Relationship between cardiac diffusion tensor imaging parameters and anthropometrics in healthy volunteers.

BACKGROUND: In vivo cardiac diffusion tensor imaging (cDTI) is uniquely capable of interrogating laminar myocardial dynamics non-invasively. A comprehensive dataset of quantative parameters and comparison with subject anthropometrics is required. METHODS: cDTI was performed at 3T with a diffusion weighted STEAM sequence. Data was acquired from the mid left ventricle in 43 subjects during the systolic and diastolic pauses. Global and regional values were determined for fractional anisotropy (FA), mean diffusivity (MD), helix angle gradient (HAg, degrees/%depth) and the secondary eigenvector angulation (E2A). Regression analysis was performed between global values and subject anthropometrics. RESULTS: All cDTI parameters displayed regional heterogeneity. The RR interval had a significant, but clinically small effect on systolic values for FA, HAg and E2A. Male sex and increasing left ventricular end diastolic volume were associated with increased systolic HAg. Diastolic HAg and systolic E2A were both directly related to left ventricular mass and body surface area. There was an inverse relationship between E2A mobility and both age and ejection fraction. CONCLUSIONS: Future interpretations of quantitative cDTI data should take into account anthropometric variations observed with patient age, body surface area and left ventricular measurements. Further work determining the impact of technical factors such as strain and SNR is required.

Parallel transmit excitation at 1.5 T based on the minimization of a driving function for device heating.

PURPOSE: To provide a rapid method to reduce the radiofrequency (RF) E-field coupling and consequent heating in long conductors in an interventional MRI (iMRI) setup. METHODS: A driving function for device heating (W) was defined as the integration of the E-field along the direction of the wire and calculated through a quasistatic approximation. Based on this function, the phases of four independently controlled transmit channels were dynamically changed in a 1.5 T MRI scanner. During the different excitation configurations, the RF induced heating in a nitinol wire immersed in a saline phantom was measured by fiber-optic temperature sensing. Additionally, a minimization of W as a function of phase and amplitude values of the different channels and constrained by the homogeneity of the RF excitation field (B1) over a region of interest was proposed and its results tested on the benchtop. To analyze the validity of the proposed method, using a model of the array and phantom setup tested in the scanner, RF fields and SAR maps were calculated through finite-difference time-domain (FDTD) simulations. In addition to phantom experiments, RF induced heating of an active guidewire inserted in a swine was also evaluated. RESULTS: In the phantom experiment, heating at the tip of the device was reduced by 92% when replacing the body coil by an optimized parallel transmit excitation with same nominal flip angle. In the benchtop, up to 90% heating reduction was measured when implementing the constrained minimization algorithm with the additional degree of freedom given by independent amplitude control. The computation of the optimum phase and amplitude values was executed in just 12 s using a standard CPU. The results of the FDTD simulations showed similar trend of the local SAR at the tip of the wire and measured temperature as well as to a quadratic function of W, confirming the validity of the quasistatic approach for the presented problem at 64 MHz. Imaging and heating reduction of the guidewire were successfully performed in vivo with the proposed hardware and phase control. CONCLUSIONS: Phantom and in vivo data demonstrated that additional degrees of freedom in a parallel transmission system can be used to control RF induced heating in long conductors. A novel constrained optimization approach to reduce device heating was also presented that can be run in just few seconds and therefore could be added to an iMRI protocol to improve RF safety.

A novel method for quantitative monitoring of transplanted islets of langerhans by positive contrast magnetic resonance imaging.

The Automatic Quantitative Ultrashort Echo Time imaging (AQUTE) protocol for serial MRI allows quantitative in vivo monitoring of iron labeled pancreatic islets of Langerhans transplanted into the liver, quantifying graft implantation and persistence in a rodent model. Rats (n = 14), transplanted with iron oxide loaded cells (0-4000 islet equivalents, IEQ), were imaged using a 3D radial ultrashort echo time difference technique (dUTE) on a Siemens MAGNETOM 3T clinical scanner up to 5 months postsurgery. In vivo 3D dUTE images gave positive contrast from labeled cells, suppressing liver signal and small vessels, allowing automatic quantification. Position of labeled islet clusters was consistent over time and quantification of hyperintense pixels correlated with the number of injected IEQs (R² = 0.898, p < 0.0001), and showed persistence over time (5 months posttransplantation). Automatic quantification was superior to standard imaging and manual counting methods, due to the uniform suppressed background and high contrast, resulting in significant timesavings, reproducibility and ease of quantification. Three-dimensional coverage of the whole liver in the absence of cardiac/respiratory artifact provided further improvement over conventional imaging. This imaging protocol reliably quantifies transplanted islet mass and has high translational potential to clinical studies of transplanted pancreatic islets.

Muscle Na+ channelopathies: MRI detects intracellular 23Na accumulation during episodic weakness.

BACKGROUND: Muscle channelopathies such as paramyotonia, hyperkalemic periodic paralysis, and potassium-aggravated myotonia are caused by gain-of-function Na+ channel mutations. METHODS: Methods: Implementation of a three-dimensional radial 23Na magnetic resonance (MR) sequence with ultra-short echo times allowed the authors to quantify changes in the total muscular 23Na signal intensity. By this technique and T2-weighted 1H MRI, the authors studied whether the affected muscles take up Na+ and water during episodes of myotonic stiffness or of cold- or exercise-induced weakness. RESULTS: A 22% increase in the 23Na signal intensity and edema-like changes on T2-weighted 1H MR images were associated with cold-induced weakness in all 10 paramyotonia patients; signal increase and weakness disappeared within 1 day. A 10% increase in 23Na, but no increase in the T2-weighted 1H signal, occurred during cold- or exercise-induced weakness in seven hyperkalemic periodic paralysis patients, and no MR changes were observed in controls or exercise-induced stiffness in six potassium-aggravated myotonia patients. Measurements on native muscle fibers revealed provocation-induced, intracellular Na+ accumulation and membrane depolarization by -41 mV for paramyotonia, by -30 mV for hyperkalemic periodic paralysis, and by -20 mV for potassium-aggravated myotonia. The combined in vivo and in vitro approach showed a close correlation between the increase in 23Na MR signal intensity and the membrane depolarization (r = 0.92). CONCLUSIONS: The increase in the total 23Na signal intensity reflects intracellular changes, the cold-induced Na+ shifts are greatest and osmotically relevant in paramyotonia patients, and even osmotically irrelevant Na+ shifts can be detected by the implemented 23Na MR technique.

ECG-gated 23Na-MRI of the human heart using a 3D-radial projection technique with ultra-short echo times.

Pathological changes in tissue often manifest themselves in an altered sodium gradient between intra- and extracellular space due to a malfunctioning Na+-K+ pump, resulting in an increase in total sodium concentration in ischaemic regions. Therefore, 23Na-MRI has the potential to non-invasively differentiate viable from non-viable tissue by detecting concentration changes of intra- and extracellular sodium. As the in vivo sodium signal shows a bi-exponential T2 decay, with a short component of less than 1 ms, the accurate quantification of the total sodium content requires imaging techniques with ultra-short echo times (TE) below 0.5 ms. A 3D-radial projection technique has been developed which allows the acquisition of ECG-triggered sodium images of the human heart with a TE of 0.4 ms. With this pulse sequence 23Na-MRI volunteer measurements of the head or the heart were performed in less than 18 min on a 1.5-T clinical scanner with an isotropic resolution of 10 mm3. The signal to noise ratio of the radial projection technique is twofold higher than that of a Cartesian gradient echo pulse sequence (TE = 3.2 ms). Radial 23Na-MRI provides a tool for clinical studies, aiming at the differentiation of viable and non-viable tissue.