Magnetic Resonance Imaging Image Quality Research Articles

Qualitative and Quantitative Assessment of Wrist MRI at 3.0t: Comparison between Isotropic 3d Turbo Spin Echo and Isotropic 3D Fast Field Echo and 2D Turbo Spin Echo

Isotropic three-dimensional (3D) magnetic resonance imaging (MRI) has been applied to various joints. However, comparison for image quality between isotropic 3D MRI and two-dimensional (2D) turbo spin echo (TSE) sequence of the wrist at a 3T MR system has not been investigated. To compare the image quality of isotropic 3D MRI including TSE intermediate-weighted (VISTA) sequence and fast field echo (FFE) sequence with 2D TSE intermediate-weighted sequence of the wrist joint at 3.0 T. MRI was performed in 10 wrists of 10 healthy volunteers with isotropic 3D sequences (VISTA and FFE) and 2D TSE intermediate-weighted sequences at 3.0 T. The signal-to-noise ratio (SNR) was obtained by imaging phantom and noise-only image. Contrast ratios (CRs) were calculated between fluid and cartilage, triangular fibrocartilage complex (TFCC), and the scapholunate ligament. Two radiologists independently assessed the visibility of TFCC, carpal ligaments, cartilage, tendons and nerves with a four-point grading scale. Statistical analysis to compare CRs (one way ANOVA with a Tukey test) and grades of visibility (Kruskal-Wallis test) between three sequences and those for inter-observer agreement (kappa analysis) were performed. The SNR of 2D TSE (46.26) was higher than those of VISTA (23.34) and 3D FFE (19.41). CRs were superior in 2D TSE than VISTA (P = 0.02) for fluid-cartilage and in 2D TSE than 3D FFE (P < 0.01) for fluid-TFCC. The visibility was best in 2D TSE (P < 0.01) for TFCC and in VISTA (P = 0.01) for scapholunate ligament. The visibility was better in 2D TSE and 3D FFE (P = 0.04) for cartilage and in VISTA than 3D FFE (P < 0.01) for TFCC. The inter-observer agreement for the visibility of anatomic structures was moderate or substantial. Image quality of 2D TSE was superior to isotropic 3D MR imaging for cartilage, and TFCC. 3D FFE has better visibility for cartilage than VISTA and VISTA has superior visibility for TFCC to 3D FFE and the visibility for scapholunate ligament was best on VISTA.

Simulation Environment to Predict the Effect of Eddy Currents on Image Quality in MRI

Gradient coils generate a magnetic field with a linear spatial variation that superimposes over the main magnetic field of a magnetic resonance imaging (MRI) system; such superimposition of the magnetic fields enables the encoding of the spatial position in MRI. A rapid change in the gradient field induces eddy currents in the conducting structures of an MRI system, resulting in the production of image artifacts. An objective of the gradient coil design phase is to predict both the coil's performance with respect to eddy currents and the image quality (IQ) before the coil is manufactured. In this paper, an integrated simulation environment is presented that combines the gradient coil design with an image formation simulation to predict the IQ. Here, an unshielded, uni-planar gradient set was simulated. Further, a study was conducted to determine the effect of frequency on the eddy currents induced in the conducting structures of the main magnet coil while exciting the uni-planar gradient set. The knowledge acquired from this study was applied to the IQ simulation, and a time-dependent simulation of a gradient echo pulse sequence was performed. The IQ of the uni-planar gradient set was predicted, and the input and reference images as well the images distorted by the eddy currents are shown.

Prospective and retrospective motion correction in diffusion magnetic resonance imaging of the human brain

Diffusion-weighting in magnetic resonance imaging (MRI) increases the sensitivity to molecular Brownian motion, providing insight in the micro-environment of the underlying tissue types and structures. At the same time, the diffusion weighting renders the scans sensitive to other motion, including bulk patient motion. Typically, several image volumes are needed to extract diffusion information, inducing also inter-volume motion susceptibility. Bulk motion is more likely during long acquisitions, as they appear in diffusion tensor, diffusion spectrum and q-ball imaging. Image registration methods are successfully used to correct for bulk motion in other MRI time series, but their performance in diffusion-weighted MRI is limited since diffusion weighting introduces strong signal and contrast changes between serial image volumes. In this work, we combine the capability of free induction decay (FID) navigators, providing information on object motion, with image registration methodology to prospectively--or optionally retrospectively--correct for motion in diffusion imaging of the human brain. Eight healthy subjects were instructed to perform small-scale voluntary head motion during clinical diffusion tensor imaging acquisitions. The implemented motion detection based on FID navigator signals is processed in real-time and provided an excellent detection performance of voluntary motion patterns even at a sub-millimetre scale (sensitivity≥92%, specificity>98%). Motion detection triggered an additional image volume acquisition with b=0 s/mm2 which was subsequently co-registered to a reference volume. In the prospective correction scenario, the calculated motion-parameters were applied to perform a real-time update of the gradient coordinate system to correct for the head movement. Quantitative analysis revealed that the motion correction implementation is capable to correct head motion in diffusion-weighted MRI to a level comparable to scans without voluntary head motion. The results indicate the potential of this method to improve image quality in diffusion-weighted MRI, a concept that can also be applied when highest diffusion weightings are performed.

Effect of butylscopolamine on image quality in MRI of the prostate

To evaluate the impact of butylscopolamine on the quality of magnetic resonance imaging (MRI) images of the prostate. Eighty-two MRI examinations of the prostate were retrospectively analysed. MRI was performed with a combined endorectal/body phased-array coil including proton density-weighted (PD) sequence, T1-weighted turbo spin-echo (TSE)-sequence, and T2-weighted TSE-sequences. Forty milligrams of butylscopolamine was administered intramuscularly in 31 patients (im-group) and intravenously in 30 patients (iv-group). Twenty-one patients did not receive premedication with butylscopolamine (ø-group). Overall image quality, delineation of the bowel wall, and visualization of the prostate, neurovascular bundle, and pelvic lymph nodes were evaluated qualitatively using a five-point scale (from 1=excellent to 5=non-diagnostic/structure not discernible). Motion artefacts within the endorectal coil were quantified by baseline adjusted signal intensities inside the endorectal coil area. Delineation of the bowel wall using the PD-sequence was significantly improved after both intramuscular and intravenous butylscopolamine administration (ø-group: 3.6+/-0.7; im-group: 2.9+/-0.7; iv-group: 2.9+/-0.7; p=0.001). However, there were no significant differences in motion artefacts measured within the endorectal coil (ø-group: 1.18+/-0.14; im-group: 1.15+/-0.11; iv-group: 1.12+/-0.06; p=0.39). There were also no significant differences in qualitative assessment of visualization of the prostate, neurovascular bundle, pelvic lymph nodes, and of overall image quality between the study groups. : In conclusion, butylscopolamine had only a small effect on image quality and is not mandatory for MRI of the prostate.

MRI of the transverse and alar ligaments in rheumatoid arthritis: feasibility and relations to atlantoaxial subluxation and disease activity

Dysfunctional transverse and alar craniovertebral ligaments can cause instability and osseous destruction in rheumatoid arthritis (RA). This study examined (1) the feasibility of high-resolution magnetic resonance imaging (MRI) of these ligaments in RA and (2) the relation between ligament high-signal changes and atlantoaxial subluxation and RA duration/severity. Consecutive RA patients (n=46) underwent clinical examination, functional radiography, and high-resolution MRI. Two blinded radiologists rated MRI image quality, graded ligament high-signal changes 0-3 on proton-weighted sequences using an existing grading system, and assessed cervical spine rheumatic changes on short tau inversion recovery images. Agreement was analyzed using kappa and relations using multiple logistic regression. MRI images had good quality in 42 (91.3%) of 46 patients and were interpretable in 44 (32 women and 12 men, median age/disease duration 60.4/9.1 years). MRI grades 2-3 changes of the transverse and alar ligaments showed moderate and good interobserver agreement (kappa 0.59 and 0.78), respectively, and prevalence 31.8% and 34.1%. Such ligament changes were more frequent with increasing anterior atlantoaxial subluxation (p=0.012 transverse, p=0.028 alar), higher erythrocyte sedimentation rate (p=0.003 transverse), positive rheumatoid factor (p=0.002 alar), and neck pain (p=0.004 alar). This first study of high-resolution MRI of these ligaments in RA showed high feasibility and relations with atlantoaxial subluxation, RA disease activity, and neck pain. The clinical usefulness of such MRI needs further evaluation.

Two Paralleled Four Quadrant DC Chopper for Gradient Coil Magnetic Fields in MRI System

This paper presents a two-paralleled four quadrant DC chopper type PWM power conversion circuit in order to generate a gradient magnetic field in the Magnetic Resonance Imaging (MRI) system. This circuit has 8-IGBTs at their inputs/outputs to realize further high-power density, high speed current tracking control, and to get a low switching ripple amplitude in a controlled current in the Gradient Coils (GCs). Moreover, the power conversion circuit has to realize quick rise/fall response characteristics in proportion to various target currents in GCs. It is proposed in this paper that a unique control scheme can achieve the above objective DSP-based control system realize a high control facility and accuracy. It is proved that the new control system will greatly enlarge the diagnostic target and improve the image quality of MRI.

BAHA Devices and Magnetic Resonance Imaging Scanners

That an osteointegrated Bone-anchored Hearing Aid (BAHA System) will show movement or displacement when exposed to a 9.4-T magnetic resonance imaging (MRI) magnet and that any movement may have implications for patient safety and image quality. BAHA System implanted patients may require MRI scans for various pathologies. The possibility of BAHA fixture and abutment movement can be explored to help clarify patients' and physicians' decisions regarding safety issues. University Medical Center (tertiary care center). BAHA System assembled fixture with abutment of 3 and 4 mm were each placed inside of a Petri dish and mounted into human cadaver temporal bones, then exposed to a 9.4-T MRI magnet. The BAHA System external processor was entered into the MRI suite. Head MRI images and charts of 2 patients with BAHA System devices who had undergone MRI scanning at 1.5 and 3.0 T, respectively, were reviewed. The BAHA System fixture with abutment moved inside of the Petri dish. The temporal bone studies revealed no sign of motion or displacement. The external BAHA processor showed strong movement toward the magnet. MRI scannings of patients were without any adverse effects. Image MRI quality was affected only around the BAHA post. The bone-mounted fixture with abutment of 3 and 4 mm resisted any magnetic forces trying to displace them up to 9.4 T. Two patients underwent scanning at 1.5 and 3.0 T with no adverse effects. MRI image quality was excellent starting at 1 cm from the fixture with abutment. The study findings lend strong evidence that patients can be safely scanned up to 9.4 T without adverse effects.

Vibration analysis and testing of a thin-walled gradient coil model

Ongoing development of magnetic resonance imaging (MRI) technology leads to high magnetic field strength (up to 7–9 T) and high-speed switching current in gradient coils for the purpose of improving MRI image quality. These two factors among others contribute largely to the high levels of structure-borne noise that surrounds current MRI scanners. In this paper, the forcing function and distribution acting on gradient coils are described (gradient coils x, y and z). Single point forces and vibration responses of harmonic, transient and impulsive excitations are investigated. Modal expansion method is used to predict the whole cylinder vibration of a thin-walled model under these excitations. Experimental testing of a thin-walled model and a single-layered gradient coil is done by using National Instruments PXI. The measured whole-body radial vibration modes show an agreement with the analytical results. This agreement indicates that modal expansion method could be used to predict the whole-body vibration modes of gradient coils in the low-frequency range.

The value of hyoscine butylbromide in pelvic MRI

To evaluate the effect of hyoscine butylbromide (HBB) on image quality and lesion and organ visualization in pelvic magnetic resonance imaging (MRI) MATERIALS AND METHODS: A prospective, ethically approved study was undertaken of 47 patients attending for pelvic MRI at a cancer centre. T2-weighted transverse and sagittal sequences were performed before and after intravenous injection of 20 mg HBB. Three radiologists independently scored anonymized image series for overall image quality, visualization of pelvic lesions and visualization of individual pelvic organs. Statistical analysis was performed to assess improvements in radiologists' scores post-HBB administration. Radiologists also assessed pre-HBB administration T1-weighted images for degree of bowel peristalsis to determine whether this could predict improvement in post-HBB T2-weighted image scores. Side effects of HBB were recorded using a patient questionnaire. Radiologists' scores for image quality and lesion visualization were significantly higher on the post-HBB administration T2-weighted series (p<0.0005). Scores for the visualization of the bladder, rectum, pelvic bowel, prostate, and seminal vesicles (all p<0.0005), cervix (p=0.019) and vagina (p=0.0001) were also significantly higher post-HBB administration. Scores for the degree of peristalsis on T1-weighted images were not related to improvement in image quality or lesion visualization on T2-weighted images post-HBB administration. Side effects of HBB were mild and self-limiting. Intravenous HBB administration improves image quality and lesion visualization in oncological pelvic MRI and is recommended for routine use.

High-resolution MRI of the wrist and finger joints in patients with rheumatoid arthritis: comparison of 1.5 Tesla and 3.0 Tesla

The goal of this study was to compare magnetic resonance (MR) image quality at different field strengths for evaluating lesions in wrist and finger joints of patients with rheumatoid arthritis (RA) in order to determine whether the higher field strength provides diagnostic gain. The hand mainly affected in 17 RA patients was examined at 1.5 Tesla (T) and 3.0 T with comparable MR imaging (MRI) protocols. MR images were reviewed twice by two experienced radiologists using the Rheumatoid Arthritis MRI Scoring System (RAMRIS) of the OMERACT (Outcome Measures in Rheumatoid Arthritis Clinical Trials) group. Image quality was rated on a five-point scale using Friedmann's test and Kendall's W-test for statistical analysis. Image comparison revealed better image quality at higher field strength. Image quality of T1-weighted images was rated 14-22% better at 3.0 T compared with 1.5 T by both readers. Moreover, the rating for the T2-weighted-images acquired at 3.0 T was one point better in the five-point scale used. Inter-reader correlation for image quality, bone erosions/defects, edema and synovitis ranged between 0.6 and 0.9 at 3.0 T and between 0.6 and 0.8 at 1.5 T. Intra-reader correlation for these parameters was high at 0.8-1.0. MRI image quality of RA hands is superior at 3.0 T, while an acceptable image quality is achieved at 1.5 T, which improves the evaluation of extent of bone edema, synovitis and identification of small bone erosions.

Atelectasis in children undergoing either propofol infusion or positive pressure ventilation anesthesia for magnetic resonance imaging

Atelectasis because of anesthesia is a recognized problem but may be affected by the anesthetic technique. We compared magnetic resonance images of atelectasis in children undergoing two types of anesthesia. Children requiring anesthesia for magnetic resonance imaging (MRI) had additional lung imaging sequences at the beginning and the end of anesthesia. Children had either i.v. propofol infusion (PI) without an artificial airway (n = 26) or positive pressure ventilation (PPV) via a tracheal tube (n = 20); the technique was chosen for clinical reasons. The extent of atelectasis was scored by two independent radiologists. The median ages (range) for PI and PPV groups were 45 months (1-77 months) and 18 months (2-74 months), respectively. The proportion of children with atelectasis was different in the first lung scan (42% vs 80%), but in the second scan atelectasis was seen frequently in both groups (82% vs 94%) with a greater extent in the PPV group. The atelectasis score was higher in young children, but all children had normal oxygen requirements and saturations. Many factors may influence the development of atelectasis but this study found less extensive atelectasis with PI than PPV. PI allows for sufficient motionlessness, required for high diagnostic image quality in pediatric MRI.

Low-dose intranasal versus oral midazolam for routine body MRI of claustrophobic patients

The purpose of this study was to assess prospectively the potential of low-dose intranasal midazolam compared to oral midazolam in claustrophobic patients undergoing routine body magnetic resonance imaging (MRI). Seventy-two adult claustrophobic patients referred for body MRI were randomly assigned to one of two treatment groups (TG1 and TG2). The 36 patients of TG1 received 7.5 mg midazolam orally 15 min before MRI, whereas the 36 patients of TG2 received one (or, if necessary, two) pumps of a midazolam nasal spray into each nostril immediately prior to MRI (in total, 1 or 2 mg). Patients' tolerance, anxiety and sedation were assessed using a questionnaire and a visual analogue scale immediately before and after MRI. Image quality was evaluated using a five-point-scale. In TG1, 18/36 MRI examinations (50%) had to be cancelled, the reduction of anxiety was insufficient in 12/18 remaining patients (67%). In TG2, 35/36 MRI examinations (97%) were completed successfully, without relevant adverse effects. MRI image quality was rated higher among patients of TG2 compared to TG1 (p<0.001). Low-dose intranasal midazolam is an effective and patient-friendly solution to overcome anxiety in claustrophobic patients in a broad spectrum of body MRI. Its anxiolytic effect is superior to that of the orally administrated form.

Parallel Operation of Full-Bridge Power Amplifiers and Its Digital Optimal Control Scheme for Ultrahigh-Speed Scanning MRI

In recent years, MRI (Magnetic Resonance Imaging) systems have been actually requested to diagnose the moving parts of a human body such as the heart or the blood in angiography, where images should be taken in milliseconds instead of minutes. The gradient power supplies which are small but vital components for advanced MRI require higher output power capability as well as faster rise/fall dynamic response characteristics under a variety of specified current reference signals.This paper presents a novel switch-mode gradient power amplifier using IGBTs which is connected in parallel with the conventional 4-switch full-bridge power conversion circuits at their inputs/outputs in order to realize further high-power density. To satisfy the design specifications which require minimized the ripple and improved the rise/fall dynamic response characteristics of the current in the gradient coil, a unique digital control scheme on the basis of optimal preview type-1 servo system is proposed and described in detail. The above effectiveness is discussed and evaluated through the computer-aided analysis. It is expected that the proposed techniques will highly enlarge the diagnostic target and improve the image quality of ultrahigh-speed scanning MRI.

Studies on developments of imaging and analytical methods and evaluations of properties mainly on image quality in MRI (in Japanese)

The aims of this thesis were to develop imaging and analytical methods in magnetic resonance imaging (MRI) to obtain morphological and functional information of the human body and to evaluate image quality of an MRI system. Various new methods are illustrated. A fast imaging method for obtaining a T1‐weighted image by using the rapid acquisition relaxation enhanced sequence combined with the variable flip angle excitation is described. Next, the thesis describes a method for performing a dynamic contrast‐enhanced T1‐weighted and study simultaneously to evaluate regional hemodynamics in brain tumors. This method can increase the amount of vascular information and reveals the hemodynamics of the brain tumor and surrounding regions in greater detail. The thesis describes a frequency analysis of cerebrospinal fluid (CSF) flow measured with an ECG gated phase contrast cine‐MRI to clarify the flow dynamics of intracranial CSF in normal pressure hydrocephalus (NPH). This frequency analysis can noninvasively provide a picture of the pathophysiology of NPH and of intracranial changes. Developments of the above imaging and analytical methods in MRI make it possible to obtain more detailed information on the human body. In addition, the thesis describes a new method to accurately measure the modulation transfer function and the signal‐to‐noise ratio in the positive and negative frequencies in MRI with a specially designed phantom. This evaluation method makes it possible to obtain more detail on the image quality of MRI, and is useful in various performance assessments such as the evaluation of pulse sequences.

Imaging myocardial strain

Measuring the local mechanical activity of the heart has lagged behind the measurement of electrical activity due to a lack of measurement tools. Myocardial wall motion abnormalities have been studied for years in the context of regional ischemia. Implanted beads and screws have been used to measure the mechanical activity of the heart in a few isolated regions. Over the past decade, precise and accurate methods for measuring local three-dimensional (3-D) myocardial motion with magnetic resonance imaging (MRI) have been developed using presaturation tagging patterns, velocity encoded phase maps, and displacement encoded phase maps. Concurrently, the quality of cardiac MRI images improved greatly with the use of customized receiver coils and the speed of acquisition has increased dramatically with the advent of undersampling techniques and new generations of MR machines with faster switching gradient coils. The use of these cardiac MRI techniques to produce an image of the local deformation of the heart in the form of a myocardial strain image is described. Using these images, the "mechanical activation" of the heart are defined, that is, the time of onset of contraction. A map of the mechanical activation over the heart is a direct analogy to an electrical activation map of the heart.

Parallel operation of four-switch full-bridge power amplifiers and a unique digital control scheme for advanced magnetic resonance imaging

In recent years, magnetic resonance imaging (MRI) systems have been in demand to diagnose moving parts in the human body such as the heart or the blood in angiography, where images should be taken in milliseconds instead of minutes. Gradient power amplifiers, which are small but vital components for advanced MRI, require higher output power capacity as well as faster rise/fall dynamic response characteristics under a variety of specified current reference signals. This paper presents a novel switch-mode gradient power amplifier using IGBTs which are connected in parallel to a conventional four-switch full-bridge power conversion circuit at their inputs/outputs in order to realize a higher power density. To satisfy the design specifications, which require minimized ripple and improved rise/fall dynamic response characteristics of the current in the gradient coil, a unique digital control scheme based on an optimal type 1 servo system is proposed and described in detail. The effectiveness of the above is discussed and evaluated through computer-aided analysis. It is expected that the proposed techniques will greatly expand the diagnostic targets and improve the image quality of MRI. © 2000 Scripta Technica, Electr Eng Jpn, 132(1): 64–72, 2000

Magnetic resonance imaging of the postoperative hip.

Magnetic resonance imaging (MRI) of the postoperative hip for delineation of various pathological conditions has been established in addition to conventional radiography and computed tomography. MRI provides superior soft-tissue contrast than the other imaging modalities, and it can be used for visualization of structures and pathological entities that cannot be depicted by conventional radiography and computed tomography. These entities include bone marrow changes such as bone marrow edema and avascular necrosis, and infiltration of the bone marrow by tumor recurrence or infections after insertion of metallic osteosynthetic material. The image quality of MRI, which is reduced as result of artifacts caused by metal alloys, can be optimized by using spin-echo or fast spin-echo sequences, and by adapting phase- and frequency-encoding directions in cases where metallic osteosynthetic materials were used. MRI, in addition to computed tomography and conventional radiography, appears to be a valuable tool for imaging the different pathological conditions of the postoperative hip, including after implantation of metallic osteosynthetic material.

Book reviews

This monograph is the proceedings of an American Association of Physicists in Medicine (AAPM) symposium in co-operation with the Bowman Gray School of Medicine held in April 1988, with contributions from a number of academic and commercial magnetic resonance imaging (MRI) scientists and engineers. Image quality in MRI is dependent upon a highly complex inter-relationship between a number of subsystems each requiring optimal adjustment. The book addresses the criteria for objective evaluation of image quality and system performance and is aimed at the clinical physicist with the responsibility for making such measurements. The first chapter provides a brief overview of the influence of particular MRI system components on image quality, whilst the next four chapters describe the main image parameters required for adequate monitoring of the sensitivity and geometric characteristics of a clinical MR imaging system.