Body MRI System Research Articles

Bi-ventricular assessment with cardiovascular magnetic resonance at 5 Tesla: A pilot study.

BackgroundCardiovascular magnetic resonance (CMR) imaging at ultra-high fields (UHF) such as 7T has encountered many challenges such as faster T relaxation, stronger B0 and B1+ field inhomogeneities and additional safety concerns due to increased specific absorption rate (SAR) and peripheral nervous stimulation (PNS). Recently, a new line of 5T whole body MRI system has become available, and this study aims at evaluating the performance and benefits of this new UHF system for CMR imaging.MethodsGradient echo (GRE) CINE imaging was performed on healthy volunteers at both 5 and 3T, and was compared to balanced steady-state-free-procession (bSSFP) CINE imaging at 3T as reference. Higher spatial resolution GRE CINE scans were additionally performed at 5T. All scans at both fields were performed with ECG-gating and breath-holding. Image quality was blindly evaluated by two radiologists, and the cardiac functional parameters (e.g., EDV/ESV/mass/EF) of the left and right ventricles were measured for statistical analyses using the Wilcoxon signed-rank test and Bland-Altman analysis.ResultsCompared to 3T GRE CINE imaging, 5T GRE CINE imaging achieved comparable or improved image quality with significantly superior SNR and CNR, and it has also demonstrated excellent capability for high resolution (1.0 × 1.0 × 6.0 mm3) imaging. Functional assessments from 5T GRE CINE images were highly similar with the 3T bSSFP CINE reference.ConclusionsThis pilot study has presented the initial evaluation of CMR CINE imaging at 5T UHF, which yielded superior image quality and accurate functional quantification when compared to 3T counterparts. Along with reliable ECG gating, the new 5T UHF system has the potential to achieve well-balanced performance for CMR applications.

Evidence for regional hippocampal damage in patients with schizophrenia.

Schizophrenia patients show cognitive and mood impairments, including memory loss and depression, suggesting damage in the brain regions. The hippocampus is a brain structure that is significantly involved in memory and mood function and shows impairment in schizophrenia. In the present study, we examined the regional hippocampal changes in schizophrenia patients using voxel-based morphometry (VBM), Freesurfer, and proton magnetic resonance spectroscopy (1H MRS) procedures. 1H MRS and high-resolution T1-weighted magnetic resonance imaging were collected in both healthy control subjects (N = 28) and schizophrenia patients (N = 28) using 3-Tesla whole body MRI system. Regional hippocampal volume was analyzed using VBM and Freesufer procedures. The relative ratios of the neurometabolites were calculated using linear combination model (LCModel). Compared to controls, schizophrenia patients showed significantly decreased gray matter volume in the hippocampus. Schizophrenia patients also showed significantly reduced glutamate (Glu) and myo-inositol (mI) ratios in the hippocampus. Additionally, significant positive correlation between gray matter volume and Glu/tCr was also observed in the hippocampus in schizophrenia. Our findings provide an evidence for a possible association between structural deficits and metabolic alterations in schizophrenia patients.

Simultaneous PET/MR imaging with a radio frequency-penetrable PET insert.

A brain sized radio frequency (RF)-penetrable PET insert has been designed for simultaneous operation with MRI systems. This system takes advantage of electro-optical coupling and battery power to electrically float the PET insert relative to the MRI ground, permitting RF signals to be transmitted through small gaps between the modules that form the PET ring. This design facilitates the use of the built-in body coil for RF transmission and thus could be inserted into any existing MR site wishing to achieve simultaneous PET/MR imaging. The PET detectors employ nonmagnetic silicon photomultipliers in conjunction with a compressed sensing signal multiplexing scheme, and optical fibers to transmit analog PET detector signals out of the MRI room for decoding, processing, and image reconstruction. The PET insert was first constructed and tested in a laboratory benchtop setting, where tomographic images of a custom resolution phantom were successfully acquired. The PET insert was then placed within a 3T body MRI system, and tomographic resolution/contrast phantom images were acquired both with only the B0 field present, and under continuous pulsing from different MR imaging sequences. The resulting PET images have comparable contrast-to-noise ratios (CNR) under all MR pulsing conditions: The maximum percent CNR relative difference for each rod type among all four PET images acquired in the MRI system has a mean of 14.0±7.7%. MR images were successfully acquired through the RF-penetrable PET shielding using only the built-in MR body coil, suggesting that simultaneous imaging is possible without significant mutual interference. These results show promise for this technology as an alternative to costly integrated PET/MR scanners; a PET insert that is compatible with any existing clinical MRI system could greatly increase the availability, accessibility, and dissemination of PET/MR.

Technical Note: Building a combined cyclotron and MRI facility: Implications for interference

With the introduction of hybrid PET∕MRI systems, it has become more likely that the cyclotron and MRI systems will be located close to each other. This study considered the interference between a cyclotron and a superconducting MRI system. Interactions between cyclotrons and MRIs are theoretically considered. The main interference is expected to be the perturbation of the magnetic field in the MRI due to switching on or off the magnetic field of the cyclotron. MR imaging is distorted by a dynamic spatial gradient of an external inplane magnetic field larger than 0.5-0.04 μT∕m, depending on the specific MR application. From the design of a cyclotron, it is expected that the magnetic fringe field at large distances behaves as a magnetic dipolar field. This allows estimation of the full dipolar field and its spatial gradients from a single measurement. Around an 18 MeV cyclotron (Cyclone, IBA), magnetic field measurements were performed on 5 locations and compared with calculations based upon a dipolar field model. At the measurement locations the estimated and measured values of the magnetic field component and its spatial gradients of the inplane component were compared, and found to agree within a factor 1.1 for the magnetic field and within a factor of 1.5 for the spatial gradients of the field. In the specific case of the 18 MeV cyclotron with a vertical magnetic field and a 3T superconducting whole body MR system, a minimum distance of 20 m has to be considered to prevent interference. This study showed that a dipole model is sufficiently accurate to predict the interference of a cyclotron on a MRI scanner, for site planning purposes. The cyclotron and a whole body MRI system considered in this study need to be placed more than 20 m apart, or magnetic shielding should be utilized.

MRI assessment of diastolic dysfunction in comparison to transthoracic echocardiography

Summary We could show that PC-CMR analysis of DD is clinically feasible and showed excellent agreement with the widely accepted EC parameters for classification of DD. PC-CMR could offer the potential of a practically and reasonably time-consuming approach for the clinically important assessment of DD with CMR. Background Over the last decade the interest in diastolic dysfunction has grown, due to the fact that at least half of all cases of newly diagnosed heart failure patients present with preserved ejection fraction. One of the most important tasks of cardiovascular imaging is the objective assessment of left ventricular (LV) systolic and diastolic function, which is routinely performed with standard transthoracic echocardiography (EC). In EC, classification of diastolic dysfunction (DD) is widely accepted and mainly assessed using three criteria: mitral blood flow (MBF, EA-curve), lateral wall velocity (LWV, S-E-A-curve) and pulmonary vein flow (PVF, S-D-AR-curve). With these three characteristic flow and velocity patterns and their ratios E/A, E/E and S/D, regular diastolic function can be clearly distinguished. Cardiovascular magnetic resonance (CMR) has excellent capabilities to assess blood flow and myocardial tissue motion using phase contrast (PC-CMR) imaging. We sought to compare the feasibility of PC-CMR with echocardiographic doppler imaging for the assessment of DD using the echocardiographic flow and velocity approach for DD-classification. Methods After acquisition of regular short axis cine SSFP volumetry and 2-3-4-chamber views in 38 patients with various cardiovascular diseases, we performed single-slice short-axis PC-CMR (60phases,velocity-encoding=100cm/ s) similarly to typical EC locations at the tip of mitral leaflets in diastole on a 1.5T whole body MRI system (Philips Achieva) to generate mitral E-and A-waves, lateral S`-E`-A-velocities, E/A- and E/E`-ratios. PC-CMR for PVF was planned orthogonally to the cine 4-chamber plane 1cm distal from pulmonary vein inflow into the left atrium. Directly after MRI, EC was performed to generate complementary data for MBF, LWV and PVF. Results EC and PC-CMR could be performed in all patients, whereas EC PVF could not be assessed in 8 patients due to reduced flow signals. Correlation with mitral E and A velocities was good (r=0.72 and 0.74, p<0.001) as well as their correlation (r=0.76, p<0.001). The correlation for E/E’, as one of the most clinically relevant parameters, was high with r=0.79 (p<0.001). 36/38 patients (95%) were categorized the same by both CMR and EC, whereas in 2 cases PC-CMR mis-diagnosed them as normal. Mean scan-time for MBF, LWV and PVF was 5.20±1.31 minutes, mean analysis time was 4.10±1.31 min. Conclusions We could show that PC-CMR analysis of DD is clinically feasible and showed excellent agreement with the widely accepted EC parameters for classification of DD. PC-CMR could offer the potential of a practically and reasonably time-consuming approach for the clinically important assessment of DD with CMR. Funding

A specific absorption rate prediction concept for parallel transmission MR

The specific absorption rate (SAR) is a limiting factor in high-field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single-channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so-called Q-matrices and further computational optimizations allow for a real-time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight-channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high-field MR.

Characterization of the failing murine heart in a desmin knock-out model using a clinical 3 T MRI scanner

The purpose of this study was to establish an MRI protocol on a clinical scanner for assessment of left (LV) and right (RV) ventricular myocardial function of the murine heart, and to apply this protocol for the first in vivo assessment of myocardial function in a mouse model of cardiomyopathy (Desmin-/-). MRI was performed on a clinical 3 T whole body MRI system using a dedicated solenoid receive-only coil. Contiguous short axis slices were acquired covering the entire heart using a spoiled cine gradient echo sequence (TR 9-12 ms, TE 3-4 ms, α 25°, 1.0 × 0.23 × 0.23 mm³). Global LV- and RV-myocardial functional parameters such as end-diastolic ventricular volume, ejection fraction (EF), LV mass and cardiac output (CO) of Desmin-/- mice and age-matched controls were determined. Global myocardial functional data of healthy controls (n = 4) were in very good agreement with previously reported data. The transgenic mice (n = 8) revealed a significantly reduced LV- and RV-EF as well as CO. Body weight-normalized LV- and RV-end-diastolic volumes and LV mass were significantly increased. In addition desmin deficient mice exhibited segmental wall thinning and akinesia, suggesting myocardial necrosis. This study demonstrates that clinical 3 T MRI-systems may reliably be used for non-invasive assessment of LV- and RV-myocardial function in normal and in genetically engineered mice with cardiomyopathies. In addition, this proof of principle study presents first in vivo MRI data of the cardiac phenotype of desmin knock-out mice.

S118 Direct visualisation of collateral ventilation in COPD with hyperpolarised Gas MRI

Introduction and ObjectivesCollateral ventilation is important in pathophysiology of Chronic Obstructive Pulmonary Disease (COPD), complicated pneumothorax, and bronchoscopic lung volume reduction surgery but limited observations of it in vivo have...

Influence of Acid Gastroesophageal Reflux Disease (GERD) on Lower Esophageal Sphincter (LES) Relaxation Reflex Kinetics in Human Infants

Background: Standard methods for assessment of meal induced gastric secretion are invasive, unphysiological and may themselves alter gastric secretion. Recently, a MRI technique for the quantitative assessment of intragastric secretion and meal dilution was developed [1]. Using this technique and a purpose-built physiologic test meal, this pilot study aimed on the non-invasive quantification of meal induced secretion and its effect on caloric emptying. Methods: Six healthy fasted volunteers (4males, age: 22-45 years (mean 31.8 years), BMI 24.2±1.9 kg/m2) were examined in right lateral position in a 1.5T whole body MRI System after ingestion of a purpose-made, secretogenic test meal. The meal consisted of a 400 ml viscous chocolate drink (450kcal) containing the following ingredients: 90 g carbohydrates, 40 g cacao powder, 100 mg Calcium, 16 g amino acids, 3 g locust bean gum, 133 μl GdDOTA. Gastric content volume (GCV), meal volume (MV) and secretion volume (SV) as well as distribution of secretion were assessed at 10 min intervals for 100 min, whereby MV emptying curves reflected caloric emptying. Previous in-vitro experiments confirmed acid stability and low buffer capacity of the test meal. Data were expressed as means ± SD. Results: The applied test meal continuously stimulated gastric secretion, resulting in a linear SV increase and maximum SV of 240ml at 100min (Figure 1). MV decreased linearly over time with a caloric emptying rate of 1.9±0.3 kcal/min, whereas GCV did not significantly change over time (Figure 1). T50 of caloric emptying was 108±34 min. Mixing of meal and secretion was heterogeneous and an increasing secretion layer on top of the meal was observed. The secretion dynamics resulted in a maximal meal dilution of 48±7 % after 100 min. Caloric emptying rate was negatively correlated to the secretion production rate (r= 0.73, p=0.06). Conclusion: A non-invasive, quantitative MR assessment of gastric secretion volume and distribution of secretions within the stomach is presented. The physiologic test meal induced large volumes of gastric secretion, attaining almost 50% of total gastric content volume at 100min. As a result changes in total gastric content volume (meal and secretion) systematically underestimated meal emptying and, therefore, caloric delivery to the small bowel. These findings highlight the need for concurrent assessment of secretion with gastric content volume if accurate assessment of meal emptying is to be obtained (at least in meals that stimulate secretion). This technique should provide new insight into the effects of acid secretion on gastric function in health and disease (e.g. gastro-esophageal reflux and peptic ulcer disease)”

Clinical Cardiac MR at 3.0 Tesla using parallel RF transmission with patient-adaptive local RF shimming: initial experience

Purpose: The use of high-field MRI scanners has introduced new challenges for cardiac imaging, associated with B1- and B0-field inhomogeneities. A dual – source RF transmission system may help reduce dielectric effects, thus improving image contrast and reducing local SAR peaks of cardiac TSE and SSFP sequences, thereby allowing a shorter minimum TR and TE of SSFP sequences. Thus, the aim of our study was to evaluate the impact of dual – source parallel RF transmission with patient adaptive RF-shimming on image quality and diagnostic confidence for routine clinical CMR using a 3.0 T dual – channel transmit whole – body MRI system.

Conceptual Design of a 3T MRI Superconducting Magnet With Non-Thrust Face Integrated Former

Recently, 3T MRI system has been one of the fastest growing segments of the MRI clinical diagnoses. In order to lower the 3T MRI cost and allow it to be more widely applied, especially in developing country like China, this paper presents a conceptual design of a 3T superconducting magnet with a non-thrust face integrated former for a whole body MRI system. The concept is an 8-coil active shielded magnet design with the minimized 82 cm standard warm bore size. The operating current is 480 A and the full field is 2.9T. The integrated former was designed to further minimize the coil internal diameter and reduce the superconducting wire length. Unlike traditional formers, there are no coil slots on the former for the coil supporting and positioning to reduce former manufacturing cost. The coils are wound and cast on a detachable mould tooling, and then bonded onto the outer surface of the integrated former. At 4.2 K, the coils remain bonded to the surface of the former; this prevents slight relative movement between the coil and the former in the coil energizing process. This coil-former structure can reduce the quench rate during the magnet ramping process to save liquid Helium and the build quality cost.

Intraindividual In Vivo Comparison of Gadolinium Contrast Agents for Pharmacokinetic Analysis Using Dynamic Contrast Enhanced Magnetic Resonance Imaging

To compare the intraindividual differences of dynamic signal characteristics for 3 gadolinium chelates (gadopentetate dimeglumine [Gd-DTPA], gadodiamide [Gd-DTPA-BMA], and gadobenate dimeglumine [Gd-BOPTA]) using dynamic contrast enhanced magnetic resonance (MR) imaging (DCE-MRI) with a preclinical beagle model at 7 Tesla. Seven beagles were scanned 3 times each with a 7-day interval between the scans on a 7T whole body MRI system (Achieva, Philips) using a T/R head coil. Three different Gd contrast agents including Gd-DTPA, Gd-DTPABMA, and Gd-BOPTA were injected in a randomized order with a power injector (Spectris, MedRad, Indianola, PA) using dose of 0.1 mmol/kg body weight and flow rate of 0.06 mL/s. During image acquisition and data analysis, the identity of the specific contrast agent used for each examination was blinded. A 3D RF-spoiled fast field echo sequence was used for dynamic scans with in-plane spatial resolution 0.47 x 0.47 mm(2), temporal resolution of 9.5 seconds, and a total of 60 time points. Regions of interest were drawn within the carotid arteries and muscle tissue to determine semiquantitative parameters including maximum enhancement ratio, area under the signal enhancement curve over 90 seconds after contrast injection (AUC_90), time to maximum signal enhancement (T(max)), and washout_score. Additionally, quantitative pharmacokinetic parameters were measured in muscle tissues by applying 3 separate 2-compartment models; (1) artery input function (AIF) based Tofts model, (2) Brix model without AIF, and (3) AIF decomposed refined Brix model. Gd-BOPTA produced higher signal to noise ratio on postcontrast T1- weighted images than the other 2 Gd based contrast agents at 7T. Quantitatively, Gd-BOPTA provided a significantly higher maximum enhancement ratio (P < 0.01), AUC_90 (P < 0.01) and washout_score (P < 0.01) in beagle musculature and cranial vasculature compared with both Gd-DTPA and Gd-DTPA-BMA. Among all the quantitative pharmacokinetic parameters, only the exchange rate constants (kep) calculated from these 3 models did not show a significant difference among the various contrast agents. Gd chelate containing MR contrast agents can be used at 7T for DCEMRI. Gd-BOPTA demonstrates stronger signal enhancement than standard Gd chelates, in concordance with the results of studies at lower fields. The observed enhancement characteristics for the 3 contrast agents demonstrate that the pharmacokinetic parameter kep is more robust in various models using DCE-MRI than the other pharmacokinetic parameters. This information is important relative to multisite clinical trials and long-term clinical studies that often use several different contrast agents and different models.

Schnelles In-vivo-19F-MRT-T1-Mapping zur Messung der Konzentration von fluorinierten Gasen in der Lunge

Ziele: Despite the great success of applying hyperpolarized 3He and 129Xe in the field of morphological and functional MR-imaging of lungs both contrast gases still remain mostly research tools. Therefore, the interest in using thermally polarized fluorinated gases, such as SF6, C2F6 and C4F8 in the MRI-investigations of lungs has grown. The short T1 relaxation time of these gases (1–50ms at 1bar) allows for a short measurement time (<10 s) if imaging is performed in a large animal. As the fraction of fluorinated gases is not linearly correlated to the spin density due to T1 weighting other functional MR parameters are necessary for gas concentration quantification. One of such parameters is the T1 relaxation time itself. The aim of this study was to prove the feasibility of the 19F T1 mapping for the estimation of local partial pressure of fluorinated gases in vivo. Methode: For all experiments, the variable flip angle (VFA) technique was used. First, a variety of investigations on phantoms containing mixtures of C4F8 and O2 in different proportions aiming to proof the VFA method for fluorinated gases were done. Next, ex-vivo measurements in rat lungs were performed to test the method efficiency. The rat was ventilated in-vivo with a mixture of 80% C4F8 with 20% of O2 and then, immediately after death, 5ml of C4F8 were injected additionally. Finally, in-vivo experiments in domestic pig (20–25kg) lungs were done using the same mixture. The scan time in all measurements during inspiratory hold was 60–70 s. All experiments were run on a Siemens Magnetom Vision Experimental 1.5T whole body MRI system using a 25cm birdcage coil. Ergebnis: In the ex-vivo T1-maps regions with various T1 can be identified corresponding to variations of C4F8 concentration. The in-vivo measurements results show essentially broad T1- time distribution (and therefore related C4F8 concentration) that obviously reflects a metabolic processes of gas exchange. In this case metabolic changes of the proportions of gas mixtures (C4F8:O2:CO2) in different lung regions could lead to corresponding variations of T1. Schlussfolgerung: In conclusion, the applicability of the VFA method of T1 measurement for the in-vivo MRI-measurement of lungs with heavy fluorinated gases was demonstrated. The sensitivity of the method is sufficient enough to detect 5–10% variations of gas mixture content in-vivo. Combined with diffusion weighted 19F imaging techniques this may open up a number of new options for the study of lung functions and pathology.

Enhanced diffusion weighting generated by selective adiabatic pulse trains

A theoretical description and experimental validation of the enhanced diffusion weighting generated by selective adiabatic full passage (AFP) pulse trains is provided. Six phantoms (Ph-1–Ph-6) were studied on a 4 T Varian/Siemens whole body MRI system. Phantoms consisted of 2.8 cm diameter plastic tubes containing a mixture of 10 μm ORGASOL polymer beads and 2 mM Gd-DTPA dissolved in 5% agar (Ph-1) or nickel(II) ammonium sulphate hexahydrate doped (56.3–0.8 mM) water solutions (Ph-2–Ph-6). A customized localization by adiabatic selective refocusing (LASER) sequence containing slice selective AFP pulse trains and pulsed diffusion gradients applied in the phase encoding direction was used to measure 1H 2O diffusion. The b-value associated with the LASER sequence was derived using the Bloch–Torrey equation. The apparent diffusion coefficients measured by LASER were comparable to those measured by a conventional pulsed gradient spin-echo (PGSE) sequence for all phantoms. Image signal intensity increased in Ph-1 and decreased in Ph-2–Ph-6 as AFP pulse train length increased while maintaining a constant echo-time. These experimental results suggest that such AFP pulse trains can enhance contrast between regions containing microscopic magnetic susceptibility variations and homogeneous regions in which dynamic dephasing relaxation mechanisms are dominant.

Adaptive data acquisition in MRI

We propose an adaptive data acquisition technique that depends on the object to be imaged in magnetic resonance (MR) imaging. In this paper, we employed a matching pursuit (MP) algorithm to achieve the adaptive data acquisition. Since the matching pursuit is a greedy algorithm to find RF and gradient waveforms which are the best match for an object-signal, the signal can be decomposed with a few iterations and thereby lead reduction of imaging time in MR. To adopt the matching pursuit algorithm to the adaptive data acquisition in MRI, we have designed a dictionary which contains a windowed Fourier basis set. Because the basis set is localized spatially, the image signal could be divided into segmented signals so that matching pursuit with the segmented signals could lead to effective and object-dependent data acquisition. To verify the proposed technique, computer simulations and experiments are performed with a 1.0 T whole body MRI system.

Multilevel wavelet-transform encoding in MRI.

This paper explores an alternative to standard Fourier MRI called wavelet-transform encoding. Spatially selective radiofrequency (RF) pulses are used to excite slice profiles in the shape of the wavelet-basis functions of a discrete wavelet transform. Our implementation on a standard commercial whole body MRI system resolves one spatial dimension through wavelet encoding and orthogonal directions through conventional frequency encoding and rectangular-slice selection. The wavelet transform is described as a method for multiresolution analysis, and we show how a high resolution MR image can be constructed from lower resolution images, representing the approximation and detail structures of the object. A multiple level architecture of the wavelet reconstruction is described, thereby allowing wavelet-encoded images to be reconstructed through several modes. Multilevel wavelet-encoded images of a gel phantom are presented that show comparable image quality to Fourier-encoded images of similar signal-to-noise ratio. However, a chemical-shift artifact is identified with this particular implementation.

NMR functional imaging using a tailored RF gradient echo sequence: a true susceptibility measurement technique.

The tailored radio frequency gradient echo (TRFGE) technique that has been used in venography (Cho et al., Magn. Reson. Med. 28, 25-38, 1992; Ro, Cho, Magn. Reson. Med. 28, 237, 1992) is applied to functional imaging. The TRFGE technique has the advantage that it is sensitive to the field gradient created by the susceptibility effect, thereby enhancing only the signals from the regions with local field gradient. In addition, the method is insensitive to the in-flow effect, especially from the arterial blood. The latter further simplifies the functional MRI data analysis. The TRFGE sequence is, therefore, suitable for functional MR imaging for which true susceptibility effect measurements are of prime importance. To examine the TRFGE functional imaging systematically, experiments with various imaging parameters such as flip angle alpha, repetition time, and echo time were performed and the results were compared with the data obtained from the conventional gradient echo functional MR imaging. The experimental results shown were all obtained from human volunteers with a 2.0T whole body MRI system.

Noninvasive temperature imaging using diffusion MRI

Efficacy and safety considerations for cancer therapy with hyperthermia require accurate temperature measurements throughout the heated volume. We report the use of molecular diffusion, whose temperature dependence is well known. A dedicated hyperthermia applicator was built, combining a MRI gradient coil and a rf coil. Diffusion and derived temperature images were obtained with a 1 x 2 mm pixel size on a polyacrylamide gel phantom using a clinical 1.5-T whole body MRI system. Temperatures determined from these images using 1 cm2 regions of interest were found to be within 0.2 degrees C of those recorded from the thermocouples and fiber-optic probes placed inside the gel.

Radio frequency field intensity mapping using a composite spin-echo sequence

A novel radio frequency (RF) field intensity mapping or imaging method using a composite NMR spin-echo sequence is proposed. A composite spin-echo RF pulse with 90° y -180° x -90° y sequence makes phase change in the final image depending on the RF field intensity on the object. The resultant phase change or phase map can be used to obtain the actual RF flip-angle map for a given condition which includes the status of tuning and RF inhomogeneity, etc. Bloch equation has been solved numerically to obtain the effects of the RF field intensity as well as the main magnetic field inhomogeneity and the results are used for the mapping (imaging) of the RF field intensity. Phantom studies have been performed using a 1.5 Tesla whole body MRI system and the results are presented.

The rf coil as a sensitive motion detector for magnetic resonance imaging

A new sensor principle for detection of patient movement in magnetic resonance imaging has been successfully applied for the reduction of motion artifacts. It uses a device that is already present in every MRI system, namely the rf coil. Patient movement within the coil causes changes in the rf impedance match of the coil, which can be measured as variations in the reflected rf power. The principle used for the detection of respiratory and cardiac motion is described, and experimental results measured with several coil arrangements are given. Images are presented which were acquired with respiratory gating derived from the rf body coil of a 2 Tesla whole body MRI system.