Solenoid surface coils in magnetic resonance imaging

A nonplanar solenoidal surface radiofrequency coil is used as a receiver with a conventional transmitter coil in a magnetic resonance imaging system. The improved signal-to-noise ratio, compared with that of conventional fixed saddle or solenoid receiver coils, permits higher resolution imaging and thinner image sections. In addition, the problem of signal dropoff that occurs in deep structures with planar and other noncircumferential surface coils is eliminated. Solenoid surface coils are particularly useful in imaging deep structures in anatomic regions that do not fit standard head and body coils, such as the neck, knees, and other smaller body parts.     

In vivo method for correcting transmit/receive nonuniformities with phased array coils.

Phased array coils are finding widespread applications in both the research and the clinical setting. However, intensity nonuniformities with such coils can reduce the potential benefits of these coils, particularly for applications such as tissue segmentation. In this work, a method is described for correcting the nonuniform signal response based on in vivo measures of both the transmission field of body coil and the reception sensitivity of phased array coils, separately. For a uniform phantom, the reception sensitivity can be calculated using both Bloch equations and transmission field maps. For a heterogeneous object such as a brain, a minimal contrast acquisition must be obtained to map the receiver nonuniformities. This transmit field/receiver sensitivity (TFRS) approach is compared with the standard methods of using the body coil to obtain a reference scan and low-pass filtering. The quantitative comparison results shows that the TFRS approach provides superior results in correcting intensity nonuniformities for a uniform phantom. This approach reduces the ratio between signal intensity SD of an image and its mean intensity from approximately 21% before correction to 13% after correction. Results are also shown demonstrating the utility of this approach in vivo with human brain images. The method is general and can be applied with most pulse sequences, any coil combination for transmission and reception, and in any anatomic region.     

High-resolution MR imaging using loop-gap resonators. Work in progress

Representative results using loop-gap resonator (LGR) configurations for local high-resolution MR imaging (0.6 mm X 0.6 mm pixel size with 3-mm section thickness) are presented. Four geometries were employed: flat single loops, coplanar (planar pair) loops, coaxial (axial pair) loops, and symmetrically angled (butterfly pair) loops. The LGRs were used as receiver coils with the body coil as transmitter. Butterfly pairs and planar pairs are intrinsically decoupled from a transmitted field of arbitrary orientation, whereas the single loops and axial pairs are decoupled from a field that is linearly polarized and orthogonal to its axis. The coils were evaluated using computer-generated and experimentally determined field distributions and by the imaging of human subjects on a 1.5-T MR system. Matching the region of sensitivity of the coil to the region of anatomic interest is emphasized.     

Dual cervical thoracic coil for spine magnetic resonance imaging

The need for repositioning of surface coils and patients in MR examinations of the cervical and thoracic spine prolongs examination time. A new receiver design is proposed which overcomes this problem. The device is composed of two actively decoupled receiver coils mounted on the frame of a Philadelphia collar. These coils may be used separately to image either the thoracic or cervical spine or together to produce larger field-of-view images of the combined region. Signal-to-noise ratios of the separate cervical and thoracic spine images are not degraded as a result of mounting the receivers together. The full cervical and thoracic region is shown to be imaged at a signal-to-noise ratio significantly higher than that afforded by the body coil. A retrospective review of our case load suggests that a time saving could be achieved in approximately 1/3 of spine examinations by using this coil.     

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.     

Radio-frequency coil selection for MR imaging of the brain and skull base

Radio-frequency coils play a crucial role in the quest for optimal magnetic resonance (MR) image resolution. Given the growing variety of specialized coils available for neuroradiologic imaging applications, it is critical that radiologists use a coherent strategy for successfully matching these coils to specific imaging situations. First, fundamental concepts of coil design are reviewed. Subsequently, a coil-selection algorithm for neuroimaging applications is described. The algorithm uses the patient's clinical history to derive a region of interest, a desired spatial resolution, and a desired contrast resolution. These factors are then used to impose anatomic coverage and imaging protocol constraints on the set of available coils. Finally, coil selection is further refined according to patient tolerance factors. The following coils are considered for use with a 1.5-T superconducting MR imager; namely, quadrature birdcage head, neurovascular phased-array, and dual single-circular-element coils, as well as investigational coils that have not yet been approved by the U.S. Food and Drug Administration: reduced-volume birdcage end-cap, temporal lobe phased-array, carotid artery phased-array, coils. Rationales are discussed regarding appropriate coil selection for screening whole brain and imaging brainstem, cranial nerves, orbits, cerebral cortex, mesial temporal lobes, and internal auditory canal, and for MR angiography.     

Application of anatomically shaped surface coils in MRI at 0.5 T

The construction and application of eight different MRI surface coils is described. The coils consist of an anatomically shaped copper wire loop as an antenna and a printed circuit board containing electronic components for tuning and matching. The electronic device for tuning and matching is interchangeable between the various coils. Surface coils for signal detection yield images with high signal-to-noise ratio in comparison to the usual saddle-shaped head or body coils. The sensitivity of a surface coil decreases with increasing distance between the coil and the object of interest and therefore the coils are constructed to fit the anatomical structure under examination as well as possible. The application of dedicated surface coils for superficial structures in the body extends the possibilities of the MRI system. Photographs of the coils positioned on the body and MR images of volunteers and patients are shown.     

Technical considerations for the use of surface coils in MRI

The signal-to-noise response characteristics for two surface coils of different construction geometries (a single-turn planar coil and a single-turn saddle-shaped coil) were measured and compared with head and body coils. Measurements were made at different gradient magnifications (0, 20, 30, and 40%, relative to the head coil) and with different numbers of signal averages (3, 8, 12, and 18). The signal-to-noise curves were used to guide the selection of surface coils for use in clinical studies. This technique is useful in determining the optimal technique for specific clinical problems evaluated by surface-coil imaging. For the planar (flat) surface coils, the signal-to-noise per pixel was found to be superior to the conventional head coil at depths equal to or less than 5 cm. For the saddle coil, signal-to-noise per pixel was superior to the head coil for depths below 8.5 cm for magnifications up to 30%. For the 40% magnification, the depth at which the signal-to-noise was equal to the head coil decreased to about 6 cm. Surface coils have demonstrated a marked improvement in signal-to-noise relative to conventional head and body coils for superficial structures.     

Simultaneous image acquisition utilizing hybrid body and phased array receiver coils.

In clinical MR imaging the design and selection of receiver coil is an important step in ensuring the highest image quality. Often this choice is based on selecting a receiver coil characterized by high spatial uniformity such as the body and head volume receiver coils or a surface coil (or array of coils) that provide high signal-to-noise ratio (SNR). In the past, it has been difficult to accomplish both high SNR and spatial uniformity as both coil types achieve one of these characteristics at the expense of the other. The purpose of this study was to achieve both high SNR and spatial uniformity through the simultaneous acquisition of the MR signal using the body and a surface coil array. Results indicate that this hybrid system can provide uniformity and SNR values comparable to those achieved by the body and surface coil arrays, respectively.     

Mitigation of susceptibility-induced signal loss in neuroimaging using localized shim coils.

Correction of magnetic field distortions is essential for obtaining accurate brain blood-oxygen-level-dependent functional magnetic resonance imaging (fMRI) activation maps. The present work introduces an active shimming method that utilizes the magnetic field generated by resistive shim coils placed in the mouth to locally homogenize the magnetic field in the inferior portion of the frontal lobe, where the field is most seriously distorted. The shimming field can be optimized in situ patient by patient for the region of interest of the scanner operator's choice. The method at 1.5 T is shown to be effective in reducing field inhomogeneity and in recovery of fMRI signal. For example, in a region of interest approximately of 149 cm3, a coil of simple geometry can reduce the root mean square of the magnetic field by more than 50% and the recovered signal increases the extent of activation detected in a breath-holding fMRI experiment.     

SENSE imaging with a quadrature half-volume transverse electromagnetic (TEM) coil at 4T

PURPOSE: To demonstrate the feasibility of high-field SENSE imaging of large objects, such as the human head, using a semicircular (half-volume) coil for both transmission and multi-channel reception. MATERIALS AND METHODS: As a proof of concept, we present experimental data obtained using a seven-element half-volume (180 degrees of arc) transmit/receive quadrature transverse electromagnetic (TEM) coil. SENSE images of the human brain were acquired with a reduction factor of R=2, using two degenerate linear modes of the same coil as independent receive channels at 4T. Since the need for additional hardware (i.e., a separate set of receive coils) is eliminated, the design can be substantially simplified. RESULTS: The experimental data demonstrate that linear modes of the half-volume TEM coil have essentially no noise correlation, and their sensitivity profiles satisfy the requirement for small g-factors. Also, this type of coil provides efficient transmission with a relatively large uniform region and a reception profile that is more uniform than that of the surface coils. CONCLUSION: We demonstrate the feasibility of SENSE imaging using a half-volume coil. Half-volume coils allow reduced total power deposition compared to full-volume coils, and may replace the latter in body imaging applications in which the target region of interest (ROI) is smaller than the entire torso.     

Computer modeling of surface coil sensitivity

A simple model is presented for the calculation of relative signal-to-noise (S/N) ratios of coils of different sizes and configurations when applied to in vivo MRS. Axial symmetry is assumed, which enables rather simple expressions to be used for the calculation of coil loading by the tissue. The model is calibrated to experiments through measurement of the loaded and unloaded coil Q's. Applications of the model demonstrate that for small, superficial regions of interest (ROI), small surface coils can provide a S/N much improved over that of a larger coil. However, for very deep ROIs, larger coils or coils producing uniform B1 provide improved S/N.     

MRI of the floor of the mouth, tongue and orohypopharynx

Magnetic resonance is the imaging modality of choice for studies of the orohypopharynx, floor of the mouth, or tongue base. The superiority of MRI soft tissue contrast can demonstrate intra- and extraorgan spread of tumor beyond that of CT. Use of T1- and T2-weighted pulse sequences allows better discrimination of pathologic masses from fat or muscle than does CT. Multiplanar capabilities allow ease of examination in the preferred planes. Various sequences or planes of imaging may be chosen to tailor the examination to the anatomic region of interest. The use of Gd-DTPA with T1-weighted images should further improve diagnostic precision of tumor location and extension and may replace the need for the longer T2-weighted sequences. Gadolinium may help differentiate tumor recurrence from fibrosis in the post-radiation patient. New improvements in surface coil technology, motion and flow compensation imaging strategies, faster scan times, and spatial resolution will further advance MRI as the modality of choice for assessment of oropharyngeal, mouth, and tongue soft tissue masses.     

Noise correlation

Calculations and experiments that provide support for our previously stated theorem are presented: If two coils simultaneously receiving magnetic resonance signals from the same anatomic region exhibit zero mutual inductance, there can be no correlation of the noise. It is shown that correlation does not exist even in the presence of mutual inductance unless the two signal paths have amplifiers prior to signal combination. It is further found that in the presence of mutual inductance with ideal amplifiers (0 dB noise figure) in the two signal paths, there is no correlation of noise. In order to satisfy the condition of zero mutual inductance, it may be necessary to employ a decoupling circuit external to the body. A novel coil assembly, which was used in the experiments, places a single-turn surface coil in the median plane between the two loops of a counter rotating current coil. The signal-to-noise ratio can be improved by combining signals. This is in analogy to quadrature receiving coils, where the mutual inductance is zero because vector reception fields are perpendicular. In the present geometry, vector reception fields are collinear, but are parallel and antiparallel on the two sides of the coil assembly, resulting in zero mutual inductance.     

MRI of the cervical spine. Creation of a surface coil. Technical and clinical results

Cervical myelopathy represents a good indication for study by Magnetic Resonance Imaging (MRI). The MRI examination may be performed without hospitalisation and without any pain or risk for the patient. It often gives sufficient information to decide whether to proceed with surgical intervention, after imaging on standard plain films and ever before cervical myelography. An efficient study of the cervical spinal cord requires special surface coils adapted to this region. We have developed a surface coil, working as a receiver, inductively coupled, tuned and matched all at once, and easy to use. The concave form of this coil has been studied so as to be comfortable for all patients. It can be directly connected to our Thomson CGR machines (Magniscan 5000). In continuous routine use for 6 months, without any problems, it has been found to be very reliable. We present here some results on different types of myelopathy and discuss methodological aspects concerning the choice of acquisition parameters in the examinations. The simplicity of its realisation and the low cost leads us to believe that it will be possible to construct other surface coils convenient on many other parts of the body.     

Methods: MRT

MRI has become the imaging method of choice in special regions of the head and neck (e.g. nasopharynx, oropharynx, oral cavity, floor of the mouth). Superconducting MR-equipment with field strengths of 1.0-1.5 T are appropriate for the evaluation of the head and neck region. Signal acquisition is optimal with circular polarized head coils or with specially designed surface coils; the body coil is insufficient.When imaging tumors we need T1 contrast, T2 contrast and contrast medium information (enhancement information). For the T1 contrast T1-spin-echo is and remains the best sequence. For T2-contast T2 turbo-spin-echo with fat suppression has replaced the T2 spin-echo sequences because it is faster and shows good contrast between tumor and saturated fat tissue. Fat saturated T1 turbo-spin-echo enables best tissue contrast after Gd-DTPA application.     

Magnetic resonance imaging with adiabatic pulses using a single surface coil for RF transmission and signal detection

In order to overcome the problems that arise from nonuniform B1 fields, there has been interest in developing pulses that are insensitive to large variations in RF power. Pulses derived from adiabatic passage principles that can execute spin inversion, excitation, and 90 degrees and 180 degrees plane rotations in the presence of B1 inhomogeneities have recently been described. When driven with optimized modulation functions, these pulses can execute uniform excitation, refocusing, and slice-selective inversion over a 10-fold or greater variation in B1 magnitude. This insensitivity to B1 strength enables the execution of T1- and/or T2-weighted spin-echo imaging experiments using coils, such as the surface coil, with extremely inhomogeneous B1 profiles. We have successfully acquired images with these pulses at 200 MHz using a single surface coil as the transmitter and receiver. Images of the slice definition, the region over which the excitation and refocusing pulses operate with a surface coil, and brain images obtained with slice planes perpendicular to the plane of the surface coil are presented. Results demonstrate that these pulses can be transmitted with a surface coil to yield high-quality T1- and/or T2-weighted images without B1 artifacts.     

Calibrated uncoupling of tightly coupled concentric surface coils for in vivo NMR

Three known active methods of detuning rf coils have been tested by determining the efficiency of uncoupling two concentric transmit surface coils in a double-coil probe. In a comparison at 80 MHz, the method based on the use of lambda/4 cables was found to have advantages. These methods utilize the introduction of an additional resonance at the frequency of interest to completely detune a coil. By offsetting the detuning of the coil (by offsetting the additional resonance) a calibrated fraction of the rf field of the detuned coil can be added to or substracted from the rf field of the neighboring coil.     

MR spectroscopy using multi-ring surface coils.

A spatially uniform B(1)-field is preferred for MR imaging and spectroscopy. Unfortunately, volume coils are sometimes unavailable, or do not provide adequate RF power or SNR for some applications. In quantitative MRS, mean metabolite concentration cannot be evaluated when the coil response is nonuniform, unless an assumption is made concerning the metabolite spatial distribution. It is well known that standard single-loop surface coils, although offering high SNR characteristics, have poor B(1) homogeneity. New multi-ring surface coils are proposed which produce a locally uniform B(1) field, with sensitivity and power requirements comparable to those of standard surface coils. MR spectroscopy using two and three-ring versions of this "local volume coil" result in spatial localization essentially identical to that obtained with a volume coil but with much improved RF power and SNR characteristics. When compared to standard surface coils, the multi-ring coil offers much improved water suppression and localization, as well as reduced outer voxel contamination, with only a small loss in SNR and moderate increase in SAR. In summary, the multi-ring coil operates midway between the volume coil and the standard surface coil, retaining the most advantageous properties of both. Magn Reson Med 42:655-664, 1999. Published 1999 Wiley-Liss, Inc.