Sridhar R. Charagundla

In vivo measurement of T1ρ dispersion in the human brain at 1.5 tesla

To measure T1ρ relaxation times and T1ρ dispersion in the human brain in vivo.

T1ρ Imaging of Murine Brain Tumors at 4 T

Abstract Rationale and Objectives The goal of this study was to evaluate the utility of T1 ρ weighting in magnetic resonance imaging of murine brain tumors. Materials and Methods S91 Cloudman melanoma was implanted in mouse brains ( n = 4). A T2-weighted spin-echo (SE) and a T1 ρ -weighted fast SE-based sequence were performed on a 4-T clinical imager. T2 and T1 ρ maps were computed. The tumor-to-normal-tissue contrast was compared between T2-weighted, T1 ρ -weighted, proton-density–weighted, and pre- and postcontrast T1-weighted SE images. Results The tumor-tissue contrast of the T1 ρ -weighted images was similar to that of the T2-weighted images but less than that of the postcontrast T1-weighted images. The T1 ρ -weighted images provided better definition of tumor boundaries than T2-weighted images. At spin-locking powers of 0.5 and 1.5 kHz, the T1 ρ of the tumor was 64.0 msec ± 0.46 and 68.65 msec ± 0.59, respectively. There was no significant inter- or intra-animal variation in T1 ρ for tumor or normal brain cortex. Conclusion T1 ρ -weighted imaging performed at low spin-lock strengths qualitatively depicted tumor borders better than proton-density or T2-weighted imaging and could be useful in treatment planning when combined with other imaging sequences.

Method for reduced SAR T1ρ-weighted MRI

A reduced specific absorption rate (SAR) version of the T1ρ‐weighted MR pulse sequence was designed and implemented. The reduced SAR method employs a partial k‐space acquisition approach in which a full power spin‐lock pulse is applied to only the central phase‐encode lines of k‐space, while the remainder of k‐space receives a low‐power spin‐lock pulse. Acquisition of high‐ and low‐power phase‐encode lines are interspersed chronologically to minimize average power deposition. In this way, the majority of signal energy in the central portion of k‐space receives full T1ρ‐weighting, while the average SAR of the overall acquisition can be reduced, thereby lowering the minimum safely allowable TR. The pulse sequence was used to create T1ρ maps of a phantom, an in vivo mouse brain, and the brain of a human volunteer. In the images of the human brain, SAR was reduced by 40% while the measurements of T1ρ differed by only 2%. The reduced SAR sequence enables T1ρ‐weighted MRI in a clinical setting, even at high field strengths. Magn Reson Med 51:1096–1102, 2004.

Pulse sequence for multislice T1ρ-weighted MRI

A 2D multislice spin‐lock (MS‐SL) MR pulse sequence is presented for rapid volumetric T1ρ‐weighted imaging. Image quality is compared with T1ρ‐weighted data collected using a single‐slice (SS) SL sequence and T2‐weighted data from a standard MS spin‐echo (SE) sequence. Saturation of longitudinal magnetization by the application of nonselective SL pulses is experimentally measured and theoretically modeled as T2ρ decay. The saturation data is used to correct the image data as a function of the SL pulse duration to make quantitative measurements of T1ρ. Measurements of T1ρ using the saturation‐corrected MS‐SL data are nearly identical to those measured using an SS‐SL sequence. The MS‐SL sequence produces quantitative T1ρ maps of an entire sample volume with the high‐SNR advantages conferred by SE‐based sequences. Magn Reson Med 51:362–369, 2004.

Indirect 17O-magnetic resonance imaging of cerebral blood flow in the rat

Proton T1ρ‐dispersion MRI is demonstrated for indirect, in vivo detection of 17O in the brain. This technique, which may be readily implemented on any clinical MRI scanner, is applied towards high‐resolution, quantitative mapping of cerebral blood flow (CBF) in the rat by monitoring the clearance of 17O‐enriched water. Strategies are derived and employed for 1) quantitation of absolute H217O tracer concentration from a ratio of high‐ and low‐frequency spin‐locked T1ρ images, and 2) mapping CBF without having to transform the T1ρ signal to H217O tracer concentration. Absolute regional blood flow was mapped in a single 3‐mm brain slice at an in‐plane resolution of 0.4 × 0.8 mm within a 5‐min tracer washout time; these data are consistent with the less localized CBF measurements reported in the literature. T1ρ‐weighted imaging yields excellent signal‐to‐noise ratios, spatiotemporal resolution, and anatomical contrast for mapping CBF. Magn Reson Med 49:479–487, 2003.

A pulse sequence for rapid in vivo spin-locked MRI.

To develop a novel pulse sequence called spin‐locked echo planar imaging (EPI), or (SLEPI), to perform rapid T1ρ‐weighted MRI.

17O-decoupled 1H detection using a double-tuned coil.

17O-decoupled proton MR spectroscopy imaging with a double-tuned radiofrequency (RF) coil at 2 T was used to detect and quantify H2 17O in tissue containing various concentrations of 17O-enriched water in 5% gelatin. The pulse sequence used in these experiments consisted of a conventional proton spin-echo sequence with RF irradiation at the 17O resonance frequency applied between the proton 90 degrees pulse and the signal acquisition window. The double-tuned coil provided several advantages over systems using separate RF coils for 17O decoupling and proton excitation/detection, including ensuring that the same (or similar) sample volumes are excited and decoupled and permitting accurate calibration of the 17O decoupling pulse amplitude. The efficiency of 17O decoupling as a function of decoupling RF amplitude, decoupling duration, and decoupling resonance offset was investigated. Finally, the specific absorption rate of the 17O decoupled pulse sequence was investigated and found to lie within federal guidelines at 1.5 T.

An MR imaging method for simultaneous measurement of gaseous diffusion constant and longitudinal relaxation time

A magnetic resonance imaging method for simultaneous and accurate determination of gaseous diffusion constant and longitudinal relaxation time is presented. The method is based on direct observation of diffusive motion. Initially, a slice-selective saturation of helium-3 (3He) spins was performed on a 3He/O2 phantom (9 atm/2 atm). A time-delay interval was introduced after saturation, allowing spins to diffuse in and out of the labeled slice. Following the delay interval a one-dimensional (1-D) projection image of the phantom was acquired. A series of 21 images was collected, each subsequent image having been acquired with an increased delay interval. Gradual spreading of the slice boundaries due to diffusion was thus observed. The projection profiles were fit to a solution of the Bloch equation corrected for diffusive motion. The fitting procedure yielded a value of D3He = 0.1562+/-0.0013 cm2/s, in good agreement with a measurement obtained with a modified version of the standard pulsed-field gradient technique. The method also enabled us to accurately measure the longitudinal relaxation of 3He spins by fitting the change of the total area under the projection profiles to an exponential. A value of T1 = 1.67 s (2 T field) was recorded, in excellent agreement with an inversion recovery measurement.

CT and MR Angiography of the Coronary Arteries

Evaluation of the coronary arteries in the patient with suspected ischemic heart disease is inherently difficult, since these vessels are small (with an average diameter of 3–4 mm) and are in constant motion. Over the past decade, technological developments in CT angiography (CTA) and magnetic resonance angiography (MRA) have made noninvasive coronary artery imaging a clinical reality. Given the widespread prevalence of coronary disease, the risks of catheter angiography, and the ability of CT and MR to visualize both vessel lumen and vessel wall, these techniques promise to occupy a central role in the evaluation of ischemic heart disease.