How thin can you go? Performance of thin copper and aluminum RF coil conductors.

PURPOSE: To evaluate the impact of emerging conductor technology on RF coils. Performance and resulting image quality of thin or alternate conductors (eg, aluminum instead of copper) and thicknesses (9-600 µm) are compared in terms of SNR. METHODS: Eight prototype RF coils (15 cm × 15 cm square loops) were constructed and bench-tested to measure quality factor. The coils used 6-mm-wide conducting strips of either copper or aluminum of a few different thicknesses (copper: 17, 32, 35, 127, 600 µm; aluminum: 9, 13, 20, 127 µm) on acetate projector sheets for backing. Corresponding image SNR was measured at 0.48 tesla (20.56 MHz). RESULTS: The coils spanned a range of unloaded quality factors from 89 to 390 and a fivefold range of losses. The image SNRs were consistent with the coils' bench-measured efficiencies (0.33-0.73). Thin aluminum conductors (9 µm) led to the highest reduction in SNR (65% that of 127 µm copper). Thin copper (<32 µm) conductors lead to a much smaller decrease in SNR (approximately 10%) compared to 127 µm copper. No performance difference was observed between 127 µm thick copper and aluminum. The much thicker 600 µm copper bars only yield a 5% improvement in SNR. CONCLUSION: Even at 0.48 tesla, copper RF coil conductors much thinner than those in conventional construction can be used while maintaining SNR greater than 50% that of thick copper. These emerging coil conductor technologies enable RF coil functionality that cannot be achieved otherwise.

High-resolution MRI encoding using radiofrequency phase gradients.

Although MRI offers highly diagnostic medical imagery, patient access to this modality worldwide is very limited when compared with X-ray or ultrasound. One reason for this is the expense and complexity of the equipment used to generate the switched magnetic fields necessary for MRI encoding. These field gradients are also responsible for intense acoustic noise and have the potential to induce nerve stimulation. We present results with a new MRI encoding principle which operates entirely without the use of conventional B0 field gradients. This new approach--'Transmit Array Spatial Encoding' (TRASE)--uses only the resonant radiofrequency (RF) field to produce Fourier spatial encoding equivalent to conventional MRI. k-space traversal (image encoding) is achieved by spin refocusing with phase gradient transmit fields in spin echo trains. A transmit coil array, driven by just a single transmitter channel, was constructed to produce four phase gradient fields, which allows the encoding of two orthogonal spatial axes. High-resolution two-dimensional-encoded in vivo MR images of hand and wrist were obtained at 0.2 T. TRASE exploits RF field phase gradients, and offers the possibility of very low-cost diagnostics and novel experiments exploiting unique capabilities, such as imaging without disturbance of the main B0 magnetic field. Lower field imaging (<1 T) and micro-imaging are favorable application domains as, in both cases, it is technically easier to achieve the short RF pulses desirable for long echo trains, and also to limit RF power deposition. As TRASE is simply an alternative mechanism (and technology) of moving through k space, there are many close analogies between it and conventional B0 -encoded techniques. TRASE is compatible with both B0 gradient encoding and parallel imaging, and so hybrid sequences containing all three spatial encoding approaches are possible.

B1 transmit phase gradient coil for single-axis TRASE RF encoding.

PURPOSE: TRASE (Transmit Array Spatial Encoding) MRI uses RF transmit phase gradients instead of B0 field gradients for k-space traversal and high-resolution MR image formation. Transmit coil performance is a key determinant of TRASE image quality. The purpose of this work is to design an optimized RF transmit phase gradient array for spatial encoding in a transverse direction (x- or y- axis) for a 0.2T vertical B0 field MRI system, using a single transmitter channel. This requires the generation of two transmit B1 RF fields with uniform amplitude and positive and negative linear phase gradients respectively over the imaging volume. MATERIALS AND METHODS: A two-element array consisting of a double Maxwell-type coil and a Helmholtz-type coil was designed using 3D field simulations. The phase gradient polarity is set by the relative phase of the RF signals driving the simultaneously energized elements. RESULTS: Field mapping and 1D TRASE imaging experiments confirmed that the constructed coil produced the fields and operated as designed. A substantially larger imaging volume relative to that obtainable from a non-optimized Maxwell-Helmholtz design was achieved. CONCLUSION: The Maxwell (sine)-Helmholtz (cosine) approach has proven successful for a horizontal phase gradient coil. A similar approach may be useful for other phase-gradient coil designs.

A comparison of MR imaging of a mouse model of glioma at 0.2 T and 9.4 T.

Both 0.2 T and 9.4 T MRI systems were used to image a mouse model of glioma. RF coils were designed for both fields. A spin-echo, multi-echo pulse sequence was used to determine T(2) relaxation times of both brain and tumor tissues. Contrast-to-noise ratio was calculated based on the selected echo time. The results showed that 0.2 T is suitable for mouse model imaging, however total scan time must be long to achieve high enough SNR. T(2) relaxation times of the tumor and brain tissues can be measured at 0.2 T and are 2.1 and 1.8 times respectively longer at 0.2 T than at 9.4 T. Contrast to noise ratio for tumor and brain was better at high field than at the low field. We concluded that 0.2 T may be used to study mouse model of glioma using spin echo pulse sequence, yet the total scan time is long (about 40 min), resolution is lower (∼250 µm × 250 µm) and slice thickness (3mm) must be large enough to obtain sufficient SNR.

A volume microstrip RF coil for MRI microscopy.

Quantitative magnetic resonance imaging (MRI) studies of small samples such as a single cell or cell clusters require application of radiofrequency (RF) coils that provide homogenous B(1) field distribution and high signal-to-noise ratio (SNR). We present a novel design of an MRI RF volume microcoil based on a microstrip structure. The coil consists of two parallel microstrip elements conducting RF currents in the opposite directions, thus creating homogenous RF field within the space between the microstrips. The construction of the microcoil is simple, efficient and cost-effective. Theoretical calculations and finite element method simulations were used to optimize the coil geometry to achieve optimal B(1) and SNR distributions within the sample and predict parameters of the coil. The theoretical calculations were confirmed with MR images of a 1-mm-diameter capillary and a plant obtained with the double microstrip RF microcoil at 11.7 T. The in-plane resolution of MR images was 24 µm × 24 µm.

An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging.

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (> or = 1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B(1) field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.

Single-point imaging with a variable phase encoding interval.

A modified single-point imaging (SPI) technique using a variable phase encoding interval is proposed. This method is based on the minimization of the phase encoding interval for further signal-to-noise ratio (SNR) optimization. This is particularly beneficial when the maximum gradient amplitude limits an optimal phase encoding interval, and the resulting SNR suffers from T(2)-related signal attenuation. Theoretical calculation of the SNR and simulation of the point spread function (PSF) for the different experimental parameters are presented. Experiments using a rubber sample (T(2)* approximately 73 micros) and a tooth (bi-exponential relaxation: T(2,1)*=111 micros and T(2,1)*=872 micros) showed a significant increase in SNR (>3 and >2, respectively) when compared with images acquired with conventional SPI.

Simple phase method for measurement of magnetic field gradient waveforms.

Magnetic field gradients play a fundamental role in MR imaging and localized spectroscopy. The MRI experiment, in particular fast MRI, relies on precise gradient switching, which has become more demanding with the constantly growing number of fast imaging techniques. Here we present a simple MR method to measure the behavior of a magnetic field gradient waveform in an MR scanner. The method employs excitation of a thin slice, followed by application of the studied gradient and simultaneous FID acquisition. Measurements of different gradient waveforms were performed with a spherical phantom filled with doped water and positioned at the isocenter of the gradient set. The presented experiments demonstrate the capability of the technique to measure different gradient waveforms with an estimated error of less than 200 microT/m.

Magnetic resonance imaging of seeds by use of single point acquisition.

In general, magnetic resonance imaging (MRI) is used to obtain a spatial representation of the water distribution in an object. Water in soft materials (living matter) often shows a high degree of translational mobility, giving rise to relatively long magnetic relaxation times. This allows the use of conventional MRI techniques such as the spin-echo, to acquire an image. However, when hydration levels become low, water becomes less mobile, resulting in much shorter magnetic relaxation times and a corresponding signal loss. To avoid problems arising from rapid decaying signals, we investigated the use of single point imaging (SPI) in the study of seeds. We were able to obtain SPI images of nonimbibed and imbibed seeds. Using SPI with shaped gradients significantly reduced the acoustic noise level.

Regenerative activity and liver function following partial hepatectomy in the rat using (31)P-MR spectroscopy.

The aim of the present study was to determine whether alterations in hepatic energy expenditure following partial hepatectomy (PHx), as documented by in vivo hepatic (31)P-MRS, correlate with standard parameters of hepatic regeneration and/or liver function. In addition, we sought to determine whether changes in hepatic energy levels are proportional to the extent of hepatic resection. Adult male Sprague-Dawley rats (4-7 per group) underwent a 40%, 70%, or 90% PHx or sham surgeries. Magnetic resonance spectroscopy (MRS) examinations were performed on each animal 24 or 48 hours thereafter. After MRS examinations, [(3)H]thymidine incorporation into hepatic DNA, proliferating cell nuclear antigen (PCNA) protein expression, and serum bilirubin determinations were performed on each rat. Twenty-four hours following surgery, rats that had undergone 70% PHx had unchanged adenosine triphosphate (ATP) levels but significantly lower ATP/inorganic phosphate (Pi) ratios (P <.05), whereas, at 48 hours post-PHx, both ATP and ATP/Pi levels were lower than in sham- and nonoperated controls (P <.05). Hepatic regeneration and liver dysfunction mirrored these changes; correlations existed between ATP/Pi ratios and [(3)H]thymidine incorporation (r = -0.61, P <.005), PCNA protein expression (r = -0.62, P <.005), and serum bilirubin (r = -0.49, P <.05). For rats that had undergone graded resections, depleted energy levels 48 hours post-PHx were proportional to the extent of resection, degree of enhanced regenerative activity, and liver dysfunction. In conclusion, (31)P-MRS-generated ATP/Pi index is a noninvasive, robust determination that correlates with standard parameters of hepatic regeneration and function.