(results and analysis cont.)Both coils exhibit the desired dual-tuned behavior;

resonance peaks at the carbon-13 and hydrogen frequencies are visible. The IR has a significant advantage over the HP coil in terms of tuning to the resonant frequency. The HP coil exhibited significant frequency splitting, which made it hard to determine the frequency of the m = 1 mode. Additionally, when combined with the LP, the hydrogen frequency of the HP shifted up nearly 20MHz due to strong LP/HP coupling. Such a drastic shift did not occur for the IR when combined with the LP, indicating much less coupling between the coils.

Going Beyond Protons in MR StudySeraina Murphy, Dr. Robert Brown

Department of Physics, Case Western Reserve University, Cleveland, OH 44106

INTRODUCTIONMagnetic resonance imaging (MRI) is the application of nuclear magnetic resonance in

image processing. Nuclear magnetic resonance is built on the phenomenon that spinning atomic nuclei precess at a certain frequency in the presence of an applied magnetic field. This frequency, called the Larmor frequency, is governed by the equation

where γ is the gyromagnetic ratio—a constant that is element dependent—and B0 is the external magnetic field about which the nuclei precess. The resonanceaspect of MRI refers to the use of radiofrequency (RF) coil elements tuned to the precession frequency of a particular element (or elements) in order to manipulate the direction of the spins. MRI is a process that exploits this property of

MATERIALS & METHODS

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ABSTRACTThis research investigates the feasibility of an innovative

dual-tuned coil design that involves two concentric birdcage resonators, each tuned to a single frequency. The birdcages in this design have each proven highly effective as separate, single-tuned devices: a “low-pass” birdcage, which is very effective for resonating with low-frequency elements, and an “inductive resonator,” which is easy to tune to high-frequencies (hydrogen), but also has greater stability than other high-frequency resonators. Ultimately this design has proven successful, as the two concentric coils have simultaneously displayed resonance behaviors at their respective frequencies. Compared to previous designs, this dual-tuned coil has comparable output and has the advantage of greater uniformity and easier tuning capability.

INTRODUCTIONIn the current market for magnetic resonance imaging

technology, coils are designed to image hydrogen, primarily due to the large natural abundance of hydrogen in the body which results in a large detectable signal in RF receive coils. However, there are instances when hydrogen is limited in the body (fig. 1), and in these cases, dual-tuned coils are necessary to obtain high-contrast images. These ‘dual-tuned’ coils resonate with both hydrogen nuclei and the nuclei of a locally injected hyperpolarized element such as carbon-13 or helium-3.

METHODSThis experiment utilized three ‘birdcage’ coils: ‘low-pass’

(LP), ‘high-pass’ (HP), and ‘inductive resonator’’ (IR). For the LP coil (fig. 2(a)), the lowest-order mode is most easily tuned to low frequencies. For HP (fig. 2(b)), the lowest-order mode is easily tuned to higher frequencies. The term ‘inductive resonator’ (fig. 2(c)) refers to the fact that the meshes of the resonator have strong mutual inductance. This configuration behaves like the HP coil, but instead the capacitors are located on the legs like the LP coil.

This design utilizes two, 16-leg, concentric birdcages (fig. 3, left): LP inside tuned to the 3-Tesla frequency of carbon-13 (32.125MHz), and IR outside tuned to for hydrogen (127.72MHz). A second coil combining HP and LP coils was also built for comparison (fig. 3, right), as such a design has previously been proven effective as a dual-tuned coil and we wanted to compare the output of the two designs.

RESULTS ANALYSISWe wanted to determine whether or not the inductive

resonator could theoretically work effectively with the low-pass birdcage. Since the IR acts like a HP coil, we decided to check some measured values of mode frequencies against calculated values. The mode frequencies ω of the HP birdcage are governed by the equation

where C is the capacitance in one leg, M is the mutual inductance, L is the self-inductance of the circuit, and m is the mode number. For a birdcage of N legs, there are N/2 unique modes [3]. Given 16 legs for each birdcage, the value of m ranges from 1 through 8.

To determine theoretical frequencies, we set L =M = 1. The C values were taken from the capacitors used on the coils—20pF. The frequency values were then graphed and compared to the measured values. The IR and HP mode behavior is very similar; because the HP/LP coil works, the IR/LP coil should theoretically work as well.

Observed output of the LP/IR coil is shown in fig. 5 (left). The resonant peaks of carbon and hydrogen are both evident indicating that the coil functions correctly as a dual-tuned resonator. The output of the LP/HP coil is shown in fig. 5 (right) for comparison.

CONCLUSIONSThe LP/IR coil successfully resonates at both carbon-

13 and hydrogen frequencies, indicating that this design can be used effectively as a dual-tuned coil. Its reduced coupling and ease of tuning indicates it is a competitive alternative design to the LP/HP combination. Further investigation would involve attaching cables and preamps to the coil and obtaining images using a 3T magnet.

REFERENCES1. E.M. Haacke, R.W. Brown, M.R. Thompson, and R. Venkatesan. Magnetic Resonance Imaging:

Physical Principles and Sequence Design. Wiley, USA (1999).2. J. Frazer. Lung Imaging Method Allows Visualization of Airways. Focus. Harvard Medical

School, 16 May (2003). <http://focus.hms.harvard.edu/2003/May16_2003/research_briefs.html>

3. J. Jin. Electromagnetic Analysis and Design in Magnetic Resonance Imaging. CRC Press LLC, FL (1999).

Fig. 2: (a) Low-pass birdcage; (b) High-pass birdcage, (c) Inductive resonator

(a) (b) (c)

Fig. 3: Dual-tuned birdcages comprised of two, staggered, concentric birdcages. (Left) HP/LP combination; (Right) IR/LP combination.

Fig. 5: (left) Output of IR/LP coil; (right) output of HP/LP coil.

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Figure 1: MRI of lungs; (Left) hydrogen imaging; (Right) hydrogen and helium-3 imaging [2].

Fig. 4: Comparison of calculated versus measured frequencies for the HP and IR coils.