Myocardial Tissue Oxygenation and Microvascular Blood Volume Measurement Using a Contrast Blood Oxygenation Level-Dependent Imaging Model.

OBJECTIVES: We propose a method of quantitatively measuring drug-induced microvascular volume changes, as well as drug-induced changes in blood oxygenation using calibrated blood oxygen level-dependent magnetic resonance imaging (MRI). We postulate that for MRI signals there is a contribution to R2* relaxation rates from static susceptibility effects of the intravascular blood that scales with the blood volume/magnetic field and depends on the oxygenation state of the blood. These may be compared with the effects of an intravascular contrast agent. With 4 R2* measurements, microvascular blood volume (MBV) and tissue oxygenation changes can be quantified with the administration of a vasoactive drug. MATERIALS AND METHODS: The protocol examined 12 healthy rats in a prospective observational study. R2* maps were acquired with and without infusion of adenosine, which increases microvascular blood flow, or dobutamine, which increases myocardial oxygen consumption. In addition, R2* maps were acquired after the intravenous administration of a monocrystalline iron oxide nanoparticle, with and without adenosine or dobutamine. RESULTS: Total microvascular volume was shown to increase by 10.8% with adenosine and by 25.6% with dobutamine ( P < 0.05). When comparing endocardium versus epicardium, both adenosine and dobutamine demonstrated significant differences between endocardial and epicardial MBV changes ( P < 0.05). Total myocardial oxygenation saturation increased by 6.59% with adenosine and by 1.64% with dobutamine ( P = 0.27). The difference between epicardial and endocardial oxygenation changes were significant with each drug (adenosine P < 0.05, dobutamine P < 0.05). CONCLUSIONS: Our results demonstrate the ability to quantify microvascular volume and oxygenation changes using calibrated blood oxygen level-dependent MRI, and we demonstrate different responses of adenosine and dobutamine. This method has clinical potential in examining microvascular disease in various disease states without the administration of radiopharmaceuticals or gadolinium-based contrast agents.

MRI simulator with object-specific field map calculations.

A new MRI simulator has been developed that generates images of realistic objects for arbitrary pulse sequences executed in the presence of static field inhomogeneities, including those due to magnetic susceptibility, variations in the applied field, and chemical shift. In contrast to previous simulators, this system generates object-specific inhomogeneity patterns from first principles and propagates the consequent frequency offsets and intravoxel dephasing through the acquisition protocols to produce images with realistic artifacts. The simulator consists of two parts. The input to part 1 is a set of "susceptibility voxels" that describe the magnetic properties of the object being imaged. It calculates a frequency offset for each voxel by computing the size of the static field offset at each voxel in the image based on the magnetic susceptibility of each tissue type within all voxels. The method of calculation is a three-dimensional convolution of the susceptibility-voxels with a kernel derived from a previously published method and takes advantage of the superposition principle to include voxels with mixtures of substances of differing susceptibilities. Part 2 produces both a signal and a reconstructed image. Its inputs include a voxel-based description of the object, frequency offsets computed by part 1, applied static field errors, chemical shift values, and a description of the imaging protocol. Intravoxel variations in both static field and time-dependent phase are calculated for each voxel. Validations of part 1 are presented for a known analytic solution and for experimental data from two phantoms. Part 2 was validated with comparisons to an independent simulation provided by the Montreal Neurological Institute and experimental data from a phantom.

Improved in vivo measurement of myocardial transverse relaxation with 3 Tesla magnetic resonance imaging.

PURPOSE: To develop practical methods at 3 Tesla (T) for measuring myocardial transverse relaxation in normal human myocardium. MATERIALS AND METHODS: Ten healthy volunteers were investigated with four multi-echo, turbo spin-echo (TSE) methods. Each method traded acquired phase encoding lines per image for echo-image sample points obtained along the T(2) decay curve. Four multi-echo turbo field-echo (TFE) methods were also tested. The TFE methods highlighted differences between achievable receiver bandwidth and echo time constraints versus the number of sample points obtained along the T(2) (*) decay curve. RESULTS: Measured transverse relaxation values were consistent in reported means across all scan methods. T(2) for the ventricular septum was measured as 58.8 +/- 7.7 ms (N = 10). T(2) (*) for the ventricular septum was 31.6 +/- 5.8 ms (N = 10). The variation of mean T(2) or T(2) (*) within an region of interest improved significantly with increases in acquired echoes. Therefore, four or more echoes may provide for clear distinctions between regions of altered tissue composition within a subject. CONCLUSION: These results suggest that the 4-echo methods are best suited for measuring variations in transverse relaxation values in the mid-ventricular septum.

Measurement of longitudinal (T1) relaxation in the human lung at 3.0 Tesla with tissue-based and regional gradient analyses.

PURPOSE: The purpose of this study is to measure the longitudinal (T1) relaxation time of human lung parenchyma at 3.0 Tesla (T), independent of large vessel signal, and to examine T1 as a function of position in gravitational, isogravitational, and radial planes. MATERIALS AND METHODS: Sixteen subjects were imaged. A series of 16-20 turbo field echo images was acquired over a 6-s period after the application of a single nonselective inversion (180 degrees ) pulse. Tissue-based segmentation was used to separate parenchymal tissue from large pulmonary vascular tissue in the resulting images. Time-intensity curves for each tissue type were constructed and spin-lattice relaxation time was determined by line-fitting the time-intensity curves. The lung slice was divided into 10 regions of interest in the gravitational, isogravitational, and radial directions and regional T1 versus position gradient analyses were performed. RESULTS: The T1 relaxation time of human lung parenchyma at 3.0T was determined to be 1374 +/- 226 ms, while the T1 of blood in large pulmonary vessels was 1623 +/- 236 ms. Whole lung T1 was found to be 1397 +/- 214 ms. T1 of lung parenchyma was found to be significantly shorter than the T1 of blood in large pulmonary vessels and whole lung T1. No regional gradient was seen in the gravitational or isogravitational directions, but a significant gradient was seen in the radial direction.

Simulation study of susceptibility gradients leading to focal myocardial signal loss.

PURPOSE: To assess the cause of a "bite"-shaped signal void artifact often seen in 1.5 Tesla (T) and 3T gradient echo MR images in myocardium along the infero-apical border of the heart, MRI simulation was used to conduct experiments impossible in reality. Two previous studies attempting to explain the origin of this artifact came to different conclusions. One suggested deoxygenated blood in the posterior vein of the left ventricle (PVLV) leads to a susceptibility gradient that causes the artifact. The other suggested the difference in susceptibility between lung tissue and myocardium was responsible. This study assessed the relative effect of each possible cause. MATERIALS AND METHODS: Anthropometric phantoms were developed for use with a previously reported MRI simulator. The images were simulated at 3T with gradient echo scans using TE = 4 ms, TR = 25 ms, and theta = 25 degrees . RESULTS: The simulations indicate that both susceptibility differences can lead to signal losses in the area of the artifact with contributions from the PVLV being more localized while lung tissue effects are stronger but more spatially distributed. CONCLUSION: The data support the conclusion that both differences together, rather than one or the other, are responsible for the artifact.

Formative assessment in physiology teaching using a wireless classroom communication system.

Systems physiology, studied by biomedical engineers, is an analytical way to approach the homeostatic foundations of basic physiology. In many systems physiology courses, students attend lectures and are given homework and reading assignments to complete outside of class. The effectiveness of this traditional approach was compared with an approach in which a wireless classroom communication system was used to provide instant feedback on in-class learning activities and reading assignment quizzes. Homework was eliminated in this approach. The feedback system used stimulated 100% participation in class and facilitated rapid formative assessment. The results of this study indicate that learning of systems physiology concepts including physiology is at least, as if not more, effective when in-class quizzes and activities with instant feedback are used in place of traditional learning activities including homework. When results of this study are interpreted in light of possible effects of the September 11, 2001 terrorist attacks on student learning in the test group, it appears that the modified instruction may be more effective than the traditional instruction.

K-space in the clinic.

Magnetic resonance imaging (MRI) sequences are characterized by both radio frequency (RF) pulses and time-varying gradient magnetic fields. The RF pulses manipulate the alignment of the resonant nuclei and thereby generate a measurable signal. The gradient fields spatially encode the signals so that those arising from one location in an excited slice of tissue may be distinguished from those arising in another location. These signals are collected and mapped into an array called k-space that represents the spatial frequency content of the imaged object. Spatial frequencies indicate how rapidly an image feature changes over a given distance. It is the action of the gradient fields that determines where in the k-space array each data point is located, with the order in which k-space points are acquired being described by the k-space trajectory. How signals are mapped into k-space determines much of the spatial, temporal, and contrast resolution of the resulting images and scan duration. The objective of this article is to provide an understanding of k-space as is needed to better understand basic research in MRI and to make well-informed decisions about clinical protocols. Four major classes of trajectories-echo planar imaging (EPI), standard (non-EPI) rectilinear, radial, and spiral-are explained. Parallel imaging techniques SMASH (simultaneous acquisition of spatial harmonics) and SENSE (sensitivity encoding) are also described.