Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-02-08
2004-11-23
Robinson, Daniel (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S307000
Reexamination Certificate
active
06823205
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to magnetic resonance imaging (MR imaging), and more particularly to computing quantitative images as well as synthetic images from scan data, with the purpose of allowing the user to perform virtual MRI scanning retrospectively and not requiring the presence of the patient.
2. Background Information
In magnetic resonance image scanning (MRI scanning), images of a subject, usually a patient's body, are produced through the interaction of a magnetic field applied to the patient's body and the magnetic moment of protons. Each proton behaves as small bar magnet, and the strength of the bar magnet is referred to as the “magnetic moment” of the proton. All protons have the same value of magnetic moment, just as each proton has the same value of electric charge. The protons are the nuclei of hydrogen atoms. The hydrogen is chemically bonded in compounds of the patient's tissue.
It is standard engineering practice in MRI imaging to apply a strong magnetic field substantially parallel to the spinal column of the patient. This magnetic field is referred to as the “longitudinal magnetic field” and is represented in symbols as B
0
. Upon the application of the longitudinal magnetic field, protons in the patient's tissue align with the magnetic field to produce a magnetization (longitudinal magnetization) of the patient's tissue. The longitudinal magnetization is a vector quantity that points along the applied longitudinal magnetic field. The magnetization of the patient's tissue may be represented as formed by many protons aligned with the longitudinal magnetic field.
The patient's magnetization is used to produce images by observing its response to a magnetic field applied by radio frequency pulses, where the radio frequency magnetic field is applied perpendicular to the longitudinal magnetic field. The frequency of the radio frequency magnetic field is chosen, along with the time duration of the application of the radio frequency magnetic field, to cause the protons to rotate, more precisely to precess, by a desired angle. The angle by which the protons rotate, or precess, is referred to as the “flip angle”. Ordinarily, radio frequency magnetic fields are applied to rotate the protons through an angle of 180 degrees so that their magnetization points in the reverse direction of the applied longitudinal magnetic field (that is into the anti-parallel direction), or to rotate the protons through an angle of 90 degrees so that their magnetization points into a plane perpendicular to the direction of the applied longitudinal magnetic field, that is into the “transverse plane”. Other values of the flip angle may also be employed in MRI imaging.
An image reproducing the proton density in the patient's body may be obtained by applying a radio frequency field for a time sufficient to rotate the proton magnetization through to a 90-degree angle, and such an application of a radio frequency magnetic field is referred to as applying a “90 degree” RF pulse. Upon application of a 90 degree RF pulse, the protons are rotated from a direction substantially parallel to the longitudinal magnetic field into a direction substantially perpendicular to the longitudinal magnetic field. The proton magnetic moments, during this rotation, remain substantially aligned with each other, so the patient magnetization becomes a vector in the plane perpendicular to the longitudinal magnetic field. The plane perpendicular to the longitudinal magnetic field is referred to as the “transverse plane”.
The patient's magnetization in the transverse plane is substantially equal in magnitude to the value that it had before application of the 90 degree RF pulse, however the patient's magnetization points in a direction in the transverse plane. For example, the transverse plane can be described with an X-axis and a Y-axis, and the X-axis may be chosen so that it is aligned with the magnetization in the transverse plane at the end of the 90-degree RF pulse. The magnetization in the transverse plane rotates in the transverse plane, and as a consequence of this rotation generates a radio frequency signal originating from the patient's tissue. This radio frequency signal is detected by a radio receiver, and is analyzed to produce an image.
A particular transverse plane is chosen for readout by applying a longitudinal magnetic field gradient, and choosing the frequency of the RF pulse to resonate with the protons in the chosen transverse plane. Ordinarily, changing the frequency of the RF pulse while longitudinal magnetic field gradient is held constant shifts the position of the desired transverse plane. The radio receiver receives the RF signal generated by the rotating magnetization, and the RF signal received by the radio receiver is spread over a frequency band, and with different phases, by the application of two transverse magnetic field gradients. For example, a transverse magnetic field gradient is applied, and a read out of emissions from the patient's tissue is obtained from the RF receiver. Again, a different transverse magnetic field gradient is applied, and a second readout is obtained from the RF receiver. A sequence of readouts is obtained, for different radio frequency values and for different phases, by applying different magnetic field gradients. A Fourier transform of the frequency and phase information received by the RF receiver is then computed. The output of the Fourier transform calculation produces the image of the patient's tissue. The image is presented as a two dimensional matrix of pixels.
After a first image is obtained, a waiting period is introduced. At the end of the waiting period a second image of the patient's tissue in the same transverse plane is obtained. The intensity of the radio frequency signal generated by the patient's tissue is reduced in the second image in comparison with the strength produced in the first image by transverse relaxation phenomena. Transverse relaxation phenomena are modeled by a transverse relaxation time, referred to as “T
2
”. This reduction in the signal in the second image is used to compute the transverse relaxation time T
2
. The values of T
2
may be computed at each pixel of the image.
Transverse relaxation phenomena, which are measured by the measured value of T
2
, are predominately caused by different protons in the transverse plane being subject to slightly different magnetic fields. The different magnetic fields throughout the transverse plane have their origin in several phenomena: the first being the magnetic field gradient which is intentionally applied to the transverse plane in order to obtain space resolution in the transverse plane; a second being different chemical environments of the protons in molecules within different regions of the transverse plane; and a third being movement of the protons through the tissue of the patient, such as caused by blood flow, etc.
Additionally, the protons in the transverse plane relax back to being parallel to the longitudinal magnetic field with a relaxation time referred to as “T
1
”, where T
1
is known as the “longitudinal relaxation time”.
A 180-degree RF pulse may be applied to the patient's tissue. The result of the 180-degree RF pulse is that the proton magnetic moments are rotated 180 degrees. This rotation points the protons away from being parallel to the longitudinal magnetic field to being anti-parallel. The protons then relax toward the parallel orientation of the longitudinal magnetic field with the transverse relaxation time “T
1
”. After a waiting time from the 180-degree pulse, a 90-degree RF pulse is then applied to the patient's tissue. The 90-degree RF pulse causes the net magnetization of the patient's tissue to rotate 90 degrees into the transverse plane where the magnetization precesses, and so produces an output RF signal from the patient's tissue. An image is again read out by using transverse magnetic field gradients a
Boston University Radiology Associates
Cesari and McKenna LLP
Robinson Daniel
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