Multi-frequency magnetic resonance imaging

The present invention relates to the field of magnetic resonance. 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 y of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (¹H) 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 (³¹P) nuclei is significant in that phosphorous is involved in many metabolic processes. Additionally, the utilization of hyper-polarized gases such as xenon (¹²⁹Xe) and helium three (³He) 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.

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 aspect of the present invention, a magnetic resonance method is provided. Hyper-polarized gas is introduced into a region of interest of the subject being examined which is placed in a substantially uniform temporally constant main magnetic field. Magnetic gradients are generated in the main magnetic field across the region of interest and, via a first tuned coil, RF signals having a frequency for exciting resonance in hydrogen dipoles are transmitted into the region of interest. Signals emitted from the region of interest by resonating hydrogen dipoles are received via the first tuned coil. Via a second tuned coil whose tuning accounts for a capacitive coupling with the first tuned coil, RF signals having a frequency for exciting resonance in hyper-polarized gas dipoles are transmitted into the region of interest, and signals emitted from the region of interest by resonating hyper-polarized gas dipoles are received. Human viewable images are reconstructed of the region of interest from the received signals.

Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>FIGURE 1 is a diagrammatic illustration of a magnetic resonance apparatus in accordance with aspects of the present invention; and</li><li>FIGURE 2 is a diagrammatic illustration of an end view of a lung coil in accordance with aspects of the present invention.</li></ul>

With reference to FIGURE 1, a main magnetic field control <b>10</b> controls superconducting or resistive magnets <b>12</b> such that a substantially uniform, temporally constant main magnetic field B₀ is created along a z axis through an examination region <b>14</b>. A couch <b>16</b> suspends a subject <b>18</b> to be examined at least partially within the examination region (i.e., so that a region of interest is in the examination region). In a preferred embodiment, the couch <b>16</b>, and consequently the subject <b>18</b>, is movable so that the subject <b>18</b> may be selectively placed in and removed from the examination region <b>14</b>.

A magnetic resonance echo means operated under the control of a sequence control circuit <b>20</b> applies a series of radio frequency (RF) and magnetic field gradient pulses to invert or excite magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, to saturate spins, and the like to generate magnetic resonance imaging and spectroscopy sequences.

More specifically, a gradient coil assembly selectively creates magnetic gradients in the main magnetic field across the examination region. Gradient current amplifiers <b>30</b> apply electrical current pulses to selected ones or pairs of whole-body gradient coils <b>32</b>. Preferably, the whole-body gradient coils <b>32</b> are self shielded gradient coils for producing magnetic gradients along three mutually orthogonal axes, x, y, and z.

A transmit/receive (T/R) switch <b>40</b> under the control of the sequence control circuit <b>20</b> electronically switches a whole-body RF coil <b>42</b> between transmit and receive modes. In the transmit mode, a RF transmitter <b>44</b> (preferably a digital transmitter) is electronically connected and transmits RF pulses or pulse packets to a whole-body RF coil <b>42</b> to excite resonance in the ¹H species. A typical RF pulse is composed of a packet of immediately continuous pulse segments of short duration which taken together achieve a selected magnetic resonance manipulation. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance in selected portions of the examination region <b>14</b>. The whole-body RF coil <b>42</b> is tuned to the ¹H resonance frequency and attached PIN diodes <b>46</b> (shown in FIGURE 2) are selectively biased to enable and disable it. The PIN diodes are biased by a selectively applied DC potential to open and closed states to open or close connections between coil segments and/or between the coil segments and ground.

For whole-body applications, the signals from excited ¹H dipoles as they relax are picked up by the whole-body RF coil <b>42</b>. In the receive mode, the T/R switch <b>40</b> electronically connects the whole-body RF coil <b>42</b> to a channel of a receiver <b>50</b> (preferably a digital multi-channel receiver) via a preamplifier <b>52</b>. The sequence control circuit <b>20</b> controls the gradient pulse amplifiers <b>30</b> and the RF transmitter <b>44</b> to generate any of a plurality of multiple echo sequences, such as echo-planar imaging, echo-volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like. For the selected sequence, the receiver <b>50</b> receives one or more resonance signals following each RF excitation pulse. Ultimately, the received RF signals are demodulated and reconstructed into an image representation by a reconstruction processor <b>60</b> which applies a two-dimensional Fourier transform or other appropriate reconstruction algorithm. The image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory <b>62</b> where it may be accessed by a display, such as a video monitor <b>64</b> which provides a human viewable display of the resultant image.

With reference to FIGURE 2 and continuing reference to FIGURE 1, an insertable lung coil <b>70</b> is utilized for imaging of the subject's <b>18</b> lungs. In a preferred embodiment, the subject <b>18</b> and the lung coil <b>70</b> rest on the couch <b>16</b> such that the subject's <b>18</b> lungs are positioned in the lung coil <b>70</b>. With arms raised behind the head, the subject <b>18</b> is in a better position to have the lung coil <b>70</b> arranged high enough for full coverage of the lungs. The lung coil <b>70</b> is formed from upper and lower sections <b>70a</b> and <b>70b</b>, respectively. To ease entry and access, the lung coil <b>70</b> is splitable at junction <b>72</b> and opens about a hinge <b>74</b>. Additionally, the lung coil <b>70</b> is fitted with a number of ¹H traps, in the illustrated embodiment parallel resonant circuits <b>76</b> including a parallel connected inductor capacitor pair, that make the lung coil <b>70</b> transparent to the transmit field of the whole-body RF coil <b>42</b>. In one embodiment, the traps are tuned to present open, signal blocking circuits at the whole body coil frequency and closed, signal passing circuits at the lung coil frequency. Optionally, other appropriate traps and/or circuits may be utilized that restrict the lung coil <b>70</b> from carrying signal of the frequency of the whole-body RF coil <b>42</b>.

Optionally, the two sections <b>70a</b> and <b>70b</b> are completely separable having a full break analogous to junction <b>72</b> in place of the hinged break <b>74</b>. The coil portions on the top and bottom halves can be connected with pins and sockets, electro magnetically coupled by the tank circuits, capacitively coupled, or the like. On the other hand, the lung coil <b>70</b> may be made out of a single nonsplitable piece for appropriate applications.

In the preferred embodiment, the lung coil <b>70</b> is a birdcage coil (or other appropriate type coil) with rigid upper and lower sections <b>70a</b> and <b>70b</b>. The rigidity of the upper and lower sections <b>70a</b> and <b>70b</b> serves to fix the geometric and spatial relationship of the lung coil <b>70</b> with respect to surrounding structures and the subject <b>18</b>. However, either or both sections <b>70a, 70b</b> may be made flexible to achieve certain other advantages such as improved compatibility with subjects <b>18</b> of differing sizes.

Like the whole-body coil <b>42</b>, the lung coil <b>70</b> is connected via a transmit/receive (T/R) switch <b>80</b>, controlled by the sequence control circuit <b>20</b>, to a RF transmitter <b>82</b> and a preamplifier <b>84</b> that feeds a second channel of the receiver <b>50</b> (optionally two separate receivers are employed). The lung coil <b>70</b> is tuned to an alternate resonance frequency for a species other than ¹H. In a preferred embodiment, the lung coil <b>70</b> is tuned to the resonance frequency for a hyper-polarized gas such as ¹²⁹Xe, ³He, or the like. In this manner, the lungs of the subject <b>18</b> being examined can be imaged when the hyper-polarized gas is introduced. In particular, the hyper-polarized gas is imaged to generate images showing the lung cavity, absorption of the hyper-polarized gas, and the like. One advantage realized is an improved image due to the relative abundance of the resonant species (i.e. the hyper-polarized gas) as compared to the ¹H present in the lung tissue. That is, the lungs are a "cavity" with inherently little tissue in which to excite resonance and generate an image.

The sequence control circuit <b>20</b> causes a selected imaging sequence to be generated for the lung coil <b>70</b> which excites dipoles of the hyper-polarized gas in the lungs of the subject <b>18</b>. Ultimately, signals generated by the dipoles as they relax are: picked-up by the lung coil <b>70</b>, received and demodulated by the receiver <b>50</b>, reconstructed by the processor <b>60</b>, and stored in the image memory <b>62</b> for selective viewing on the monitor <b>64</b>.

In operation, when the lung coil <b>70</b> is in transmit mode currents tend to be induced in the whole-body RF coil <b>42</b> by their inductive coupling. These induced currents could, potentially, produce uncontrollable effects by forward conduction of the PIN diodes <b>46</b>. Such induced currents could be carried down stream and damage preamplifiers and other downstream components. To protect against strong transmit signals from the lung coil <b>70</b> from being received by a disabled whole-body RF coil <b>42</b> despite the PIN diodes <b>46</b>, the whole-body RF coil <b>42</b> is enabled. The sequence control circuit <b>20</b> puts the T/R switch <b>40</b> into the transmit mode such that the preamplifier <b>52</b> is not connected to the whole-body RF coil <b>42</b>. Optionally, traps and/or band pass filters <b>48</b> that pass the ¹H resonant frequency signal while restricting other frequencies are connected between the whole body RF coil <b>42</b> and the downstream components, such as the preamplifier <b>52</b>.

This results in a small, but not negligible, capacitance being inductively coupled into the lung coil <b>70</b>. The lung coil <b>70</b> is tuned, via adjustable capacitor <b>71</b> or other appropriate tuning circuit, with the whole-body RF coil <b>42</b> enabled in order to account for the effective capacitance from the whole-body RF coil and achieve optimum performance. To maintain the correct tuning when the lung coil is in the receive mode, the whole-body RF coil <b>42</b> is enabled during the receiving operation of the lung coil <b>70</b>. Additionally, when the lung coil <b>70</b> is in receive mode, the whole-body RF coil <b>42</b> is also in receive mode to protect the second channel of the receiver <b>50</b> (the channel connected to the lung coil <b>70</b>) from incoming noise from the RF transmitter <b>44</b>.

One advantage of the illustrated lung coil for imaging hyper-polarized gas in an MRI scanner is that it simultaneously images at least two different species. Another advantage is that the insertable RF coil can be installed into an MRI apparatus having a built-in RF body coil tuned to hydrogen and work with, rather than in spite of, the built-in RF body coil. Another advantage is that it provides an alternately tuned coil of sufficient size for imaging a subject's lungs. Another advantage is that it protects downstream reception components from damage. Another advantage resides in its improved performance for lung imaging.

While preferred embodiments above have been described with reference to a lung coil for imaging hyper-polarized gas, the invention herein is also amenable to other applications. The invention may be utilized to image other anatomical regions and/or other species. For example, a head coil could excite a hyper-polarized gas, such as ¹²⁹Xe, dissolved in a subject's blood for brain imaging studies. The coil could be tuned to the resonant frequency for ³¹P to study metabolic processes of, for example, the heart or other muscle of interest.

Additionally, while illustrated for use in a central bore horizontal field type MRI apparatus, the invention is also applicable to open geometry MRI systems having an examination region defined between opposing pole pieces that are connected by a ferrous flux return path. Open geometry MRI systems provide certain advantages particularly in the case of interventional MRI applications. In either case, in at least one preferred embodiment of the invention herein, it is utilized for interventional applications such as an MRI guided lung biopsy procedure.