We report that visual stimulation produces an easily detectable (5-20%) transient increase in the intensity of water proton magnetic resonance signals in human primary visual cortex in gradient echo images at 4-T magnetic-field strength. The observed changes predominantly occur in areas containing gray matter and can be used to produce highspatial-resolution functional brain maps in humans. Reducing the image-acquisition echo time from 40 msec to 8 msec reduces the amplitude of the fractional signal change, suggesting that it is produced by a change in apparent transverse relaxation time T2. The amplitude, sign, and echo-time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral blood flow and concomitantly increases venous-blood oxygenation.Magnetic-resonance imaging (MRI) of rodent brains at high (7-T) magnetic-field strength shows proton signal-intensity alterations related to blood oxygenation in regions close to local blood vessels (1-3). We have termed this phenomenon blood oxygenation-level-dependent (BOLD) contrast and have demonstrated that the underlying mechanism is a magnetic-susceptibility variation caused by deoxyhemoglobin, an endogenous paramagnetic contrast agent. It was further demonstrated that this magnetic-susceptibility effect could be used to measure in vivo changes in hemodynamics. For example, pharmacologically induced changes in cerebral blood flow and oxygen utilization produce measurable changes in BOLD contrast in the rat cerebral cortex. Similar results have recently been demonstrated in cat brain (4).There is increased evidence that a local elevation in human-brain venous-blood oxygenation accompanies an increase in neuronal activity (5-8). For example, positron emission tomography imaging experiments demonstrate stimulation-produced increases in regional cerebral blood flow without significantly changing local oxygen use, thus predicting an elevation in venous-blood oxygenation (6, 7). This result suggested that BOLD contrast imaging could be used to map human mental operations. To examine whether detectable intrinsic magnetic-susceptibility changes are produced in the human brain in response to neuronal activation, we studied the effect of visual stimulation on gradient echo images of human visual cortex acquired at high-magneticfield strength. In general, high-field strength increases the magnitude of susceptibility contrast effects, accentuating BOLD contrast. MATERIALS AND METHODSMRI experiments were done with a 4-T whole-body imaging system with actively shielded gradient coils [Sisco (Sunnyvale, CA)/Siemens (Erlangen, F.R.G.)]. Approval for these human experiments was obtained from the institutional review board of the University of Minnesota Medical School. Radiofrequency power deposition was kept two orders of magnitude below Food and Drug Administration specificabsorption rate guidelines. A snugly fitted head holder with a curved-surface radiofrequency coil (14 cm in diameter) was used to limit...

It recently has been demonstrated that magnetic resonance imaging can be used to map changes in brain hemodynamics produced by human mental operations. One method under development relies on blood oxygenation level-dependent (BOLD) contrast: a change in the signal strength of brain water protons produced by the paramagnetic effects of venous blood deoxyhemoglobin. Here we discuss the basic quantitative features of the observed BOLD-based signal changes, including the signal amplitude and its magnetic field dependence and dynamic effects such as a pronounced oscillatory pattern that is induced in the signal from primary visual cortex during photic stimulation experiments. The observed features are compared with the results of Monte Carlo simulations of water proton intravoxel phase dispersion produced by local field gradients generated by paramagnetic deoxyhemoglobin in nearby venous blood vessels. The simulations suggest that the effect of water molecule diffusion is strong for the case of blood capillaries, but, for larger venous blood vessels, water diffusion is not an important determinant of deoxyhemoglobin-induced signal dephasing. We provide an expression for the apparent in-plane relaxation rate constant (R2*) in terms of the main magnetic field strength, the degree of the oxygenation of the venous blood, the venous blood volume fraction in the tissue, and the size of the blood vessel.

A hemispheric asymmetry in the functional activation of the human motor cortex during contralateral (C) and ipsilateral (I) finger movements, especially in right-handed subjects, was documented with nuclear magnetic resonance imaging at high field strength (4 tesla). Whereas the right motor cortex was activated mostly during contralateral finger movements in both right-handed (C/I mean area of activation = 36.8) and left-handed (C/I = 29.9) subjects, the left motor cortex was activated substantially during ipsilateral movements in left-handed subjects (C/I = 5.4) and even more so in right-handed subjects (C/I = 1.3).

Purpose To develop a strategy for training a physics‐guided MRI reconstruction neural network without a database of fully sampled data sets. Methods Self‐supervised learning via data undersampling (SSDU) for physics‐guided deep learning reconstruction partitions available measurements into two disjoint sets, one of which is used in the data consistency (DC) units in the unrolled network and the other is used to define the loss for training. The proposed training without fully sampled data is compared with fully supervised training with ground‐truth data, as well as conventional compressed‐sensing and parallel imaging methods using the publicly available fastMRI knee database. The same physics‐guided neural network is used for both proposed SSDU and supervised training. The SSDU training is also applied to prospectively two‐fold accelerated high‐resolution brain data sets at different acceleration rates, and compared with parallel imaging. Results Results on five different knee sequences at an acceleration rate of 4 shows that the proposed self‐supervised approach performs closely with supervised learning, while significantly outperforming conventional compressed‐sensing and parallel imaging, as characterized by quantitative metrics and a clinical reader study. The results on prospectively subsampled brain data sets, in which supervised learning cannot be used due to lack of ground‐truth reference, show that the proposed self‐supervised approach successfully performs reconstruction at high acceleration rates (4, 6, and 8). Image readings indicate improved visual reconstruction quality with the proposed approach compared with parallel imaging at acquisition acceleration. Conclusion The proposed SSDU approach allows training of physics‐guided deep learning MRI reconstruction without fully sampled data, while achieving comparable results with supervised deep learning MRI trained on fully sampled data.

The sensitivity of contrast-enhanced MR first pass perfusion imaging in detection and quantification of hypoperfused myocardium was evaluated using an instrumented, closed-chest dog model where graded regional hypoperfusion was induced by applying predetermined levels of stenosis to the left anterior descending artery (LAD). All measurements were performed at rest and under stress induced by dipyridamole (DIP). Myocardial perfusion was assessed both with MR and radiolabeled microspheres injected immediately before the administration of the MR contrast agent. Ultrafast MR imaging was performed using a Turbo FLASH sequence with a 180 degrees inversion prepulse. A Gd-DTPA bolus was injected into the left atrium and T1-weighted images were acquired with every heart beat. Signal intensity measured from the images in regions of the LAD and left circumflex (LCx) perfusion beds was plotted against time to generate signal intensity versus time curves (SI time curve). Various flow indices were derived according to the indicator dilution theory, and compared with and without volume correction due to vasodilation to the myocardial blood flow (MBF) calculated from radiolabeled microspheres. Correlation of the MR and MBF data demonstrated that different transmural and regional myocardial perfusion levels can be easily visualized in the perfusion images and accurately monitored by the SI time curves. Detection of the impairment of myocardial perfusion improved significantly after administration of DIP. The inverse mean transit time calculated from the SI time curve was found to yield a linear correlation to absolute MBF derived from the microsphere data. These results suggest that with intracardiac injections of exogenous contrast agent, myocardial perfusion can be assessed parametrically with first pass contrast enhanced ultrafast MRI.

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We report the use of high-speed magetic resonance imaging to follow the changes in image intens in the human visual cortex during stimulation by a flashing checkerboard stimulus. Measurements were made in a 2.1-T, 1-m-diameter magnet, part of a Bruker Biospec spectrometer that we had programmed to do echo-planar imaging. A 15-cm-diameter surface coil was used to transmit and receive signals. Images were acquired during periods of stimulation from 2 s to 180 s. Images were acquired in 65.5 ms in a 10-mm slice with in-plane voxel size of 6 x 3 mm. Repetition time (TR) was generally 2 s, although for the long flashing periods, TR = 8 s was used. Voxels were located onto an inversion recovery image taken with 2 x 2 mm in-plane resolution. Image intensity increased after onset ofthe stimulus. The mean change in signal relative to the prestimulation level (AS/S) was 9.7% (SD = 2.8%, n = 20) with an echo time of 70 ms. Irrespective of the period of stimulation, the increase in magnetic resonance signal intensity was delayed relative to the stimulus. The mean delay measured from the start of stimulation for each protocol was as follows: 2-s stimulation, delay = 3.5 s (SD = 0.5 s, n = 10) (the delay exceeds stimulus duration); 20-to 24-s stimulation, delay = 5 s (SD = 2 s, n = 20).Functional mapping of the brain by nuclear magnetic resonance (NMR) methods has recently been demonstrated by two techniques. The first method uses a bolus of paramagnetic contrast agent (gadolinium/diethylenetriaminepentaacetic acid) as a tracer for blood volume (1). The agent is injected into the arm and produces variations in local blood volume magnetic susceptibility as the bolus passes through the capillary bed of the brain. The changes in susceptibility are monitored by using a transverse relaxation time (T2)-weighted, fast magnetic resonance-imaging sequence, such as echo-planar imaging (EPI) (2) or fast low-angle shot (FLASH) (3). This method is subject to the same assumptions as all kinetic tracer methods (4), but within these restrictions it gives a quantitative result. With this method Belliveau et al. (5) have made measurements in the human visual cortex and demonstrated an increase in blood volume during stimulation. The main drawback of the technique is the need for at least two injections of contrast agent to measure the resting and activated brain states.The second method relies solely on the paramagnetic effects of hemoglobin. In the oxygenated state (HbO2), the molecule is diamagnetic, but once dissociated from oxygen (deoxyhemoglobin; Hb), it becomes paramagnetic. Thus, changes in Hb concentration will also result in magnetic susceptibility changes in the capillaries. Complications may arise because the paramagnetic agent is not evenly distributed throughout the blood but is compartmentalized inside the erythrocytes. Ogawa et al. (6) have studied the effect of inhaled 02 content on the intensity of gradient-echo images ofrat brain at 7 T. They have shown that a decrease in oxygen content produces dark streaks in the i...

Background Osteochondrosis (OC) is a common developmental orthopedic disease affecting both humans and animals. Despite increasing recognition of this disease among children and adolescents, its pathogenesis is incompletely understood because clinical signs are often not apparent until lesions have progressed to end-stage, and examination of cadaveric early lesions is not feasible. In contrast, both naturally-occurring and surgically-induced animal models of disease have been extensively studied, most notably in horses and swine, species in which OC is recognized to have profound health and economic implications. The potential for a translational model of human OC has not been recognized in the existing human literature. Objective The purpose of this review is to highlight the similarities in signalment, predilection sites and clinical presentation of naturally-occurring OC in humans and animals and to propose a common pathogenesis for this condition across species. Study Design Review Methods The published human and veterinary literature for the various manifestations of OC was reviewed. Peer-reviewed original scientific articles and species-specific review articles accessible in PubMed (US National Library of Medicine) were eligible for inclusion. Results A broad range of similarities exists between OC affecting humans and animals, including predilection sites, clinical presentation, radiographic/MRI changes, and histological appearance of the end stage lesion, suggesting a shared pathogenesis across species. Conclusion This proposed shared pathogenesis for OC between species implies that naturally-occurring and surgically-induced models of OC in animals may be useful in determining risk factors and for testing new diagnostic and therapeutic interventions that can be used in humans.