Effect of body coil electric field distribution on receive-only surface coil heating

Although in the design of transmit RF coils, B(1) homogeneity is crucial for good image quality, discussion of electric field (E-field) distribution in the literature has been mostly limited to specific absorption rate (SAR) and patient loading (dielectric) effects. In this work, we report on a different aspect of E-field: the receive-only surface coil heating resulting from the voltage drop across the blocking (decoupling) networks and cable traps that are used to minimize the transmit field distortion. The results show that the z-component (parallel to the coil cable) of the E-field has a significant effect on the temperature rise in the surface coil. Therefore, in the receive-only coil designs, it is not sufficient to consider only the induced voltage on the coil loop due to the B(1) field, as is generally done in blocking network analysis calculations. The body coil E-field distribution must be considered as well.     

Quadrature detection surface coil

A surface coil assembly consisting of two interleaved coplanar resonators that are intrinsically decoupled from each other is described for imaging at 1.5 T. Vector reception fields on-axis at 3 cm depth are orthogonal and of equal magnitude. Both components of magnetization were received and combined resulting in a 2 1/2 improvement in signal-to-noise for temporomandibular joint images.     

Bronchoscopy-guided removal of intrabronchial coil migration after coil embolization of pulmonary arteriovenous malformation

Pulmonary arteriovenous malformations develop in approximately 50% of hereditary hemorrhagic telangiectasia patients. Pulmonary arteriovenous malformations are often treated with coil embolization therapy. We report a case of a 45-year-old female with multiple pulmonary arteriovenous malformations due to underlying hereditary hemorrhagic telangiectasia who had undergone 14 coil embolization procedures over 16 years. She presented with sudden onset severe, unremitting, nonproductive cough from a foreign body sensation in the airway. Computed tomography of the chest demonstrated a metallic foreign body extending from the left lower lobe of the lung into the left mainstem bronchus and trachea. Bronchoscopy-guided removal of the foreign body revealed an intact embolization coil placed 8 years prior to presentation had partially migrated through the vessel and airway walls into the airway lumen, extending from the left lower lobe bronchus to the left mainstem bronchus. Coil migration is a rare, but potentially dangerous, complication of coil embolization therapy.     

Modular gradient coil: A new concept in high-performance whole-body gradient coil design.

A new concept in high-performance MR gradient coil design is presented which we have called the Modular Gradient Coil (MGC). This novel design approach results in an actively shielded whole-body gradient coil containing multiple and independent elements, integrated onto a single former, for generating gradient fields along each of the three axes (x, y, and z). These elements can be energized in a number of configurations, using a single gradient power supply unit (PSU), to generate a whole range of gradient performance levels. The design criteria for the MGC also include a requirement to prohibit peripheral nerve stimulation in all of its modes of operation. This requirement is achieved, while simultaneously providing high performance, by specifying different volumes of gradient linearity for each of the operating modes. Magn Reson Med 42:561-570, 1999.     

MR gradient coil heat dissipation

The temperature responses of five different gradient coil designs were modeled using simplified engineering equations and measured. The model predicts that the coil temperature approaches a maximum as an inverse exponential, where the maximum temperature is governed by two parameters: a local power density and a cooling term. The power density term is a function of position and is highest where the current paths have minimum widths and are closely packed. The cooling parameter consists of convective, conductive, and radiative components which can be controlled by (1) providing forced cooling, (2) having a coil former with high thermal conductivity and thin walls, and (3) varying the emissivity of the coil surfaces. For a given gradient strength, the average temperature rise is minimized by designing a coil with a small radius and thick copper. The model predicted the local temperature rise, which is also dependent on the current density, to within 5°C of measured values.     

Surface coil holder for Signa

A surface coil holder is described for the General Electric Signa spectrometer. This allows for accurate positioning of the 31P coil relative to that of the 1H coil used for tissue slice localization.     

Eight-channel phased array coil and detunable TEM volume coil for 7 T brain imaging.

An eight-channel receive-only brain coil and table-top detunable volume transmit coil were developed and tested at 7 T for human imaging. Optimization of this device required attention to sources of interaction between the array elements, between the transmit and receive coils and minimization of common mode currents on the coaxial cables. Circular receive coils (85 mm dia.) were designed on a flexible former to fit tightly around the head and within a 270-mm diameter TEM transmit volume coil. In the near cortex, the array provided a fivefold increase in SNR compared to a TEM transmit-receive coil, a gain larger than that seen in comparable coils at 3 T. The higher SNR gain is likely due to strong dielectric effects, which cause the volume coil to perform poorly in the cortex compared to centrally. The sensitivity and coverage of the array is demonstrated with high-resolution images of the brain cortex.     

前列腺癌的MR波谱成像中体线圈与直肠内线圈的比较

目的 比较体线圈(BODY)与直肠内线圈(ERC)MRS对前列腺癌的定性诊断效能,探讨BODY MRS临床应用的可行性.方法 12例取得穿刺病理结果的前列腺外周带癌患者,其中6例为前列腺癌,6例除外了前列腺癌.12例全部完成BODY MRS检查,其中7例同时完成ERC MRS检查.以六分区法对照穿刺结果进行MRS定量分析,分别测量每个区域内癌与非癌区(胆碱+肌酸)/枸橼酸盐[(Cho+Cre)/Cit]的比值,并采用Wilcoxon符号等级检验进行比较,计算BODY MRS对前列腺癌的诊断准确性.结果 (1)癌区与非癌区体素(Cho+Cre)/Cit比较:BODY组(7例)癌区体素比值中位数1.744(0.295～7.998),非癌区中位数0.412(0.112～2.113),两者差异有统计学意义(Z=-9.159,P＜0.01).ERC组(7例)癌区体素比值中位数为1.975(0.479～7.998),非癌区中位数为0.400(0.104～2.232),两者差异有统计学意义(Z=-9.200,P＜0.01).BODY与ERC组间比较:癌区体素组间比较差异无统计学意义(Z=-0.105,P＞0.05),非癌区体素组间比较差异无统计学意义(Z=-0.692,P＞0.05).(2)ROC分析:7例BODY与ERC组曲线下面积(Az值)分别为0.931、0.935,两者比较差异无统计学意义(P=0.851);应用BODY MRS 12例患者组Az值为0.935,敏感度、特异度分别为82.2%和88.2%.结论 BODY与ERC的MRS对前列腺外周带癌定性诊断效能接近,临床应用具有可行性.     

Simultaneous reception from a whole-volume coil and a surface coil on a clinical MR system

Simultaneous acquisition of signals from the same anatomic region with use of both the head or body coil and a surface coil was demonstrated on healthy volunteers with a commercial magnetic resonance imaging system and a hybrid combiner. An improvement in the signal-to-noise ratio (S/N) over that which is obtained with the surface coil alone was demonstrated at depths of 3-4 cm and greater from the surface of the body. The practical necessity of use of the hybrid combiner to add the signals created several problems, such as the inability to obtain the predicted increase in S/N in certain areas and an actual decrease in S/N close to the surface coil. These problems would have been avoided with the use of two separate data channels. The only problem intrinsic to the method is a "wraparound" of structures and motion artifacts onto the field of view.     

Simultaneous image acquisition from the head (or body) coil and a surface coil

Any number of coils can in principle be used simultaneously and independently in magnetic resonance imaging if the mutual inductances are sufficiently small. Surface coils and head or body coils have equal sensitivity at some crossover depth of the order of 6 to 10 cm. Using a 7.5-cm-diameter surface coil that was intrinsically isolated from the head coil, images were acquired simultaneously from both coils and combined to improve the signal-to-noise ratio at 6 cm depth by 2 1/2. A similar experiment with the body coil showed 2 1/2 improvement at about 8 cm depth.     

Analytical approach to noncircular section birdcage coil design: verification with a Cassinian oval coil.

A general analytical framework is presented for the design of birdcage radiofrequency resonators on cylindrical formers having arbitrary cross-sectional shape. The primary objective of such shapes would be to improve the sensitivity of the NMR experiment to noncircular regions of the human anatomy while maintaining field homogeneity and quadrature polarization comparable to those of standard circular birdcage coils. The shape of the corresponding radiofrequency screen, which is required to decouple the coil from the rest of the NMR system and which is key to the performance, is also provided by this methodology. The theory was tested by constructing a 3-T, quadrature, proton coil on a shape conforming to the anthropomorphic mean of the human head, namely, the oval of Cassini. Both bench tests (Q) and in vivo spectral and imaging comparisons of the Cassinian coil with an equivalently dimensioned and constructed circular birdcage coil, respectively, predicted and demonstrated in vivo an improvement in SNR of approximately 24% over the circular section coil. The experimental RF field homogeneity and quadrature performance were comparable for both coil geometries, with the circular coil being marginally superior.     

Steel coil embolization of hepatoportal fistulae

Two patients with portal hypertension secondary to hepatoportal fistulae, of nontraumatic origin, were treated by transcatheter embolization of feeding hepatic artery branches with Gianturco-Anderson-Wallace steel coils. Both patients demonstrated immediate improvement of the symptoms related to their portal hypertension, 1 patient needing no further treatment after 3 years. It is concluded that using steel coils for this purpose is more effective and safer than other materials such as Gelfoam, and may cure portal hypertension in some cases.     

A 16-element phased-array head coil

Volume-array coils offer increased signal-to-noise ratio (SNR) over standard volume coils near the array elements while preserving the SNR at the center of the volume. As the number of array elements is increased, the SNR advantage as well as the complexity of actually constructing the array increases also. In this study, a 16-channel receive-only array for imaging of the brain is demonstrated and compared to a circularly polarized (CP) head coil of similar shape and diameter. The array was formed from a 2 × 8 grid of square elements placed on a cylindrical form. Mutual coupling was minimized by a combination of overlapping element placement and current-reducing matching networks. Simultaneous data acquisition from the 16 individual elements was performed using a four-channel receiver system with each channel time domain multiplexed by a factor of 4. Theoretical and experimental comparisons between the array and a standard CP head coil show that the array offers an increase in SNR of nearly a factor of 3 near its surface while maintaining a comparable SNR to that of the CP head coil in the center of the region of interest.     

Hexagonal zero mode TEM coil: a single-channel coil design for imaging multiple small animals.

A novel hexagonal coil design for simultaneous imaging of multiple small animals is presented. The design is based on a coaxial cavity and utilizes the magnetic field formed between two coaxial conductors with hexagonal cross-sections. An analytical solution describing the B(1) field between conductors of the hexagonal coil was found from the Biot-Savart law. Both experimental results and analytical calculations showed a variation in the B(1) field within the imaging region of less than 10%. Numerical calculations predicted approximately 35% improvement in B(1) field homogeneity with the hexagonal coil design compared to a cylindrical coaxial cavity design. The experimentally-measured signal-to-noise ratio (SNR) of the hexagonal coil loaded with six 50-mM phantoms was only 4-5% lower than that of a single parallel plate resonator loaded with one phantom. In vivo spin-echo (SE) images of six 7-day-old rat pups acquired simultaneously demonstrated sufficient SNR for microimaging. The construction scheme of the coil, simple methods for tuning and matching, and an anesthesia device and animal holder designed for the coil are described. The hexagonal coil design utilizes a single receiver and allows for simultaneous imaging of six small animals with no significant compromise in SNR.     

A numerical approach to non-circular birdcage RF coil optimization: verification with a fourth-order coil.

It is demonstrated that birdcage resonators, satisfying conditions of quadrature operation and radiofrequency field homogeneity, can be realized in practice on formers of non-circular cross section described by an equation of the form (x/a)n + (y/b)n = 1 where a and b are constants and n > or = 2 is an integer. Using a ladder network analogous to that of a conventional circular birdcage, optimization algorithms were employed to determine the elemental current distribution on the non-circular cylindrical surfaces. A comparison of circular, elliptical, symmetric and asymmetric fourth-order (n = 4) section birdcage current distributions is presented. A short, asymmetric fourth-order cage was constructed and tested experimentally at 3 T and compared with a conventional circular-section head coil.     

Dedicated phased-array coil for peripheral MRA

In this paper we introduce a phased-array coil dedicated for MRA of peripheral arteries which covers the upper and lower legs. The structure of this coil includes a solid cabinet with four flexible wings forming a “T.” The flexibility of the wings allows adaptation to the individual leg size. There are eight circularly polarized channels, four on each side. This coil is compatible with other surface coils. For MRA of peripheral arteries, it is combined with the body phased-array coil and the spine array coil which cover the lower abdomen and the pelvis. We examined six patients using this coil combination. The image quality, the signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of these examinations were compared with that of peripheral MRA examinations obtained with the body resonator. Image quality with the array coil was considerably improved in comparison with the body resonator examinations. The SNR and CNR increased approximately 100 %. The handling of this coil was very quick and simple, similar to the procedure with other surface coils. The use of dedicated phased-array coils for peripheral MRA may be an important step toward the establishment of MR digital subtraction angiography (DSA) as a non-invasive alternative to intra-arterial DSA in the visualization of peripheral arteries. Its potential has to be evaluated in future studies. Received: 27 October 1999; Revised: 10 April 2000; Accepted: 14 April 2000     

Minimum maximum temperature gradient coil design

Ohmic heating is a serious problem in gradient coil operation. A method is presented for redesigning cylindrical gradient coils to operate at minimum peak temperature, while maintaining field homogeneity and coil performance. To generate these minimaxT coil windings, an existing analytic method for simulating the spatial temperature distribution of single layer gradient coils is combined with a minimax optimization routine based on sequential quadratic programming. Simulations are provided for symmetric and asymmetric gradient coils that show considerable improvements in reducing maximum temperature over existing methods. The winding patterns of the minimaxT coils were found to be heavily dependent on the assumed thermal material properties and generally display an interesting “fish‐eye” spreading of windings in the dense regions of the coil. Small prototype coils were constructed and tested for experimental validation and these demonstrate that with a reasonable estimate of material properties, thermal performance can be improved considerably with negligible change to the field error or standard figures of merit. Magn Reson Med 70:584–594, 2013. © 2012 Wiley Periodicals, Inc.