MRI methods and systems

Details MRI methods and systems MRI methods and systems

1998-11-25

2002-06-04

Arana, Louis (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S309000

Reexamination Certificate

active

06400157

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods and systems for conducting magnetic resonance imaging and, more particularly, to the control of the individual parameters of a pulse sequence controlling a magnetic resonance imaging scanning sequence by medical personnel within an operating room during surgery.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging (“MRI”) is a well known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures.
Magnetic resonance imaging uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue. During a magnetic resonance imaging procedure, the patient is inserted into an imaging volume of a magnet which generates a static magnetic field. Gradient coils are provided within the imaging volume to generate time-varying, linear magnetic fields along the x, y, z axis within the imaging volume, as well. Within the imaging volume, the vector of the angular momentum or spin of nuclei of elements containing an odd number of protons or neutrons tends to align with the direction of the magnetic field. The spins of the nuclei are referred to as the “spin system”.
Exciting the spin system of the tissue within the imaging volume by radio frequency energy at the resonant or Larmor frequency shifts the spin system out of alignment with the applied magnetic field and into phase with each other. The spins of the nuclei then turn or “precess” around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance (“MR”) signals, at the resonant or Larmor frequency, which are detected by a radio frequency antenna tuned to that frequency. The gradient magnetic fields applied during the pulse sequence spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another and back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay. The MR signals are detected by a radio frequency receiving antenna positioned within the imaging volume proximate the patient and are amplified, digitized and processed by the MRI system. Hydrogen is the most commonly detected element because it is the most abundant nuclei in the human body and emits the strongest MR signal.
The rate of decay or relaxation of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By known mathematical techniques involving correlation of the gradient magnetic fields and the particular frequency of the radio frequency waves applied at various times with the decay rate of the MR signals emitted by the patient, the environment, as well as the concentrations, of the nuclei of interest at various locations within the patient's body may be determined. This information is typically displayed as an image with varying intensities which are a function of the concentration and environment of the nuclei of interest.
A magnetic resonance imaging procedure typically comprises one or more image scanning sequences, each of which comprises a precisely timed and orchestrated series of pulses of radio-frequency energy, and variation of the three orthogonal magnetic field gradients and the data sampling window. Each scanning sequence is defined by a pulse sequence, which is a series of values for parameters corresponding to particular characteristics of the scanning sequence and the resulting MR images. An image is derived from many repetitions of the pulse sequence, where small changes are typically introduced in the phase encoding gradient of the pulse sequence parameters between repetitions to provide additional spatial encoding. Other changes to the pulse sequences may be introduced, as well.
The relaxation rate of the MR signals has two components which are responsible for the contrast in intensities in MR images, T
1
, or the spin-lattice relaxation time, and T
2
, or the spin-spin relaxation time. T
1
, is a function of the rate at which the radio frequency energy absorbed by the nuclei to cause the shift of the spin system is dissipated as the magnitude of the magnetization vector of the spin system returns to its original alignment with the magnetic field. T
2
is a measure of the dephasing or loss of phase coherence of the spin system. The relaxation times T
1
, T
2
are inherent characteristics of the particular sample being irradiated.
Since different types of tissue have different relaxation times, the degree T
1
and T
2
contribute to an image, and hence the image contrast, can be altered by timing parameters of the pulse sequence. TR is the repetition time of the pulse sequence. TR varies the contribution of T
1
of different tissue types to the image. Another timing parameter, TE, referred to as the echo time, is the time interval between injecting a radio frequency pulse and the appearance of a spin echo MR signal, which affects the contribution of T
2
to the image. The TR and TE timing parameters of the pulse sequence are executed by an NMR controller of the MRI system. Scanning at short TR and TE times enables faster image acquisition than scanning at long TR and TE values. It has been found that short TR and TE times yield images with better anatomical detail than images taken at longer TR/TE times, while longer TR and TE times yield images with the better contrast information than shorter TR/TE times. Better contrast information may result in better detail for diagnostic purposes. Abnormal tissue, such as diseased or cancerous tissue, is more readily identified on images taken at longer TR and TE times.
Other characteristics of the MR image controlled by the values of parameters of the pulse sequence include the orthogonal alignment of the slice axis of the image in the sagittal, axial or coronal axis of the patient, the oblique angle of the scan through the region of interest of the patient, the field of view of the image and the position of the slice.
The orthogonal alignment of the slice axis of the image is determined by the orientation of the slice select gradient field with respect to the x-y-z axis. The orientation of the slice axis of the image gradient field is controlled by a gradient controller in the MRI system. The oblique angle of the scan is varied by suitably controlling the gradient magnetic fields to define a gradient plane of constant magnetic field at the desired angle. See, for example, U.S. Pat. No. 4,871,966, assigned to the assignee of the present invention and incorporated by reference herein.
The field of view of the image is determined by the magnitudes of the gradient fields, which are also controlled by the gradient controller, and the sampling period, which is controlled by the NMR controller.
The slice position is determined by the center frequency of the radio frequency pulse, which is controlled by the NMR controller. The thickness of the slice is determined by the bandwidth of the radio frequency pulse and the magnitudes of the gradient fields, which are controlled by the NMR controller and the gradient controller, respectively.
Typically, the MRI system is controlled by a MR technologist outside the room containing the system and patient. The technologist sits at a console with a monitor and uses a mouse or keyboard to click on or type in a limited set of options in a menu driven program. The technologist may select a particular predetermined pulse sequence or may select values for particular parameters of the pulse sequence from a list, based on the portion of the body to be scanned and the instructions of a doctor. When one scanning sequence or serie

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