Thanh D. Nguyen

Respiratory and cardiac self-gated free-breathing cardiac CINE imaging with multiecho 3D hybrid radial SSFP acquisition

A respiratory and cardiac self‐gated free‐breathing three‐dimensional cine steady‐state free precession imaging method using multiecho hybrid radial sampling is presented. Cartesian mapping of the k‐space center along the slice encoding direction provides intensity‐weighted position information, from which both respiratory and cardiac motions are derived. With in plan radial sampling acquired at every pulse repetition time, no extra scan time is required for sampling the k‐space center. Temporal filtering based on density compensation is used for radial reconstruction to achieve high signal‐to‐noise ratio and contrast‐to‐noise ratio. High correlation between the self‐gating signals and external gating signals is demonstrated. This respiratory and cardiac self‐gated, free‐breathing, three‐dimensional, radial cardiac cine imaging technique provides image quality comparable to that acquired with the multiple breath‐hold two‐dimensional Cartesian steady‐state free precession technique in short‐axis, four‐chamber, and two‐chamber orientations. Functional measurements from the three‐dimensional cardiac short axis cine images are found to be comparable to those obtained using the standard two‐dimensional technique. Magn Reson Med 63:1230–1237, 2010.

In vivo quantification of contrast agent concentration using the induced magnetic field for time‐resolved arterial input function measurement with MRI

For pharmacokinetic modeling of tissue physiology, there is great interest in measuring the arterial input function (AIF) from dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) using paramagnetic contrast agents. Due to relaxation effects, the measured signal is a nonlinear function of the injected contrast agent concentration and depends on sequence parameters, system calibration, and time-of-flight effects, making it difficult to accurately measure the AIF during the first pass. Paramagnetic contrast agents also affect susceptibility and modify the magnetic field in proportion to their concentration. This information is contained in the MR signal phase which is discarded in a typical image reconstruction. However, quantifying AIF through contrast agent susceptibility induced phase changes is made difficult by the fact that the induced magnetic field is nonlocal and depends upon the contrast agent spatial distribution and thus on organ and vessel shapes. In this article, the contrast agent susceptibility was quantified through inversion of magnetic field shifts using a piece-wise constant model. Its feasibility is demonstrated by a determination of the AIF from the susceptibility-induced field changes of an intravenous bolus. After in vitro validation, a time-resolved two-dimensional (2D) gradient echo scan, triggered to diastole, was performed in vivo on the aortic arch during a bolus injection of 0.1 mmol/kg Gd-DTPA. An approximate geometrical model of the aortic arch constructed from the magnitude images was used to calculate the spatial variation of the field associated with the bolus. In 14 subjects, Gd concentration curves were measured dynamically (one measurement per heart beat) and indirectly validated by independent 2D cine phase contrast flow rate measurements. Flow rate measurements using indicator conservation with this novel quantitative susceptibility imaging technique were found to be in good agreement with those obtained from the cine phase contrast measurements in all subjects. Contrary to techniques that rely on intensity, the accuracy of this signal phase based method is insensitive to factors influencing signal intensity such as flip angle, coil sensitivity, relaxation changes, and time-of-flight effects extending the range of pulse sequences and contrast doses for which quantitative DCE-MRI can be applied.

Effective Motion-Sensitizing Magnetization Preparation for Black Blood Magnetic Resonance Imaging of the Heart

To investigate the effectiveness of flow signal suppression of a motion‐sensitizing magnetization preparation (MSPREP) sequence and to optimize a 2D MSPREP steady‐state free precession (SSFP) sequence for black blood imaging of the heart.

A fast navigator-gated 3D sequence for delayed enhancement MRI of the myocardium: comparison with breathhold 2D imaging.

To develop a rapid navigator‐gated three‐dimensional (3DNAV) delayed‐enhancement MRI (DE‐MRI) sequence for myocardial viability assessment, and to evaluate its performance with breathhold two‐dimensional (2DBH) DE‐MRI sequence as the reference standard.

Demand-supply of neurointerventionalists for endovascular ischemic stroke therapy

Objective: To estimate the needed workforce of trained neurointerventionalists (NIs) to perform endovascular therapy (ET) for eligible patients with acute ischemic stroke (AIS). Method: Population and ischemic stroke incidence data were extracted with use of US Census and Centers for Disease Control and Prevention 2009 estimates. The annual “demand” is defined as the proportion of AIS patients who would meet inclusion criteria and clinical standards for ET. The “supply” is defined as the number of trained NIs and NIs in training. The “workforce” is the number of NIs needed to meet the demand (the number of eligible AIS patients) within an accessible geographic diameter. Data on NIs and NI fellowships were collected (Society of Neurointerventional Surgery [SNIS], Society of Vascular & Interventional Neurology [SVIN], Concentric Medical, and Penumbra Inc.). Results: The estimated number of NIs is close to 800, practicing within a 50-mile radius of major metropolitan areas in the United States, covering more than 95% of the US population. Approximately 40 NI fellows are graduating yearly from US training programs. In 5 years and 10 years, the number of NIs may reach 1,000 and 1,200, respectively. Currently, there are approximately 14,000 thrombectomy procedures performed in the United States each year. However, the percentage of AIS patients who may be eligible for ET in our estimation is 4% to 14%, or about 25,000 to 95,000 patients. This means that cases will occur at a rate of 26 to 97 per year in 5 years, or 22 to 81 per year in 10 years, for each NI. Providing 24/7 AIS coverage requires 2 to 3 NIs per medical center, adding to the challenge of providing manpower without diluting experience in areas of lower population density. Conclusion: The current and projected number of NIs would adequately supply the future need if the proportion of patients requiring AIS endovascular therapy increases. However, 2 to 3 NIs per comprehensive stroke center would be needed to provide 24/7 AIS therapy with a sufficient number of cases per NI. A tertiary stroke center model similar to the trauma model may provide the manpower solution without compromising the quality of care.

The impact of additional unlicensed spectrum on wireless services competition

The FCC in the U.S. has recently increased the amount of spectrum available for wireless broadband data services by permitting unlicensed access to television white-spaces. While this additional unlicensed spectrum allows for market expansion, it also influences competition among providers and can increase congestion (interference) among consumers of wireless services. We study the value (social welfare) obtained by adding unlicensed spectrum to an existing allocation of licensed spectrum among incumbent Service Providers (SPs). We assume a population of customers who choose a provider based on minimum delivered price. Here, delivered price is the price of the service plus a congestion cost, which depends on the number of subscribers in a band. For the model considered, we find that the social welfare depends on the amount of additional unlicensed spectrum, and can actually decrease over a significant range of unlicensed bandwidths.

Quantitative mapping of cerebral metabolic rate of oxygen (CMRO2 ) using quantitative susceptibility mapping (QSM).

To quantitatively map cerebral metabolic rate of oxygen ( CMRO2 ) and oxygen extraction fraction ( OEF ) in human brains using quantitative susceptibility mapping (QSM) and arterial spin labeling‐measured cerebral blood flow (CBF) before and after caffeine vasoconstriction.

Mitral Apparatus Assessment by Delayed Enhancement CMR: Relative Impact of Infarct Distribution on Mitral Regurgitation

OBJECTIVES This study sought to assess patterns and functional consequences of mitral apparatus infarction after acute myocardial infarction (AMI). BACKGROUND The mitral apparatus contains 2 myocardial components: papillary muscles and the adjacent left ventricular (LV) wall. Delayed-enhancement cardiac magnetic resonance (DE-CMR) enables in vivo study of inter-relationships and potential contributions of LV wall and papillary muscle infarction (PMI) to mitral regurgitation (MR). METHODS Multimodality imaging was performed: CMR was used to assess mitral geometry and infarct pattern, including 3D DE-CMR for PMI. Echocardiography was used to measure MR. Imaging occurred 27 ± 8 days after AMI (CMR, echocardiography within 1 day). RESULTS A total of 153 patients with first AMI were studied; PMI was present in 30% (n = 46 [72% posteromedial, 39% anterolateral]). When stratified by angiographic culprit vessel, PMI occurred in 65% of patients with left circumflex, 48% with right coronary, and only 14% of patients with left anterior descending infarctions (p <0.001). Patients with PMI had more advanced remodeling as measured by LV size and mitral annular diameter (p <0.05). Increased extent of PMI was accompanied by a stepwise increase in mean infarct transmurality within regional LV segments underlying each papillary muscle (p <0.001). Prevalence of lateral wall infarction was 3-fold higher among patients with PMI compared to patients without PMI (65% vs. 22%, p <0.001). Infarct distribution also impacted MR, with greater MR among patients with lateral wall infarction (p = 0.002). Conversely, MR severity did not differ on the basis of presence (p = 0.19) or extent (p = 0.12) of PMI, or by angiographic culprit vessel. In multivariable analysis, lateral wall infarct size (odds ratio 1.20/% LV myocardium [95% confidence interval: 1.05 to 1.39], p = 0.01) was independently associated with substantial (moderate or greater) MR even after controlling for mitral annular (odds ratio 1.22/mm [1.04 to 1.43], p = 0.01), and LV end-diastolic diameter (odds ratio 1.11/mm [0.99 to 1.23], p = 0.056). CONCLUSIONS Papillary muscle infarction is common after AMI, affecting nearly one-third of patients. Extent of PMI parallels adjacent LV wall injury, with lateral infarction-rather than PMI-associated with increased severity of post-AMI MR.

T2prep three‐dimensional spiral imaging with efficient whole brain coverage for myelin water quantification at 1.5 tesla

Quantitative assessment of myelination is important for characterizing tissue damage and evaluating response to therapy in white matter diseases such as multiple sclerosis. Conventional multicomponent T2 relaxometry based on the two‐dimensional (2D) multiecho spin echo sequence is a promising method to measure myelin water fraction, but its clinical utility is impeded by the prohibitively long data acquisition and limited brain coverage. The objective of this study was to develop a signal‐to‐noise ratio efficient 3D T2prep spiral gradient echo (3D SPIRAL) sequence for full brain T2 relaxometry and to validate this sequence using 3D multiecho spin echo as reference standard in healthy brains at 1.5 T. 3D SPIRAL was found to provide similar myelin water fraction in six selected white and gray matter areas using region‐of‐interest signal averaging analysis (N = 7, P > 0.05). While 3D multiecho spin echo only provided partial brain coverage, 3D SPIRAL enabled whole brain coverage with a fivefold higher acquisition speed per imaging slice and similar signal‐to‐noise ratio efficiency. Both 3D sequences provided superior signal‐to‐noise ratio efficiency when compared to the conventional 2D multiecho spin echo approach. Magn Reson Med, 2012.

Fast 3D contrast enhanced MRI of the liver using temporal resolution acceleration with constrained evolution reconstruction

Time‐resolved imaging is crucial for the accurate diagnosis of liver lesions. Current contrast enhanced liver magnetic resonance imaging acquires a few phases in sequential breath‐holds. The image quality is susceptible to bolus timing errors, which could result in missing the critical arterial phase. This impairs the detection of malignant tumors that are supplied primarily by the hepatic artery. In addition, the temporal resolution may be too low to reliably separate the arterial phase from the portal venous phase. In this study, a method called temporal resolution acceleration with constrained evolution reconstruction was developed with three‐dimensional volume coverage and high‐temporal frame rate. Data is acquired using a stack of spirals sampling trajectory combined with a golden ratio view order using an eight‐channel coil array. Temporal frames are reconstructed from vastly undersampled data sets using a nonlinear inverse algorithm assuming that the temporal changes are small at short time intervals. Numerical and phantom experimental validation is presented. Preliminary in vivo results demonstrated high spatial resolution dynamic three‐dimensional images of the whole liver with high frame rates, from which numerous subarterial phases could be easily identified retrospectively. Magn Reson Med, 2013.