Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-11-08
2002-04-23
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06377048
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally in the field of Nuclear Magnetic Resonance (NMR) based techniques, and relates to a device and method for magnetic resonance imaging (MRI). Although not limited thereto, the invention is particularly useful for medical purposes, to acquire images of cavities in a human body, but may also be used in any industrial application.
BACKGROUND OF THE INVENTION
MRI is a known imaging technique, used especially in cases where soft tissues are to be differentiated. Alternative techniques, such as ultrasound or X-ray based techniques, which mostly utilize spatial variations in material density, have inherently limited capabilities in differentiating soft tissues.
NMR is a term used to describe the physical phenomenon in which nuclei when placed in a static magnetic field, respond to a superimposed alternating (RF) magnetic field. It is known that when the RF magnetic field has a component perpendicular in direction to the static magnetic field, and when this component oscillates at a frequency known as the resonance frequency of the nuclei, then the nuclei can be excited by the RF magnetic field. This excitation is manifested in the temporal behavior of nuclear magnetization following the excitation phase, which in turn can be detected by a reception coil and termed the NMR signal. A key element in the utilization of NMR for imaging purposes is that the resonance frequency, known as the Larmor frequency has a linear dependence on the intensity of the static magnetic field in which the nuclei reside. By applying a static magnetic field, of which the intensity is spatially dependent, it is possible to differentiate signals received from nuclei residing in different magnetic field intensities, and therefore in different spatial locations. The techniques, which utilize NMR phenomena for obtaining spatial distribution images of nuclei and nuclear characteristics, are termed MRI.
In conventional MRI techniques, spatial resolution is achieved by superimposing a stationary magnetic field gradient on a static homogeneous magnetic field. By using a series of excitations and signal receptions under various gradient orientations a complete image of nuclear distribution can be obtained. Furthermore, it is a unique quality of MRI that the spatial distribution of chemical and physical characteristics of materials, such as biological tissue, can be enhanced and contrasted in many different manners by varying the excitation scheme, known as the MRI sequence, and by using an appropriate processing method.
The commercial application of MRI techniques suffers from the following two basic drawbacks: the expenses involved with purchasing and operating an MRI setup; and the relatively low signal sensitivity which requires long image acquisition time. Both of these drawbacks are linked to the requirement, in standard MRI techniques, to image relatively large volumes, such as the human body. This necessitates producing a highly homogeneous magnetic field and field gradients over the entire imaged volume, leading to extensive equipment size. Additionally, the unavoidable distance between a signal receiving coil and most of the imaging volume significantly reduces imaging sensitivity.
In order to better understand the inter-relation between hardware limitations and the characteristics of the obtained image, the typical MRI system components are described below in more detail. Such a system must include at least the following four components; (1) a strong static (DC) magnetic field source creating the primary static substantially homogeneous magnetic field in the entire volume to be imaged; (2) a transmission antenna (coil) and a transmission channel for transmitting No excitation pulses; (3) a reception antenna (coil) and a receiving channel for receiving the so-excited NMR signal; and (4) a magnetic field gradient source to spatially encode-the signals originating from the imaged volume.
The resolution of an image depends on many parameters, some of which are related to the imaging hardware, and some relate to the imaging technique or pulse-sequence used. Practically, however, the resolution is generally governed by two parameters: gradient field strength and signal to noise ratio (SNR) per volume cell (voxel).
It is known and disclosed for example in “
Principles of Nuclear Magnetic Resonance Microscopy
”, P. T. Callaghan, Oxford Science Publications, 1995, that in a noise-free (ideal) setting for 2-D Fourier spin-echo imaging, if the required resolution in the gradient dimension is &Dgr;x, then the resolution requirement can be written as follows:
&ggr;·G
max
·&Dgr;x·T
grad
≈&pgr;
wherein &ggr; is the gyromagnetic ratio, G
max
is the maximum achievable gradient (in Tesla/m) and T
grad
is the gradient pulse length. For typical gradient pulse lengths of about 1 msec (limited by signal decay, etc.), the required G
max
for a resolution of about 0.5×0.5 mm is about 2-3 Gauss/cm. It tuns out that these high gradient values are hard to achieve over large volumes (typically 50×50×50 cm), especially since large gradient coils having large inductance values are reluctant to develop large currents over short periods of time. Moreover, creating high field gradients over large portions of the patient's body can induce discomforting and even dangerous nerve activation, let alone unbearable acoustic noise during gradient transmission coming from the MRI machine itself.
The other factor that governs the resolution limit is the SNR per voxel, which is strongly related to reception coil sensitivity. Conventional, MRI machines have the reception antennas (coils) installed in the main body of the machine, and thus geometrically far from internal organs, which need to be imaged. This problem has been addressed in the past and partially solved by some remarkable innovations, disclosed for example in U.S. Pat. Nos; 5,699,801; 5,476,095; 5,365,928; 5,307814 and 5,050,607. Generally, these innovations consist of using an application-specific reception coil to be located in the vicinity of the tissue to be imaged (sometimes external and sometimes internal to the body), thus increasing receiving sensitivity, SNR and, eventually, the image resolution.
U.S. Pat. No. 5,572,132 discloses a concept of combining the static magnetic field source with the RF coil in a self contained intra-cavity medical imaging probe. Here, several permanent magnet configurations are proposed for creating a homogeneous magnetic field region external to the imaging probe itself, and several RF and gradient coil configurations that may be integrated in the imaging probe in order to allow autonomous imaging capabilities. The limitations of the autonomous probe of U.S. Pat. No. 5,572,132 are the fact that the imaging technique still requires a region of substantially homogeneous field, which, unfortunately, can be created in a very limited volume externally to the probe itself limiting a field of view (FOV) of the device, and the fact that the static field values created in this very limited region are substantially low. This limitation stems mostly from the fact that the static field sources can create sufficiently strong static magnetic field in a listed region around them. Further away from the magnets the static field strength drops significantly, up to a point where there is no sufficient SNR per voxel, and therefore no imaging feasibility. These limitations make it practically impossible to use the device for imaging, as opposed to measurement, purposes.
The use of a portable receiver coil in conjunction with external MRI machines (as disclosed in the above-mentioned U.S. Pat. Nos. 5,699,801; 5,476,095; 5,365,928; 5,307814 and 5,050,607) is not subject to this limitation, because the external field sources are capable of creating very strong magnetic field (typically 0.5 to 4 Tesla in medical imaging) over very large volumes (again: typically 50×50×50 cm's). Although the receiver sensitivity “barrier” is lifted by using an internal coil, other problems, such as di
Alexandrowicz Gil
Blank Aharon
Golan Erez
Arana Louis
Browdy and Neimark
TopSpin Medical (Israel) Limited
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