Lung coil for imaging hyper-polarized gas in an MRI scanner

Details Lung coil for imaging hyper-polarized gas in an MRI scanner Lung coil for imaging hyper-polarized gas in an MRI scanner

1998-05-08

2001-04-03

Oda, Christine (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

Reexamination Certificate

active

06211677

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic magnetic resonance imaging and spectroscopy and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to magnetic resonance imaging and spectroscopy for other applications.
In magnetic resonance imaging (MRI), a substantially uniform temporally constant main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the magnetic field by transmitting radio frequency (RF) excitation signals into the examination region. Specifically, RF pulses transmitted via a radio frequency coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The RF magnetic resonance signal is received by the RF coil assembly, and from received signals, an image representation and/or spectrum is reconstructed for display on a human viewable display.
The appropriate frequency for exciting resonance in selected dipoles is governed by the Larmor equation. That is to say, the precession frequency of a dipole in a magnetic field, and hence the appropriate frequency for exciting resonance in that dipole, is a product of the gyromagnetic ratio &ggr; of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (
1
H) dipoles have a resonance frequency of approximately 64 MHz. Generally in magnetic resonance imaging, the hydrogen species is excited because of its abundance and because it yields a strong MR signal. As a result, typical magnetic resonance imaging apparatus are equipped with built-in whole-body RF coils tuned to the resonant frequency for hydrogen.
However, it has become diagnostically advantageous to excite and receive magnetic resonance signals from other species for imaging and spectroscopy applications in addition to or in conjunction with the hydrogen signal. For example, the analysis of magnetic resonance signals produced by phosphorous (
31
P) nuclei is significant in that phosphorous is involved in many metabolic processes. Additionally, the utilization of hyper-polarized gases such as xenon (
129
Xe) and helium three (
3
He) also present certain advantages. Exciting Xe dissolved in a subjects blood is useful for brain images. Exciting the hyper-polarized gas introduced into a subjects lungs is useful for lung imaging and measuring of lung capacity.
However, different species have markedly different resonance frequencies. Phosphorous, xenon, and helium three have resonant frequencies of approximately 26 MHz, 17.6 MHz, and 49 MHz respectively in the same 1.5 T magnetic field. In order to excite and receive magnetic resonant signals from these species, a radio frequency coil tunable to each specific resonant frequency is employed.
Traditionally, double-tuned localized or surface coils have been employed for this purpose. However, such coils were limited in size and did not accommodate larger sections of a patient's anatomy. An increase in the size of the doubly-tuned radio frequency coils presents additional drawbacks due in part to the doubly-tuned RF coils' close proximity to the built-in RF coil tuned to the hydrogen resonant frequency. In larger doubly-tuned RF coils, strong coupling would occur between the built-in RF coil and the doubly-tuned RF coil which caused mode splitting in which neither mode would be at the hydrogen frequency. Furthermore, when the inserted coil was in transmit mode, voltages would be induced in the built-in, whole-body RF coil due to the coupling. Left unchecked, this presented the risk of potential damage to reception components such as the preamplifier, receiver, and the like.
The present invention contemplates a new and improved magnetic resonance apparatus which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a magnetic resonance imaging apparatus is provided. It includes a main magnet for generating a substantially uniform temporally constant main magnetic field through an examination region defined by the main magnet. A couch suspends a region of interest of a subject to be examined in the examination region. A gradient coil assembly generates substantially linear magnetic gradients in the main magnetic field across the examination region. A body RF coil situated at a periphery of the examination region is tuned to a first Larmor frequency corresponding to hydrogen nuclei. The body coil is selectively enabled and disabled. A first transmitter transmits RF signals at the first Larmor frequency. A first switch electronically switches the body RF coil between (i) a transmit mode in which the body RF coil is electronically connected to the first transmitter for exciting resonance in hydrogen nuclei disposed within the examination region, and (ii) a receive mode in which the body RF coil is electronically connected to a first receiver channel for receiving and demodulating magnetic resonance signals emitted from excited hydrogen nuclei as they relax. An insertable RF coil is positioned inside the body RF coil adjacent thereto. The insertable RF coil is tuned, while the body RF coil is enabled, to a second Larmor frequency corresponding to a non-hydrogen nuclei. A second transmitter is provided for transmitting RF signals at the second Larmor frequency. A second switch electronically switches the insertable RF coil between (i) a transmit mode in which the insertable RF coil is electronically connected to the second transmitter for exciting resonance in non-hydrogen nuclei disposed within the examination region, and a (ii) receive mode in which the insertable RF coil is electronically connected to a second receiver channel for receiving and demodulating magnetic resonance signals emitted from excited non-hydrogen nuclei as they relax. A reconstruction processor connected with the first and second receiver channels reconstructs the magnetic resonance signals from excited hydrogen and non-hydrogen nuclei into image or spectroscopy representations.
In accordance with another more limited aspect of the present invention, the non-hydrogen nuclei is one of xenon (
129
Xe), helium three (
3
He), and phosphorous (P).
In accordance with another more limited aspect of the present invention, the magnetic resonance imaging apparatus further includes a sequence control circuit which operates the first and second T/R switches such that the body RF coil is in the transmit mode when the insertable RF coil is in the transmit mode, and the body RF coil is in the receive mode when the insertable RF coil is in the receive mode.
In accordance with another more limited aspect of the present invention, the insertable RF coil is a lung coil for imaging the subject's lungs.
In accordance with another more limited aspect of the present invention, the insertable RF coil is a splitable birdcage coil having upper and lower sections. The splitable birdcage coil includes a number of trap circuits arranged such that the splitable birdcage coil is restricted from supporting signals of the first Larmor frequency.
In accordance with another more limited aspect of the present invention, the trap circuits are parallel resonant circuits tuned to the first Larmor frequency including parallel connected inductor capacitor pairs.
In accordance with another more limited aspect of the present invention, the splitable birdcage coil rests on the couch and has rigid upper and lower sections.
In accordance with another more limited aspect of the present inventi

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