Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-02-12
2002-09-03
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S318000
Reexamination Certificate
active
06445182
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the field of magnetic resonance imaging (MRI). More particularly, the present invention relates to the field of geometric distortion correction in MRI.
BACKGROUND ART
Neurosurgical stereotaxis refers to a collection of techniques used to superimpose radiological volume images intra-operatively to the patient's skull, brain, and cerebrovasculature. Typical stereotactic procedures begin with the collection of a three dimensional (3D) computer tomography (CT) image containing fiducial markers. The resulting 3D image space may then be used to determine an optimal craniotomy or skull opening site and to model virtually the potential intra-operative orientation of a surgical tool, such as a biopsy needle for example. Using such images helps reduce the size of the craniotomy and intracranial traverse of surgical tools.
For a typical procedure, a stereotactic frame or frameless fiducial imaging markers are mounted on the patient's head, often by screwing either into the skull. These fiducial markers are then used by a three dimensional (3D) digitizer to register the image-based surgical plan to the patient. Using the fiducials, the region of interest can be mapped to the coordinate system in the pre-operative image. The digitizer can be used to guide the craniotomy and initial insertion of surgical tools virtually from the pre-operative image.
Typical stereotactic surgical procedures emphasize accurate location of anatomical structures within pre-operative images. The accuracy of a neurosurgical navigational system depends on, for example, the mechanical accuracy of the 3D digitizing probe, the image registration algorithm, geometrical distortion of the pre-operative images, movement of the patient during scanning, movement of the patient with respect to the patient space determined with the 3D digitizing system, and movement of the brain between scanning and surgery.
Using CT for stereotactic pre-operative imaging helps ensure relatively high spatial fidelity is obtained between the patient's images and the patient's anatomy in the operative stereotactic guidance device. CT techniques use a beam of high-energy X-ray photons to probe the patient and display an image based on the internal influences on the passage of photons through the patient. CT may be used to generate a series of images containing a distribution of the X-ray attenuation co-efficient, which relates to the electron density of the atoms, of a 3D volume. Such image reconstruction is based on the principles of line-of-sight ray optics. CT therefore provides images of relatively high positional accuracy.
CT is useful in imaging bony structures yet has limited ability to differentiate between components of inhomogeneous soft tissue structures, such as the brain for example. CT is capable of differentiating tissues only insofar as the region of interest's X-ray attenuation coefficient differs from its surroundings. Diverse adjacent soft tissue structures, such as muscles, mucosa, nerves, and blood vessels for example, typically appear as a solid mass when imaged. As CT also involves X-ray exposure, its frequency of use must be limited to avoid tissue burning and genetic damage or mutation, for example, as a result of ionizing radiation overexposure.
Magnetic resonance (MR) is based on the presence of magnetic moments in some nuclei of atoms of the patient or other sample being imaged. The protons in many nuclei behave as tiny bar magnets or magnetic dipoles. In the absence of any external influences, these magnetic dipoles are randomly oriented with zero net magnetization. When placed in an external magnetic field, the nucleus precesses around that field at a gravitational field. The precession frequency, &ohgr;, depends on the strength of the external magnetic field B
0
as follows:
&ohgr;=&ggr;
B
0
Equation 1
where &ohgr; is the resonance or Larmor frequency of the nuclei of interest, &ggr; is the gyromagnetic ratio of the nuclei of interest, and B
0
is the applied magnetic field strength.
In magnetic resonance imaging (MRI), the sample is exposed to a radio frequency (RF) signal of the same precession frequency. The sample absorbs energy and changes its alignment relative to the applied field. Upon removal of the excitation signal, the magnetic dipoles relax and fall back to their original entropic alignment. As this occurs, each material in the sample emits a characteristic resonant frequency which can be detected and measured. The relaxation time required for each tissue sample depends on its proton environment as protons that may pass on the energy to their neighboring atoms with relative ease can return to their original state relatively faster.
MRI maps the proton density distribution weighted by T
1
and T
2
relaxation effects in the sample. Hydrogen, an abundant element in live tissue, has a nucleus comprising a single proton. Because of the freedom of hydrogen-containing-molecules to precess, MR images of living samples highlight different time dependent development of recovery of magnetization of the heterogeneous soft tissues in such samples.
Typical MRI techniques apply three orthogonal linearly varying magnetic field gradients to the nuclear magnetic dipoles of the sample in the presence of a homogeneous static magnetic field to define the location of the proton density. the sample is divided into consecutive planes or slices. The protons in an image slice can be selectively excited by a combination of a frequency selective pulse and a field gradient perpendicular to the image plane, termed the slice select gradient. The bandwidth of the RF pulse and the amplitude of the slice select gradient determine the thickness of the slice as follows:
Δ
z
=
BW
rf
γG
z
Equation
2
where &Dgr;z is the slice thickness, BW
rf
is the bandwidth of the RF pulse, and G
z
, is the slice select gradient. The slice thickness can be reduced by increasing the gradient strength or decreasing the RF bandwidth.
After the slice selection, a second magnetic field gradient can frequency encode the position of the proton density signal in one direction within the plane. The frequency encoding gradient, also known as the read-out gradient, causes the magnetic field to vary linearly from one end of the image plane to the other in the applied direction. As an extension of Equation
1
, the precession frequency of the net magnetization vector varies from point to point along this direction as follows:
&ohgr;(
y
)=&ggr;(
B
0
+yG
y
) Equation 3
where y is the position along the frequency encoded direction and G
y
is the frequency encoding gradient.
The second dimension in the image plane is encoded by phase encoding. Phase of the magnetization vector can be defined as the state of precession of the vector at a given point in time. The application of a third orthogonal gradient, also known as the phase encoding gradient, causes the magnetization vectors in that direction to precess at different frequencies. When the phase encoding gradient is removed after a short time, the vectors precess again at the same frequency although the phase of each vector has been changed in proportion to the applied gradient.
Within a slice plane, the phase encoding gradient and frequency encoding gradient are applied in orthogonal directions resulting in a data matrix of amplitude points. The phase and frequency at point (x,y) are related to their spatial position and gradient strength as follows:
&PHgr;
x
=&ggr;
x
G
x
&Dgr;t
x
x
Equation 4
where &PHgr;
x
is the phase at point (x,y), G
x
is the phase encoding gradient, &Dgr;t
x
is the phase encoding period, and x is the position along the phase encoded direction, and
&ohgr;
y
=&ggr;
y
G
y
&ggr; Equation 5
where &ohgr;
y
is the frequency at point (x,y), G
y
is the frequency encoding gradient, and y is the position along the frequency encoded direction.
For one typical MRI technique, a human subject is
Dean David
Duerk Jeffrey L.
Kamath Janardhan K.
Case Western Reserve University
Fay Sharpe Fagan Minnich & McKee LLP
Lefkowitz Edward
Shrivastav Brij B.
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