusion deficits are attributed to anatomical vulnerabilities of these lobes. During a mild TBI both of these lobes are susceptible to grazing impacts with underlying bony ridges. We propose a relation between mTBI and fatigue arising from oxidative stress in mTBI affecting ATP generation and altering endothelial homeostasis for both micro-and-large vasculatures. The tracer leakage observed around jugular veins is due to posture induced changes in venous cross-sections and wall pressure as well as from compromised endothelium post TBI induced oxidative stress.