Jaladhar Neelavalli

Clinical applications of neuroimaging with susceptibility-weighted imaging

Susceptibility‐weighted imaging (SWI) consists of using both magnitude and phase images from a high‐resolution, three‐dimensional, fully velocity compensated gradient‐echo sequence. Postprocessing is applied to the magnitude image by means of a phase mask to increase the conspicuity of the veins and other sources of susceptibility effects. This article gives a background of the SWI technique and describes its role in clinical neuroimaging. SWI is currently being tested in a number of centers worldwide as an emerging technique to improve the diagnosis of neurological trauma, brain neoplasms, and neurovascular diseases because of its ability to reveal vascular abnormalities and microbleeds. J. Magn. Reson. Imaging 2005.

Susceptibility Mapping as a Means to Visualize Veins and Quantify Oxygen Saturation

To create an orientation‐independent, 3D reconstruction of the veins in the brain using susceptibility mapping.

Susceptibility-weighted imaging to visualize blood products and improve tumor contrast in the study of brain masses.

To evaluate the diagnostic value of susceptibility‐weighted imaging (SWI) for studying brain masses.

Removing background phase variations in susceptibility-weighted imaging using a fast, forward-field calculation.

To estimate magnetic field variations induced from air–tissue interface geometry and remove their effects from susceptibility‐weighted imaging (SWI) data.

Improving Susceptibility Mapping Using a Threshold-Based K-Space/Image Domain Iterative Reconstruction Approach

To improve susceptibility quantification, a threshold‐based k‐space/image domain iterative approach that uses geometric information from the susceptibility map itself as a constraint to overcome the ill‐posed nature of the inverse filter is introduced. Simulations were used to study the accuracy of the method and its robustness in the presence of noise. In vivo data were processed and analyzed using this method. Both simulations and in vivo results show that most streaking artifacts inside the susceptibility map caused by the ill‐defined inverse filter were suppressed by the iterative approach. In simulated data, the bias toward lower mean susceptibility values inside vessels has been shown to decrease from around 10% to 2% when choosing an appropriate threshold value for the proposed iterative method. Typically, three iterations are sufficient for this approach to converge and this process takes less than 30 s to process a 512 × 512 × 256 dataset. This iterative method improves quantification of susceptibility inside vessels and reduces streaking artifacts throughout the brain for data collected from a single‐orientation acquisition. This approach has been applied to vessels alone as well as to vessels and other structures with lower susceptibility to generate whole brain susceptibility maps with significantly reduced streaking artifacts. Magn Reson Med, 2013.

New algorithm for quantifying vascular changes in dynamic contrast‐enhanced MRI independent of absolute T1 values

In this work, we present a new method for predicting changes in tumor vascularity using only one flip angle in dynamic contrast‐enhanced (DCE) imaging. The usual DCE approach finds the tissue initial T1 value T1(0) prior to injection of a contrast agent. We propose finding changes in the tissue contrast agent uptake characteristics pre‐ and postdrug treatment by fixing T1(0). Using both simulations and imaging pre‐ and postadministration of caffeine, we find that the relative change (NR50) in the median of the cumulative distribution (R50) is almost independent of T1(0). Fixing T1(0) leads to a concentration curve c(t) more robust to the presence of noise than calculating T1(0). Consequently, the NR50 for the tumor remains roughly the same as the ideal NR50 when T1(0) is exactly known. Further, variations in eating habits are shown to create significant changes in the R50 response for both liver and muscle. In conclusion, analyzing data with fixed T1(0) leads to a more stable measure of changes in NR50 and does not require knowledge of T1(0). Both caffeine and eating introduce major changes in blood flow that can significantly modify the NR50 and lead to incorrect conclusions regarding drug treatment. Magn Reson Med 58:463–472, 2007.

Imaging the Vessel Wall in Major Peripheral Arteries using Susceptibility Weighted Imaging

To demonstrate a novel contrast mechanism for imaging the vessel wall and vessel wall calcification using susceptibility‐weighted imaging (SWI).

EuII-containing cryptates as contrast agents for ultra-high field strength magnetic resonance imaging

The relaxivity (contrast-enhancing ability) of Eu(II)-containing cryptates was found to be better than a clinically approved Gd(III)-based agent at 7 T. These cryptates are among a few examples of paramagnetic substances that show an increase in longitudinal relaxivity, r(1), at ultra-high field strength relative to lower field strengths.

Quantification of punctate iron sources using magnetic resonance phase

Iron‐mediated tissue damage is present in cerebrovascular and neurodegenerative diseases and neurotrauma. Brain microbleeds are often present in these maladies and are assuming increasing clinical importance. Because brain microbleeds present a source of pathologic iron to the brain, the noninvasive quantification of this iron pool is potentially valuable. Past efforts to quantify brain iron have focused on content estimation within distributed brain regions. In addition, conventional approaches using “magnitude” images have met significant limitations. In this study, a technique is presented to quantify the iron content of punctate samples using phase images. Samples are modeled as magnetic dipoles and phase shifts due to local dipole field perturbations are mathematically related to sample iron content and radius using easily recognized geometric features in phase images. Phantoms containing samples of a chitosan‐ferric oxyhydroxide composite (which serves as a mimic for hemosiderin) were scanned with a susceptibility‐weighted imaging sequence at 11.7 T. Plots relating sample iron content and radius to phase image features were compared to theoretical predictions. The primary result is the validation of the technique by the excellent agreement between theory and the iron content plot. This research is a potential first step toward quantification of punctate brain iron sources such as brain microbleeds. Magn Reson Med, 2010.

Susceptibility mapping of air, bone, and calcium in the head.

To demonstrate the mapping of structures with high susceptibility values, such as the sinuses, bones and teeth, using short echo times.