Titus Lanz

Practical approaches to the evaluation of signal‐to‐noise ratio performance with parallel imaging: Application with cardiac imaging and a 32‐channel cardiac coil

In this work, two practical methods for the measurement of signal‐to‐noise‐ratio (SNR) performance in parallel imaging are described. Phantoms and human studies were performed with a 32‐channel cardiac coil in the context of ultrafast cardiac CINE imaging at 1.5 T using steady‐state free precession (SSFP) and TSENSE. SNR and g‐factor phantom measurements using a “multiple acquisition” method were compared to measurements from a “difference method”. Excellent agreement was seen between the two methods, and the g‐factor shows qualitative agreement with theoretical predictions from the literature. Examples of high temporal (42.6 ms) and spatial (2.1 × 2.1 × 8 mm3) resolution cardiac CINE SSFP images acquired from human volunteers using TSENSE are shown for acceleration factors up to 7. Image quality agrees qualitatively with phantom SNR measurements, suggesting an optimum acceleration of 4. With this acceleration, a cardiac function study consisting of 6 image planes (3 short‐axis views, 3 long‐axis views) was obtained in an 18‐heartbeat breath‐hold. Magn Reson Med, 2005.

Serial cine-magnetic resonance imaging of left ventricular remodeling after myocardial infarction in rats.

The purpose of the present study was the serial investigation of morphological and functional changes after left coronary artery ligation in the intact rat using cine‐magnetic resonance imaging (MRI). MRI studies were performed 4, 8, 12, and 16 weeks after myocardial infarction (MI) with an echocardiogram (ECG)‐triggered cine‐fast low‐angle shot (FLASH)‐sequence in a 7‐Tesla magnet. MI‐size, left ventricular (LV) mass and volumes, cardiac index, ejection fraction (EF), and remote wall and scar thickness of 11 Wistar rats were compared to four sham‐operated rats. Stress MRI with dobutamine (10 μl/kg × minute) was performed at 16 weeks. In MI groups (small MI < 30%, N = 5, large MI > 30%, N = 6), there was significant increase of LV mass (small MI + 47.8% increase, large MI + 74.1%) and wall thickness (large MI 1.21 ± 0.03 to 1.84 ± 0.07 mm). Scar thickness declined from four to 16 weeks (large MI 0.92 ± 0.06 to 0.38 ± 0.02mm, P < 0.05). End‐diastolic volume of both MI groups was significantly elevated but increased further only in animals with large MI from four to 16 weeks (657.1 ± 38.6 to 869.7 ± 60.7 μL, P < 0.05). Compared to sham, EF was significantly depressed in MI (large MI 31.5 ± 2.0%). Wall thickening declined from four to 16 weeks post‐MI (large MI 50.9 ± 9.9 to 28.9 ± 4.4%, P < 0.05). During stress, sham and MI rats increased wall thickening from 66.5 ± 8.2 to 111.2 ± 6.7% and from 30.8 ± 4.3 to 47.5 ± 5.8%, respectively (P < 0.05). Hypertrophy was found in all animals with MI throughout the entire period of observation, whereas dilatation after four weeks was only detected in animals with large MI. These morphologic changes were accompanied by an early decline of EF; myocardial function characterized by wall thickening deteriorated later. J. Magn. Reson. Imaging 2001;14:547–555.

Cardiac CINE MR imaging with a 32-channel cardiac coil and parallel imaging: impact of acceleration factors on image quality and volumetric accuracy.

To assess the impact of parallel imaging algorithms on image quality and volumetric accuracy of CINE magnetic resonance imaging (MRI) with high temporal and spatial resolution using a new 32‐channel dedicated cardiac phased array coil.

In vivo time-resolved quantitative motion mapping of the murine myocardium with phase contrast MRI.

Myocardial motion of healthy mice and mice with myocardial infarction was assessed in vivo by phase contrast (PC) cine MRI. The imaging module was a segmented fast low angle shot (FLASH) sequence with velocity compensation in all three gradient directions. To accomplish additional motion encoding, the spin phase was prepared using bipolar gradient pulses, which resulted in a linear dependence between the voxel velocity and spin phase. This method provided accurate quantification of the velocity magnitude and direction of the murine myocardium at a spatial resolution of 234 μm and a temporal resolution of about 10 ms. The acquisition was EKG‐gated and the mice were anesthetized by inhalation of 1.5–4.0 vol.% isoflurane at 1.5 l/min oxygen flow. To validate the MRI measurements, an experiment with a calibrated rotating phantom was performed. Deviations between MR velocity measurements and optical assessment by a light detector were lower than 1.6%. During our study, myocardial motion velocities between 0.4 cm/s and 1.7 cm/s were determined for the healthy murine myocardium across the heart cycle. Areas with myocardial infarction were clearly segmented and showed a motion velocity which was significantly reduced. In conclusion, the method is an accurate technique for the assessment of murine myocardial motion in vivo. Magn Reson Med 49:315–321, 2003.

Fabrication of NMR - Microsensors for nanoliter sample volumes

Abstract The fabrication of micro-sensors for NMR-spectroscopy on both glass and GaAs is presented. Planar coils with inner diameter from 50 μm to 400 μm including a coplanar wave-guide leading to the bonding pads were combined with a chamber for liquid samples of 200–500 μm diameter on the backside of the substrate. The microcoil served as a receiver in a 1 H-NMR experiment at 11T (500 MHz). In initial experiments, the spectrum of 60 nl-volumes of pure silicone-oil were detected by the microcoil.

7 Tesla MR imaging of the human eye in vivo

To develop a protocol which optimizes contrast, resolution and scan time for three‐dimensional (3D) imaging of the human eye in vivo using a 7 Tesla (T) scanner and custom radio frequency (RF) coil.

Ultra-fast and accurate assessment of cardiac function in rats using accelerated MRI at 9.4 Tesla

MRI can accurately and reproducibly assess cardiac function in rodents but requires relatively long imaging times. Therefore, parallel imaging techniques using a 4‐element RF‐coil array and MR sequences for cardiac MRI in rats were implemented at ultra‐high magnetic fields (9.4 Tesla [T]). The hypothesis that these developments would result in a major reduction in imaging time without loss of accuracy was tested on female Wistar rats under isoflurane anesthesia. High‐resolution, contiguous short‐axis slices (thickness 1.5 mm) were acquired covering the entire heart. Two interleaved data sets (i) with the volume coil (eight averages) and (ii) with the four‐element coil array (one average) were obtained. In addition, two‐, three‐, and fourfold accelerated data sets were generated through postprocessing of the coil array data, followed by a TGRAPPA reconstruction, resulting in five data sets per rat (in‐plane voxel size 100 × 100 μm). Using a single blinded operator, excellent agreement was obtained between volume coil (acquisition time: 88 min) and the fourfold accelerated (<3 min) data sets (e.g., LV mass 436 ± 21 mg vs 433 ± 19 mg; ejection fraction 74 ± 5% vs 75 ± 4%). This finding demonstrates that it is possible to complete a rat cine‐MRI study under 3 min with low variability and without losing temporal or spatial resolution, making high throughput screening programs feasible. Magn Reson Med 59:636–641, 2008.

Preamplified planar microcoil on GaAs substrates for microspectroscopy

In the course of the publication the design, fabrication, and experimental evaluation of a planar microcoil for nuclear magnetic resonance (NMR) spectroscopy with a 360 μm inner diameter integrated to a low-noise metal–semiconductor field-effect transistor is presented. The impedance matching of the coil and transistor was achieved by adjusted coil geometry, while an appropriate microfabrication process including air bridge contacts and a multilayer metal stack was developed for the necessary susceptibility matching of the coil metallization. The NMR spectra of experiments in an 11.75 T magnet (corresponding to 500 MHz) show the amplification of the signal by the integrated transistor. Due to previous experiments, we wanted to ensure that the whole signal emerged from the sensitive volume of the microcoil.

A flexible 32-channel receive array combined with a homogeneous transmit coil for human lung imaging with hyperpolarized 3He at 1.5 T.

Parallel imaging presents a promising approach for MRI of hyperpolarized nuclei, as the penalty in signal‐to‐noise ratio typically encountered with 1H MRI due to a reduction in acquisition time can be offset by an increase in flip angle. The signal‐to‐noise ratio of hyperpolarized MRI generally exhibits a strong dependence on flip angle, which makes a homogeneous B1+ transmit field desirable. This paper presents a flexible 32‐channel receive array for 3He human lung imaging at 1.5T designed for insertion into an asymmetric birdcage transmit coil. While the 32‐channel array allows parallel imaging at high acceleration factors, the birdcage transmit coil provides a homogeneous B1+ field. Decoupling between array elements is achieved by using a concentric shielding approach together with preamplifier decoupling. Coupling between transmit coil and array elements is low by virtue of a low geometric coupling coefficient, which is reduced further by the concentric shields in the array. The combination of the 32‐channel array and birdcage transmit coil provides 3He ventilation images of excellent quality with similar signal‐to‐noise ratio at acceleration factors R = 2 and R = 4, while maintaining a homogeneous B1+. Magn Reson Med, 2011.

Half‐fourier‐acquisition single‐shot turbo spin‐echo (HASTE) MRI of the lung at 3 Tesla using parallel imaging with 32‐receiver channel technology

To assess the feasibility of half‐Fourier‐acquisition single‐shot turbo spin‐echo (HASTE) of the lung at 3 Tesla (T) using parallel imaging with a prototype of a 32‐channel torso array coil, and to determine the optimum acceleration factor for the delineation of intrapulmonary anatomy.