Field Magnetic Resonance Imaging Research Articles

T1 and proton density at 7 T in patients with multiple sclerosis: an initial study

Magnetic resonance imaging of cortical lesions due to multiple sclerosis (MS) has been hampered by the lesions' small size and low contrast to adjacent, normal-appearing tissue. Knowing cortical lesion T1 and proton density (PD) would be highly beneficial for the process of developing and optimizing dedicated magnetic resonance (MR) sequences through computer modeling of MR tissue responses. Eight patients and seven healthy control subjects were scanned at 7 T using a series of inversion recovery turbo field echo scans with varying inversion times. Regions of interest were drawn in white matter, gray matter, cortical lesions, white matter lesions and cerebrospinal fluid. White matter and gray matter T1s were significantly higher in MS patients than in controls. Cortical and white matter lesion T1 and PD are also presented for the first time. The advantages of ultrahigh field MR imaging will be important for future investigations in MS research and sequence optimization for the detection of cortical lesions.

Finite-length shim coil design using a fourier series minimum inductance and minimum power algorithm

High field magnetic resonance (MR) imaging and spectroscopy are sensitive to magnetic field inhomogeneities caused by susceptibility differences between tissues, bone, and air. These inhomogeneities cause MR image artifacts and line broadening in MR spectra. To correct for these inhomogeneities, specialized shim coils are used in combination with the usual gradient systems. To produce realistic coils for use in human or small-animal studies, direct control over the length of the coils is necessary. In this article, a simple method for the design of shim coils of arbitrary order and with predetermined length is presented. The method is based on a simple Fourier series expansion of the current density and either power or inductance can be minimized subject to a series of field constraints. The method is mathematically simple, easy to implement and computationally fast. A quantitative comparison of figures of merit for inductance and resistance was made as a function of shim coil length. Coils of 40 cm diameter were designed with lengths of 50, 60, 80, and 100 cm. Comparison of results obtained using the two design methods across all shim axes and coil lengths indicate very little performance difference between the two methods. The decreased complexity of the minimum power designs appears to outweigh any small benefits obtainable using the minimum inductance algorithm for the range of coils investigated. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 37B: 245–253, 2010.

Epstein-Barr virus, 9.4 T MRI and phosphodiesterase inhibitors in multiple sclerosis

This month’s Journal Club will focus on three articles published recently in the field of multiple sclerosis (MS). The first deals with the role of Epstein-Barr virus in the pathogenesis of MS. Don Gilden and co-workers performed a state-of-the-art study, which shows that EpsteinBarr virus is most likely not directly or indirectly involved in the pathogenesis of MS because they did not find any evidence of active Epstein-Barr virus infection. The second article, with a rather misleading title but interesting findings, demonstrates that high field (9.4 T) magnetic resonance imaging reveals changes in cortical grey matter lesions in multiple sclerosis; this, however, was a study on tissue samples and not an in vivo study. The final article gives some preliminary evidence that ibudilast, a phosphodiesterase inhibitor, might have a neuroprotective effect in MS; the conclusion drawn by the authors is, however,

Deep erosions of the palmar aspect of the navicular bone diagnosed by standing magnetic resonance imaging

Erosion of the palmar (flexor) aspect of the navicular bone is difficult to diagnose with conventional imaging techniques. To review the clinical, magnetic resonance (MR) and pathological features of deep erosions of the palmar aspect of the navicular bone. Cases of deep erosions of the palmar aspect of the navicular bone, diagnosed by standing low field MR imaging, were selected. Clinical details, results of diagnostic procedures, MR features and pathological findings were reviewed. Deep erosions of the palmar aspect of the navicular bone were diagnosed in 16 mature horses, 6 of which were bilaterally lame. Sudden onset of lameness was recorded in 63%. Radiography prior to MR imaging showed equivocal changes in 7 horses. The MR features consisted of focal areas of intermediate or high signal intensity on T1-, T2*- and T2-weighted images and STIR images affecting the dorsal aspect of the deep digital flexor tendon, the fibrocartilage of the palmar aspect, subchondral compact bone and medulla of the navicular bone. On follow-up, 7/16 horses (44%) had been subjected to euthanasia and only one was being worked at its previous level. Erosions of the palmar aspect of the navicular bone were confirmed post mortem in 2 horses. Histologically, the lesions were characterised by localised degeneration of fibrocartilage with underlying focal osteonecrosis and fibroplasia. The adjacent deep digital flexor tendon showed fibril formation and fibrocartilaginous metaplasia. Deep erosions of the palmar aspect of the navicular bone are more easily diagnosed by standing low field MR imaging than by conventional radiography. The lesions involve degeneration of the palmar fibrocartilage with underlying osteonecrosis and fibroplasia affecting the subchondral compact bone and medulla, and carry a poor prognosis for return to performance. Diagnosis of shallow erosive lesions of the palmar fibrocartilage may allow therapeutic intervention earlier in the disease process, thereby preventing progression to deep erosive lesions.

Technical Considerations and Potential Clinical Advantages of Musculoskeletal Imaging at 3.0 Tesla

High field magnetic resonance imaging at 3.0 T is rapidly gaining clinical acceptance as the preferred platform for magnetic resonance (MR) imaging. This is spurred in part because advances in the manufacture of magnet technology have brought the cost of 3.0-T magnets into the range of previous 1.5-T machines, as well as ongoing research demonstrating numerous advantages of 3.0 T over 1.5 T in neurological imaging. Many factors are responsible for improved imaging at higher field strength, including increased signal-to-noise and contrast-to-noise ratios. The impact of 3.0-T imaging of the musculoskeletal system has been less dramatic because its optimization is more complicated in the musculoskeletal system than in the brain. Many issues must be considered beyond what might be expected from simply doubling the field strength, including hardware design, protocol modifications because of changes in tissue characteristics at higher fields, artifact reduction, and safety. This article addresses many of these concerns, focusing on techniques to optimize high field MR imaging of the musculoskeletal system.

Correlation between High Field MR Images and Histopathological Findings of Rat Transplanted Cancer Immediately after Partial Microwave Coagulation

To investigate the immediate effects of microwave coagulation on rat tumors in various magnetic resonance (MR) images at high magnetic field strength using histopathological examinations as reference. Tumors implanted in rat femurs were partially thermocoagulated by microwave. Immediately after, T1- and T2-weighted images, diffusion-weighted images (DWIs), and contrast-enhanced T1 weighted images (CE-T1WIs) were acquired with a 7-tesla MR scanner. After measurements, tumors were examined histopathologically with hematoxylin-eosin (HE) staining and histochemically for acid phosphatase activity. Without contrast, boundaries of coagulated areas were unclear on MR images, including apparent diffusion coefficient (ADC) maps. CE-T1WIs clearly showed immediate contrast enhancement of untreated areas of tumor, and the area of enhancement gradually enlarged in 5 min. Quantitative analyses were conducted by classifying tumor areas by contrast enhancement results. Signal intensities of the areas in the MR images showed no significant differences, but at the periphery, ADC values were significantly higher in areas with delayed enhancement than those with immediate enhancement. Compared with histopathological findings, with microwave thermocoagulation, increased ADC value seemed to derive from collection of extracellular fluid in the outer zone, where acid phosphatase activity was attenuated. ADC values in the areas with delayed enhancement of CE-T1WIs were higher than those in non-affected areas, but MR images could not show areas of coagulation within tumors. Clear detection of the boundaries of coagulated areas required contrast enhancement, even at magnetic field strength of 7T.

Development of a Patch Antenna Array RF Coil for Ultra-high Field MRI

In radiofrequency (RF) coil design for ultra-high-field magnetic resonance (MR) imaging, short RF wavelengths present various challenges to creating a big volume coil. When imaging a human body using an ultra-high magnetic field MR imaging system (magnetic flux density of 7 Tesla or more), short wavelength may induce artifacts from dielectric effect and other factors. To overcome these problems, we developed a patch antenna array coil (PAAC), which is a coil configured as a combination of patch antennas. We prototyped this type of coil for 7T proton MR imaging, imaged a monkey brain, and confirmed the coil's utility as an RF coil for ultra-high-field MR imaging.

Development of an MR-Compatible ECG Amplifier

The purpose of the present study is to develop a magnetic resonance (MR)-compatible electrocardiogram (ECG) amplifier. If ECG signals are measured simultaneously with the acquisition of MR images, there can be a mutual interference effect. The present system was designed to block noises caused by the main magnetic field, gradient magnetic field and radiofrequency (RF) pulse when MR images are acquired. It uses analogue elements in order to remove any possible effect on MR images. In addition, a radio-frequency-interference (RFI)-free optical data link using the pulse-width modulation (PWM) technique is adopted in order to transmit ECG signals measured inside the MR room.

Feasibility of MR Imaging–guided Percutaneous Drainage of Pancreatic Fluid Collections

To assess the feasibility and safety of magnetic resonance (MR) imaging-guided percutaneous drainage of pancreatic fluid collections in an open configuration low field MR imaging system. Ten patients with pancreatic fluid collections were examined prospectively. Five of the fluid collections were symptomatic pseudocysts and five were pancreatic abscesses. All percutaneous drainages were performed solely under MR imaging guidance with a 0.23-T open configuration C-arm shaped MR imaging scanner with interventional optical tracking. Every step of the procedure was monitored using balanced fast field echo sequences. In each case, the drainage of the fluid collection was performed with a MR imaging-compatible drainage kit using the Seldinger technique. The kit included an 18-gauge needle, a 0.035-inch stiff guide wire, 6-F and 8-F dilators, and an 8-F pigtail drainage catheter. All drainage catheters could successfully be placed into the pancreatic fluid collections under MR imaging guidance. Visualization of the needle, dilator, and drainage catheter was excellent. However, visualization of the guide wire was suboptimal. The mean time needed for the MR-guided drainage procedure was 44 minutes. No immediate complications occurred. The clinical success rate of the percutaneous drainage was 70%; three patients were subsequently treated surgically. There were no deaths. The average duration of catheterization was 40 days. MR imaging-guided percutaneous drainage of pancreatic fluid collections is feasible and safe. The presented technique has limitations--lack of real-time imaging control and small selection of MR imaging-compatible devices--that necessitate further technical developments before the procedure can be recommended for routine clinical use.

High field body MR imaging: Preliminary experiences

Whole-body magnetic resonance (MR) scanners at high field strengths (≥3 T) have been introduced in expectation of a larger signal-to-noise ratio (SNR), which would decrease the length of scan time or improve the spatial resolution. In this paper, the advantages and disadvantages of the high field MR imaging are discussed. Although the building of the radio frequency (RF) coil, safety and the specific absorption rate (SAR) are issues, the application of high field MR imaging is promising. The optimization of all parameters including injection rate of Gd–DTPA is necessary for high field MR imaging to obtain maximal results; however, we hope that high field MR imaging can be used in routine clinical applications in the future.

Relative assessment of brain iron levels using MRI at 3 tesla

High field MR (magnetic resonance) images can be made sensitive to the relative concentration of tissue iron through the use ofT*2-weighted contrast. This has enabled tissue iron levels to be assessed noninvasively by quantification of transverse relaxation rates. High field MRI may provide a new method to investigate neurological diseases which result in alteration of brain iron levels in specific areas of the human brain. Parkinson's disease (PD) results in an increase in iron concentration within the lateral region of the substantia nigra (SN), and provides one potential application of this methodology. Preliminary results of our findings are that there is a significant difference in SN iron levels in PD patients compared with age-matched controls, assessed by quantification of the reversible line-broadening component of transverse relaxation rate,R′2.

Low field (0.02 T) MR imaging of the whole body.

The diagnostic utility of extremely low field magnetic resonance (MR) (0.02 T) was evaluated in various parts of the body outside the head. The areas investigated included the spine (104 patients), kidneys (19 patients), female pelvis (21 patients), and hips (15 patients). The results were compared with those of other imaging modalities, e.g., ultrasound, CT, and scintigraphy. Available pulse sequences and typical examination times for different types of studies are reported. Poor spatial resolution and long imaging times limit the application of this technique to the whole body. Due to poor signal-to-noise ratio, image quality is not comparable with that obtained at higher field strengths. In spite of these restrictions it was possible to demonstrate lumbar disk herniations, to differentiate renal and ovarian cysts from tumors, and to detect early avascular necrosis of the femoral head. Extremely low field MR imaging may eventually be used in screening spinal pathology and in the diagnosis of early avascular necrosis of the femoral head. At present, this technique can not replace ultrasound or CT in abdominal examinations.

The utility of contrast media in MRI at 0.02 T. A preliminary report.

The effect of 0.1 mmol gadolinium-DTPA on ultralow field magnetic resonance (MR) images of intracranial neoplasms was studied on 20 patients. Tumors that characteristically were enhanced markedly on computed tomography (CT) (meningiomas, acoustic neuromas) demonstrated moderate to marked contrast enhancement with inversion recovery sequences and with saturation recovery sequences with a short repetition time. Standard spin-echo sequences failed to show any contrast effect. The enhancement of gliomas and metastases was weak and often absent. Contrast enhancement, when present, was helpful in delineating the tumor from surrounding edema but the differentiation of living from necrotic tumor was not successful because of low signal intensity. The low effect of gadolinium-DTPA at 0.02 T may be due to a relatively higher T2 weighting of available pulse sequences. The relaxing effect of macromolecules may also change the proportional effect of paramagnetic substances in an ultralow magnetic field.

Parkinson plus syndrome: diagnosis using high field MR imaging of brain iron.

The distribution of iron in the brain was analyzed using high field strength (1.5 T) magnetic resonance (MR) imaging in 14 healthy control individuals and six patients with Parkinson plus syndromes (multisystem atrophy and progressive supranuclear palsy) who were unresponsive to antiparkinsonian therapy. The normal topographic distribution of iron in the brain as indicated by high field MR images coincided precisely with the distribution of iron in the brain as determined by Perls staining for ferric iron. In Parkinson plus syndromes, there were abnormally increased concentrations of iron (decreased T2 relaxation times) in the putamen, and less prominent increases in the caudate nucleus and lateral pars compacta of the substantia nigra. In high field strength MR images of normal patients, the decreased signal intensity in the globus pallidus is more prominent than that of the putamen. In MR images of patients with Parkinson plus syndromes, the decreased signal intensity of the putamen is more prominent than that of the globus pallidus.

Choroidal melanomas: correlation of NMR spectroscopy and MR imaging.

Six freshly enucleated, unfixed human eyes with choroidal melanomas were imaged on a 1.4-T superconducting magnetic resonance (MR) imaging system. Immediately thereafter the eyes were sectioned, and tumor samples were removed for study on a variable-field (0.19-1.4 T) nuclear MR spectroscopy unit. Shorter T1 and T2 relaxation times were observed in those tumors with the greater concentrations of melanin. This is believed to result from the paramagnetic effect of radicals known to exist in melanin. High magnetic field MR imaging can enable one to distinguish between pigmented melanomas; proteinaceous effusions; fresh and subacute hematomas; and nonmelanotic tumors, including amelanotic melanomas; but may not enable melanotic melanoma to be distinguished from fat or amelanotic melanoma from other nonpigmented tumors.

High field MR imaging of cerebral venous thrombosis.

High field magnetic resonance (MR) imaging enables us to demonstrate the evolution of cerebral venous thrombosis. Initially, absence of a flow void and collateral venous channels are seen on T1 weighted images (WI). On T2WI thrombus appears hypointense. Hyperintensity is noted in an intermediate stage of thrombosis first on T1WI and later on T2WI. In the late stages recanalization of the vessel occurs with reappearance of the flow void. These findings are specific for venous thrombosis. High field MR may be the imaging modality of choice in the diagnosis of venous thrombosis.

Intracranial hematomas studied by MR imaging at 0.17 and 0.02 T.

The contrast in magnetic resonance (MR) images relies mainly on the relaxation time differences between the tissues. The relative differences in relaxation times T1 are bigger at lower field strengths, although the absolute values of T1 are smaller. A shorter T1 is also advantageous for the contrast of the T2 and proton density weighted images because of the more complete recovery of the spin system during the repetition time TR. Scrutiny of the clinical results of MR shows some unsolved problems in the specificity of diagnosing fresh intracranial hematomas. Low field MR imaging at 0.02 T seems to offer new vistas in this sense. Fresh subdural hematoma was more easily detected and differentiated at 0.02 T than at 0.17 T. The T2 of fresh intracranial hematomas was rather short compared with cerebrospinal fluid and edema and, unlike T1, was not highly dependent on magnetic field strength. The different visualization of acute versus late intracerebral hematoma and the changes during the resorption were demonstrated in follow-up studies of two patients at 0.17 T and of one at 0.02 T. In one patient the same lesion was imaged successively at both field strengths, showing the divergent contrast in the inversion recovery images at 0.02 and 0.17 T.

Use of spherical receiver coils in MR imaging of the brain.

Spherical receiver coils suitable for magnetic resonance (MR) imaging of the brain are described. Spherical geometry permits high field uniformity with close proximity of the coil to the patient's head giving a good filling factor. Patient acceptability has been good in more than 800 examinations of the brain. Spherical receiver coils of appropriate size can provide a simple and worthwhile improvement over conventional saddle designs for low field MR imaging.