Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-10-24
2003-09-09
Paik, Sang (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06618608
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to systems and methods for imaging tissue using magnetic resonance imaging, and more particularly to systems and methods for performing thermal-sensitive imaging of both fat and muscle tissue using focused magnetic resonance imaging.
BACKGROUND
A number of methods have been proposed for directing heat to a target tissue region within a patient, such as a cancerous or benign tumor, to necrose or otherwise treat the tissue region with thermal energy. For example, a piezoelectric transducer located outside the patient's body may be used to focus high intensity acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about twenty kilohertz (20 kHz), and more typically between one and five Megahertz (1-5 MHz)), at an internal tissue region of a patient to therapeutically treat the tissue region. The ultrasonic waves may be used to ablate a tumor, thereby obviating the need for invasive surgery. In an alternative method, laser fibers may be introduced into the patient's body from an entry site that are used to guide coherent optical heat sources to an internal tissue region.
During such procedures, it is often desirable to image the tissues being treated. For example, ultrasound imaging systems may be used for imaging, as well as for generating therapeutic ultrasound waves. Alternatively, magnetic resonance imaging (or “MRI”) may be used instead of ultrasound imaging, as MRI provides excellent quality images of tissue, and is not limited to “windows” that exclude bone or other structures that may interfere with or otherwise limit ultrasound imaging.
An MRI system may be used to plan a procedure, for example, before surgery or a minimally invasive procedure, such as an ultrasound ablation procedure. A patient may initially be scanned in an MRI system to locate a target tissue region and/or to plan a trajectory between an entry point and the tissue region in preparation for a procedure. Such preparation may be particularly useful because a tumor may be more visible in an magnetic resonance (“MR”) image than using direct examination. Due to differences in relaxation times of tumorous and other tissue, MRI images may provide a contrast not available using direct visualization, particularly since tumorous tissue may visually appear similar to normal tissue, or the field of view may be obscured, for example, by blood.
Once the target tissue region has been identified, MRI may be used during the procedure, for example, to image the tissue region and/or to guide the trajectory of an external ultrasound beam to a target tissue region being treated, or to guide laser energy. In addition, an MRI system may be used to monitor the temperature of the tissue region during the procedure, for example, to ensure that only the target tissue region is necrosed during an ablation procedure without damaging surrounding healthy tissue. Generally, this involves using a separate scanning sequence that provides temperature information, in addition, to a scanning sequence that provides tissue information.
For example, before applying sufficient energy to necrose tissue, a lower level of energy may be directed towards the target tissue region, generally in a pulsed or oscillating manner to minimize the effect of thermal diffusion. As the tissue region is heated, a temperature-sensitive magnetic resonance (“MR”) pulse sequence may be used to acquire a temperature “map” to ensure that the energy is applied to the target tissue region and not to the surrounding healthy tissue. The imaging system may also be used in a separate scan sequence to create an image of the tissue intended to be destroyed, and then the two images may be superimposed upon one another to identify the location of the energy relative to the target tissue region.
The placement of the energy may then be adjusted to direct the energy more accurately towards the target tissue region. For example, a focal zone of ultrasonic.energy emitted by an ultrasound transducer may be moved by mechanically adjusting the position of the transducer relative to the patient's body. Alternatively, the focal zone may be moved electronically. e.g., by controlling a phase component and/or relative amplitude of drive signals to the transducer elements, or a combination of mechanical and electronic positioning may be used, as is known in the art.
MRI systems exploit the property that free unpaired spinning protons in the nucleus of a molecule of a specific tissue, such as hydrogen molecules, align themselves in a magnetic field such that their axes precess about the magnetic field. Such unpaired protons have non-zero “spin” and consequently behave like a small magnetic dipole. The net sum of the population of dipoles results in a bulk magnetization vector M that is aligned with a static magnetic field B
0
, shown in
FIG. 1
in a reference frame X′Y′Z′, and rotating about the static magnetic field axis at a frequency equal to the precession of the spins (the Larmor frequency).
The magnetic dipoles forming the net magnetization vector M ordinarily are aligned with the applied magnetic field. However, these magnetic dipoles have an excited state that opposes this applied magnetic field. Pulses resulting from an RF excitation at the Larmor frequency will cause the magnetic dipoles to transition from the aligned state to the opposing state. An MR imaging device uses a radio frequency (RF) transmitter to “flip” the magnetic dipoles into the excited state by transmitting RF energy at the Larmor frequency. For example, a one hundred eight degree (180°) pulse from the RF transmitter will rotate or “flip” the magnetization vector M down to align along the −Z′ axis. This behavior is generic to any orientation of the magnetization vector.
Regardless of the flip angle excited by the RF transmitter, it is possible, by applying a magnetic field gradient, to selectively choose spins for excitation. This selection is necessary for imaging of a tissue structure. A one-dimensional linear magnetic field gradient, conventionally denoted in the Z direction, is applied during the RF excitation pulse. Because of the linear gradient, only spins located in a particular slice or plane through the patient will respond to a given RF pulse.
FIG. 2
illustrates the resulting net magnetization vector M after application of a ninety degree (90°) pulse. This vector aligns with the Y
2
axis and thus is entirely in the transverse X′Y′ plane. Two time constants, T
1
and T
2
, govern the relaxation of this perturbed or excited magnetic field vector back to the equilibrium state of
FIG. 1. T
1
relates to the time necessary for the decay in the longitudinal component of the excited magnetization vector. T
2
relates to the time necessary for the decay in the transverse component of the excited magnetization vector. Because two factors contribute to the decay of transverse magnetization, a combined time constant T
2
* is generally used to represent the two contributions.
One commonly used pulse sequence in MRI systems is known as a spin-echo sequence. In its traditional form, a ninety degree (90°) RF pulse is first applied to the spins, as discussed with respect to FIG.
2
. Because the spins are all in slightly different environments, the transverse magnetic field begins to dephase. During this dephasing period, a one hundred eighty degree (180°) pulse is applied. This pulse causes the transverse magnetic field to partially rephase such that a signal is produced called an echo, which is a function of both time constants T
1
and T
2
*. Alternatively, other sequences may be used, such as a gradient echo sequence, a gradient refocused echo sequence, as are well known to those skilled in the art.
To image the spins within the slice created by the slice or Z-axis gradient discussed earlier, two additional gradients are typically applied. The first gradient, called the phase encoding gradient, is applied along one of the sides of the image plane, i.e., convent
Cline Harvey E.
Watkins Ronald D.
Bingham & McCutchen LLP
Paik Sang
Txsonics, Ltd.
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