Solenoid surface coils in magnetic resonance imaging

The combined use of DTI and MR elastography for monitoring microstructural changes in the developing brain of a neurodevelopmental disorder model: Poly (I:C)-induced maternal immune-activated rats

Early neuropathology mechanisms in neurodevelopmental disorders are partially understood because routine anatomical magnetic resonance imaging (MRI) cannot detect subtle brain microstructural changes in vivo during postnatal development. Therefore, we investigated the potential value of magnetic resonance elastography (MRE) and diffusion tensor imaging (DTI) in a rat model of neurodevelopmental disorder induced by maternal immune activation. We studied 12 offspring of mothers injected with polyriboinosinic-polyribocytidylic acid (poly (I:C), 4 mg/kg) on gestational day 15, plus 8 controls. T2-weighted anatomical MR images, MRE (800 Hz) and DTI (30 gradient directions, b = 765.8 s/mm2, 5 images, b = 0 s/mm2) were collected when the rats were 4 and 10 weeks old, and results were compared with histological analysis performed at week 10. Ventricles were ~1.4 fold larger from week 4 in poly (I:C) rats than in controls. No other morphological abnormalities were detected in poly(I:C) rats. At week 4, larger ventricles were correlated with lower external capsule fractional anisotropy and internal capsule radial diffusion (Pearson, r = -0.53, 95% confidence intervals (CI) [-0.79 to -0.12], and r = -0.45, 95% CI [-0.74 to -0.01], respectively). The mean and radial diffusion of the corpus callosum, the mean and axial diffusion of the internal capsule and the radial diffusion properties in the external capsule increased with age for poly (I:C) rats only (Sidak's comparison, P<0.05). Cortical stiffness did not increase with age in poly (I:C) rats, in contrast with controls (Sidak's comparison, P = 0.005). These temporal variations probably reflected abnormal myelin content, decreased cell density and microglia activation observed at week 10 after histological assessment. To conclude, MRE and DTI allow monitoring of abnormal brain microstructural changes in poly (I:C) rats from week 4 after birth. This suggests that both imaging techniques have the potential to be used as complementary imaging tools to routine anatomical imaging to assist with the early diagnosis of neurodevelopmental disorders and provide new insights into neuropathology.

Vertical Field MR Imaging of Upper Thorax and Spine in Small Children

To improve image quality in a vertical field MR imaging unit, operating at low field strength (0.3 T), we have designed a half-elliptical coil for use in the upper thoracic region of small children. Our intention was also to shorten the examination time, which until now has been long, because several scans with different coils have been necessary to cover the thoracic region. The experimental coil is designed so that a child's shoulders fit into the central region. The coil consists of 2 serially connected cable-loops, mounted on a foam rubber vest. The coil performance was tested in a phantom and improvements relative to standard coils were demonstrated in in vivo studies. The results indicate that by using the half-elliptical coil, the signal-to-noise (S/N) ratio can be improved by a factor of 2 to 3 in the thoracic region of a child.

Coil Selection for Magnetic Resonance Imaging of the Cervical and Thoracic Spine Using a Vertical Magnetic Field

In order to optimize the coil selection for cervical and thoracic spine imaging the signal characteristics of two different solenoidal surface coils (15 cm and 30 cm diameter, respectively) as well as the head coil and body coil were determined using a 0.3 T MR scanner with a vertical magnetic field. Signal-to-noise ratio curves were obtained for each coil using tube phantoms and a human-like phantom. The findings were compared with images obtained in two healthy volunteers. The head coil was found to be superior for imaging of the cranio-cervical junction while the 15 cm surface coil gave better results in the remaining part of the cervical spine and the upper thoracic spine. The body coil was superior for imaging of the thoracic region at the level of the shoulders (T4-T6) but the 30 cm surface coil was better for the more caudal part of the thoracic spine. Combined phantom and in vivo studies are also recommended for evaluation of future, improved coils.

Emergency Magnetic Resonance Examination of Patients with Spinal Cord Symptoms

Eighteen consecutive patients with spinal cord symptoms of sudden or relatively sudden onset were examined with magnetic resonance imaging (MRI). The examinations were performed on a 0.3 tesla permanent/resistive imaging system using solenoidal surface coils. MRI revealed epidural tumour in five patients, intramedullary tumour in one, epidural abscess in one, myelitis in two, spontaneous intraspinal epidural haematoma in two, disc herniation in two, traumatic lesions in four and no abnormality in one patient. MRI was found to be capable of non-invasively and painlessly detecting and exactly defining the extent of intraspinal and paraspinal lesions. In some cases the nature of the lesion could be inferred from specific signal characteristics, which is a unique property of MRI. The results strongly suggest that MRI is superior to myelography and other imaging methods and should be regarded as the examination of choice in the emergency examination of patients with spinal cord symptoms.

Low Flip Angle Gradient Echo Magnetic Resonance Imaging of the Cervical Spine at 0.3 Tesla

The influence of flip angle and TR on signal to noise ratio and contrast between cerebrospinal fluid (CSF) and cord was evaluated in cervical spine imaging in 5 volunteers, using gradient echo technique. All experiments were performed on a 0.3 tesla Fonar β-3000 M scanner using solenoidal surface coils. The most useful sequence was considered to be TR/TE=300/12 ms and 10° flip angle. This sequence provided images with a ‘myelographic appearance’ with good delineation of cord, CSF and epidural space. The grey and white matter was also regularly visualized. The acquisition time was considerably shorter than would have been necessary if a long TR/TE spin echo sequence had been used to obtain the same contrast pattern and the sequence was not as sensitive to motion as was the spin echo sequence. The sequence was also evaluated in 10 patients with degenerative disease and in 5 with lesions in the cord. The gradient echo sequence was found to be equal to or better than short and long TR/TE spin echo sequences in demonstrating narrowing of the spinal canal and cord lesions. The drawback is the limited signal to noise ratio.

A combined solenoid-surface RF coil for high-resolution whole-brain rat imaging on a 3.0 Tesla clinical MR scanner

Rat brain models effectively simulate a multitude of human neurological disorders. Improvements in coil design have facilitated the wider utilization of rat brain models by enabling the utilization of clinical MR scanners for image acquisition. In this study, a novel coil design, subsequently referred to as the rat brain coil, is described that exploits and combines the strengths of both solenoids and surface coils into a simple, multichannel, receive-only coil dedicated to whole-brain rat imaging on a 3.0 T clinical MR scanner. Compared with a multiturn solenoid mouse body coil, a 3-cm surface coil, a modified Helmholtz coil, and a phased-array surface coil, the rat brain coil improved signal-to-noise ratio by approximately 72, 61, 78, and 242%, respectively. Effects of the rat brain coil on amplitudes of static field and radiofrequency field uniformity were similar to each of the other coils. In vivo, whole-brain images of an adult male rat were acquired with a T(2)-weighted spin-echo sequence using an isotropic acquisition resolution of 0.25 x 0.25 x 0.25 mm(3) in 60.6 min. Multiplanar images of the in vivo rat brain with identification of anatomic structures are presented. Improvement in signal-to-noise ratio afforded by the rat brain coil may broaden experiments that utilize clinical MR scanners for in vivo image acquisition.

Development of a solenoid RF coil for animal imaging in 3 T high-magnetic-field MRI

The purpose of this study was to develop a solenoid coil for use with small animals in a 3 Tesla horizontal magnetic resonance imaging (MRI) system, and to investigate image quality by examination of parameters such as signal-to-noise ratio (SNR) and Q-factor. A receiver solenoid coil was formed by winding three separate coils of copper tape around an acryl cylinder. The cylinder was supported at each end. A euthanized rat weighing 240 g was used as a subject animal for imaging. A cylindrical plastic tube containing a solution of 0.7 g/L CuSO(4) was used as a phantom. Measured SNRs were 985 in the phantom image 995 in the rat. The Q-factor was 89 in the phantom and 84 in the rat, in the loaded condition. The homogeneity of the radiofrequency (RF) field was good and the resolution of the image was sufficient to distinguish internal organs from one another in the abdomen of a rat. This study has demonstrated that a solenoid coil may be used to produce good quality images of small animals.

Improved receiver coil for upper thoracic spine imaging in a vertical magnetic field

To improve image quality in the upper thoracic spine, an anatomically shaped copper wire loop coil, made to fit over the patient's shoulders, was constructed. The coil was permanently mounted on a foam-rubber vest to facilitate attachment to the patient. Phantom and in vivo studies of the performance of the coil in healthy volunteers showed as much as a two times greater signal-to-noise ratio relative to that of standard coils for the upper thoracic spine. In a patient with lesions in the upper thoracic cord, the coil gave better image quality in the region of interest than did the standard coils. The coil has been integrated into the authors' routine imaging equipment and has been the coil of choice for imaging of the upper thoracic spine on their 0.3-T vertical field system.

Sparing of visual field perception in neonatal but not adult cerebral hemispherectomized cats. Relationship with oxidative metabolism of the superior colliculus

Following cerebral hemispherectomy and using a lick-suppression test, adult-lesioned cats showed a complete contralateral hemianopsia, while neonatal-lesioned animals reliably responded to stimuli presented out to 45 degrees in the visual field contralateral to the lesion. In adult-lesioned cats oxidative metabolism of the superior colliculus ipsilateral to the hemispherectomy was markedly depressed as compared to the contralateral colliculus. In contrast, in the neonatal-lesioned cats this metabolic imbalance was mild. We propose that this ipsilateral depression of oxidative metabolism reflects the loss of excitatory corticotectal input which has been postulated as being partly responsible for the hemianopsia following a unilateral visual cortex ablation. Here we demonstrate, in addition, that this phenomenon is affected by developmental factors and suggest that differential age-at-lesion anatomical effects may also be involved.

Low cost MRI of the paranasal sinuses

The use of automated tuning, automated MR protocol sequences, surface coils, and grouping of patients with similar examinations, patient throughput can be greatly accelerated for MRI scanning. These measures coupled with high volume scanning permit significant reductions in cost so that MRI can be performed at a price range equal to or less than plain films for some examinations. The ability of MRI to perform axial, sagittal and coronal scans with excellent tissue contrast raises the possibility of substituting MRI for sinus x-rays. Based on our preliminary experience we have the opinion that MR is equal to/or better than CT in this region. This preference is currently being investigated. Only the cost issue will be addressed in this report.

Magnetic resonance imaging of head and neck tumors

Magnetic resonance imaging has revolutionized the imaging of head and neck malignancies and is now rapidly replacing computed tomography as the study of choice in the majority of lesions in the head, neck, larynx, hypopharynx, oropharynx, paranasal sinuses, paranasopharynx, and skull base. CT scanning can be used in the same region; however, inflammation obtained in CT is not as clearly demonstrated and in some situations, such as malignancies of the tongue, the lesions may be missed entirely. There are still occasional difficult clinical problems when the two studies are complementary, but this situation will definitely be rare.

Automated MR imaging protocols for improved patient throughput

The total time spent on each patient during an MR study determines patient throughput and ultimately influences MR scan charges. Actual data acquisition often represents only a small portion of this examination time. Other functions such as patient loading and unloading, positioning, coil placement, scan setup, and image reconstruction time also contribute to the total examination time. We describe a technique for creating automated imaging protocols for standard MR examinations which optimize these functions, thereby decreasing operator interventions and shortening the overall examination time. A prototype sequence for examining the larynx and neck was developed which allowed three sequential multiple slice acquisitions to be obtained in three orthogonal planes with a single operator setup step. All tuning and scan placement after this first step was done under computer control. The new sequence allowed larynx and neck studies to be performed in less than 30 min for spin echo and under 16 min for field echo instead of the 50-60 min required by previous methods. The new technique should be useful for improving patient throughput for a cariety of standardized examinations including the temporal bone, head and neck, joints, and other regions.

Parallel-plate split-conductor surface coil: analysis and design

A split-conductor circular parallel-plate resonator with one end shorted to ground can be used to one's advantage as a surface coil for NMR imaging experiments. It can be easily constructed by a printed-circuit method and possesses a relatively high loaded Q with negligible frequency detuning. From the proposed equivalent circuit, an equation was derived that relates the self-resonance frequency of the resonator to the split distance measured from a reference point. An example of a 101-mm-diameter surface coil operating at 26 MHz for 31P in vivo spectroscopy is given to illustrate the concordance between calculated and experimental results. Complementary equations and formulas are also included to assist researchers in designing their own antennas to meet specific requirements.

An optimized head coil design for MR imaging at 0.15 T

Receive-only head coils employing a modified solenoid design were developed for a 0.15-T imager with horizontal main static and radiofrequency transmit fields. This coil design uses three or five identical horseshoe-shaped elements, constructed of 1/2-in. outside diameter copper tubing, lying one above the other in equally spaced horizontal planes. Coils of this design were compared to the half-saddle coil supplied with our imager and to the Hammersmith Hospital spherical coil (G.M. Bydder et al., J. Comput. Assist. Tomogr. 9, 987 (1985]. Compared to the half-saddle coil, one of our new head coils has the same field of view while improving the signal-to-noise ratio (S/N) by 80% in the upper brain and 20% in the cervical vertebrae; compared to the spherical coil the S/N was up to 60% lower but the field of view was larger and patient comfort was better.