Mirtech (United States)

In the US, 1.4 million people have implanted ICDs for reducing the risk of sudden death due to ventricular arrhythmias. Cardiac MRI (cMR) is of particular interest in the ICD patient population as cMR is the optimal imaging modality for distinguishing cardiac conditions that predispose to sudden death, and it is the best method to plan and guide therapy. However, all ICDs contain a ferromagnetic transformer which imposes a large inhomogeneous magnetic field in sections of the heart, creating large image voids that can mask important pathology. A shim system was devised to resolve these ICD issues. A shim coil system (CSS) that corrects ICD artifacts over a user-selected Region-of-Interest (ROI), was constructed and validated. A shim coil was constructed that can project a large magnetic field for distances of ~15 cm. The shim-coil can be positioned safely anywhere within the scanner bore. The CSS includes a cantilevered beam to hold the shim coil. Remotely controlled MR-conditional motors allow 2 mm-accuracy three-dimensional shim-coil position. The shim coil is located above the subjects and the imaging surface-coils. Interaction of the shim coil with the scanner's gradients was eliminated with an amplifier that is in a constant current mode. Coupling with the scanners' radio-frequency (rf) coils, was reduced with shielding, low-pass filters, and cable shield traps. Software, which utilizes magnetic field (B0) mapping of the ICD inhomogeneity, computes the optimal location for the shim coil and its corrective current. ECG gated single- and multiple-cardiac-phase 2D GRE and SSFP sequences, as well as 3D ECG-gated respiratory-navigated IR-GRE (LGE) sequences were tested in phantoms and N = 3 swine with overlaid ICDs. With all cMR sequences, the system reduced artifacts from >100 ppm to <25 ppm inhomogeneity, which permitted imaging of the entire left ventricle in swine with ICD-related voids. Continuously acquired Gradient recalled echo or Steady State Free Precession images were used to interactively adjust the shim current and coil location. The shim system reduced large field inhomogeneities due to implanted ICDs and corrected most ICD-related image distortions. Externally-controlled motorized translation of the shim coil simplified its utilization, supporting an efficient cardiac MRI workflow.

In this work, we report initial results from a light-weight, low field magnetic resonance device designed to make relative pulmonary density measurements at the bedside. The development of this device necessarily involves special considerations for the magnet, RF and data acquisition schemes as well as a careful analysis of what is needed to provide useful information in the ICU. A homogeneous field region is created remotely from the surface of the magnet such that when the magnet is placed against the chest, an NMR signal is measured from a small volume in the lung. In order to achieve portability, one must trade off field strength and therefore spatial resolution. We report initial measurements from a ping-pong ball size region in the lung as a function of lung volume. As expected, we measured decreased signal at larger lung volumes since lung density decreases with increasing lung volume. Using a CPMG sequence with ΔTE=3.5 ms and a 20 echo train, a signal to noise ratio ~1100 was obtained from an 8.8mT planar magnet after signal averaging for 43 s. This is the first demonstration of NMR measurements made on a human lung with a light-weight planar NMR device. We argue that very low spatial resolution measurements of different lobar lung regions will provide useful diagnostic information for clinicians treating Acute Respiratory Distress Syndrome as clinicians want to avoid ventilator pressures that cause either lung over distension (too much pressure) or lung collapse (too little pressure).

Obesity has become an epidemic in children, associated with an increase in insulin resistance and metabolic dysfunction. Mitochondrial function is known to be an important determinant of glucose metabolism in adults. However, little is known about the relationship between mitochondrial function and obesity, insulin resistance, energy expenditure, and pubertal development in children. Seventy-four participants, 37 overweight (> or = 85th percentile body mass index for age and sex) and 37 normal-weight (< 85th percentile) without personal or family history of diabetes mellitus were enrolled. Subjects were evaluated with an oral glucose tolerance test, metabolic markers, resting energy expenditure, Tanner staging, and (31)P magnetic resonance spectroscopy of skeletal muscle for mitochondrial function. Overweight and normal-weight children showed no difference in muscle ATP synthesis [phosphocreatine (PCr) recovery after exercise] (32.4 +/- 2.3 vs. 34.1 +/- 2.1, P = 0.58). However, insulin-resistant children had significantly prolonged PCr recovery when compared with insulin-sensitive children, by homeostasis model assessment for insulin resistance quartile (ANOVA, P = 0.04). Similarly, insulin-resistant overweight children had PCr recovery that was prolonged compared with insulin-sensitive overweight children (P = 0.01). PCr recovery was negatively correlated with resting energy expenditure in multivariate modeling (P = 0.03). Mitochondrial function worsened during mid-puberty in association with insulin resistance. Reduced skeletal muscle mitochondrial oxidative phosphorylation, assessed by PCr recovery, is associated with insulin resistance and an altered metabolic phenotype in children. Normal mitochondrial function may be associated with a healthier metabolic phenotype in overweight children. Further studies are needed to investigate the long-term physiological consequences and potential treatment strategies targeting children with reduced mitochondrial function.

Gas imaging has opened the new field of direct imaging of pulmonary ventilation by MRI. The use of hyperpolarised 3He gas for MRI of the lung has been pioneered by a number of groups worldwide. Due to the enormous progress in the fields of hyperpolarisation technology, administration of hyperpolarised 3He, MR hardware, and MR pulse sequences significant progress has been made and the translation into the clinical arena has been accomplished. This chapter gives an overview of the technical methods for HP 3He MRI for human lung imaging, focusing on gas polarisation methods, background physics, MRI hardware considerations, MRI pulse sequence considerations, safety considerations for imaging inhaled 3He, and pulse sequence design for probing lung physiology and anatomy. Where possible the methods will be highlighted with reference to the literature and illustrated with clinical examples of images from the author’s home group. For further discussion on the growing clinical applications the reader is referred to Chaps. 8, 9 and 10. Hyperpolarized xenon-129 (129Xe) has enormous potential to provide noninvasive functional information about the lung. In particular, because inhaled xenon follows the same pathway as oxygen, diffusing from alveolar gas spaces to septal tissue and blood, gas exchange parameters can be measured. For example, by measuring the time-dependent septal uptake of 129Xe, information about alveolar surface area, septal thickness and vascular transit times can be obtained. A principal obstacle to the development and application of this technology to humans has been the lack of a polarizer that can provide sufficient quantities of highly polarized 129Xe gas. This problem, however, has recently been solved. To obtain quantitative measures of pulmonary function with 129Xe, two methods have been studied. One is a direct method that measures the magnetic resonance signal from the gas spaces and from the different septal tissue compartments. In principal this is straightforward since the signal from each compartment can be distinguished due to a unique chemical shift frequency. In practice, however, there are limitations because of the very small signal available to measure in the septal tissue. Thus, the direct measurement of 129Xe interphase diffusion in humans has been principally confined to whole lung measurements. To obtain regional maps of 129Xe interphase diffusion, the Xenon Transfer Contrast (XTC) method has been utilized. XTC is an indirect method that measures the attenuation in the gas phase magnetization due to interphase diffusion between the gas phase and septal tissue compartments. Using XTC, regional maps of interphase diffusion in humans has been demonstrated. Fluorine MRI of the lung is an interesting new approach that may have the potential for broader use than MRI based on hyperpolarized gases like He-3 or Xe-129. Although in general the image quality is worse in fluorine MRI than that obtained with hyperpolarized gases, the latter approach has the advantage of very simple requirements: only an MRI system with non-proton imaging capabilities and a dedicated fluorine-19 MRI coil are required. Fluorinated gases do not need complex treatment before use – this makes their application less demanding on the local infrastructure and, potentially, may also reduce costs. However, currently, the most significant drawback of these gases is that they have not yet been approved for human application. The direct visual assessment of the lung parenchyma and imaging of lung ventilation using proton MRI is considerably more difficult than MRI of most other organs due to the very low signal intensity of the lung parenchyma. The low signal intensity is caused by the low average proton density and the short T2* relaxation time of lung tissue. Several methods for proton-MRI-based ventilation measurements have been proposed in order to overcome these difficulties. Currently the most established technique is oxygen-enhanced MRI of the lung, employing inhaled molecular oxygen as a T1-reducing contrast agent, which enhances the signal of the protons in the lung. The clinical application of oxygen-enhanced lung MRI has been assessed in several studies. Main advantages of oxygen- enhanced MRI are the general availability of oxygen and the relative safety of oxygen administration. Potential limitations of oxygen-enhanced lung MRI are the relatively low signal enhancement corresponding to a T1 reduction of about 10 %, and the complex contrast mechanism with contributions due to ventilation, perfusion, and oxygen-diffusion properties of the lung. Newer techniques based on non-enhanced dynamic MR acquisitions appear to be a promising tool for ventilation assessment that may be available in the near future. Other proposed techniques such as imaging after administration of aerosolized gadolinium contrast agents or after infusion of water- in-perfluorocarbon emulsions into the lung require still considerably more research before they might become applicable in clinical MR imaging.

We have proposed a two-dimensional PERiodic-Linear (PERL) magnetic encoding field geometry B(x,y) = g(y)y cos(q(x)x) and a magnetic resonance imaging pulse sequence which incorporates two fields to image a two-dimensional spin density: a standard linear gradient in the x dimension, and the PERL field. Because of its periodicity, the PERL field produces a signal where the phase of the two dimensions is functionally different. The x dimension is encoded linearly, but the y dimension appears as the argument of a sinusoidal phase term. Thus, the time-domain signal and image spin density are not related by a two-dimensional Fourier transform. They are related by a one-dimensional Fourier transform in the x dimension and a new Bessel function integral transform (the PERL transform) in the y dimension. The inverse of the PERL transform provides a reconstruction algorithm for the y dimension of the spin density from the signal space. To date, the inverse transform has been computed numerically by a Bessel function expansion over its basis functions. This numerical solution used a finite sum to approximate an infinite summation and thus introduced a truncation error. This work analytically determines the basis functions for the PERL transform and incorporates them into the reconstruction algorithm. The improved algorithm is demonstrated by (1) direct comparison between the numerically and analytically computed basis functions, and (2) reconstruction of a known spin density. The new solution for the basis functions also lends proof of the system function for the PERL transform under specific conditions.