Pulsed main field nuclear magnetic resonance imaging system

Claim 2

In a method for imaging a region of the body using nuclear magnetic resonance where the main magnet has a controllable field amplitude the steps of: applying gradient fields; increasing the amplitude of the main magnetic field during the image data acquisition time interval including the excitation and reception intervals for a time shorter than the thermal time constant of the main magnet; decreasing the amplitude of the main magnetic field during times that image data is not being acquired; and processing the nuclear magnetic resonance signals to provide spatial information for creating an image.

Claim 3

Apparatus for producing nuclear magnetic images of a region of the body comprising: an electromagnet sufficiently large to encompass the human body for providing the main field of the imaging system; means for applying gradient fields; data acquisition means for acquiring nmr imaging data; data processing means for processing the nmr data to produce an image; and means for controlling the amplitude of the main magnetic field where it is increased during the time data is acquired including the excitation and reception intervals, for a time shorter than the thermal time constant of the electromagnet, and decreased during times that image data is not being acquired whereby the signal-to-noise ratio is improved.

Claim 4

Apparatus for producing nuclear magnetic images of a region of the body comprising: an electromagnet sufficiently large to encompass the human body for providing the main field of the imaging system; means for applying gradient fields; data acquisition means for acquiring nmr imaging data; data processing means for processing the nmr data to produce an image; and means for controlling the amplitude of the main magnetic field where it is increased during the time data is acquired including the excitation and reception intervals, for a time shorter than the thermal time constant of the electromagnet, and decreased during times that image data is not being acquired whereby the signal-to-noise ratio is improved.

Claim 5

Apparatus as recited in claim 2 where the field is turned off during times that image data is not being acquired.

Claim 6

Apparatus as recited in claim 2 where the field is reduced, during times that image data is not being acquired, to a value which results in an average power dissipation in the electromagnet below that of the maximum dissipation allowed.

Claim 7

Apparatus as recited in claim 2 where the time that the main magnetic field is at the increased level corresponds to the time required to collect all of the imaging data for the region.

Claim 8

Apparatus as recited in claim 2 where the time that the main magnetic field is at the increased level corresponds to the time of acquiring data for a single projection interval including both excitation and reception intervals.

Claim 9

Apparatus as recited in claim 2 where the rate of change of the increasing and decreasing magnetic field is limited to a value which prevents excess voltage from appearing across the electromagnet and causing breakdown.

Claim 10

Apparatus as recited in claim 2 further comprising: means for increasing the magnetic field to a value higher than that used to collect imaging data; means for acquiring nmr data from a subregion substantially smaller than the region being imaged; and means for processing the nmr data from the subregion using spectral analysis to characterize the subregion.

Claim 11

Apparatus as recited in claim 2 wherein the means for controlling the amplitude of the main magnetic field includes means for periodically increasing the field at a fixed repetition rate.

Claim 12

In a method for characterizing the tissue of a subregion of a region of the body being imaged using nuclear magnetic resonance the steps of: pulsing the main magnetic field to a first value during the image data acquisition time including both excitation and reception intervals having a duration shorter than the thermal time constant of the main magnet; applying gradient fields; acquiring and processing image data from the region; pulsing the main magnetic field to a second value which is higher than the first value for a time shorter than the thermal time constant of the main magnet; acquiring nmr signals from the subregion; and spectrally analyzing the signals from the subregion.

Claim 13

In a method for characterizing the tissue of a subregion of a region of the body being imaged using nuclear magnetic resonance the steps of: pulsing the main magnetic field to a first value during the image data acquisition time including both excitation and reception intervals having a duration shorter than the thermal time constant of the main magnet; applying gradient fields; acquiring and processing image data from the region; pulsing the main magnetic field to a second value which is higher than the first value for a time shorter than the thermal time constant of the main magnet; acquiring nmr signals from the subregion; and spectrally analyzing the signals from the subregion.

Claim 14

In a method for imaging a region using nuclear magnetic resonance where the main magnet can contain the human body and has a controllable field amplitude the steps of: applying gradient fields; increasing the amplitude of the main magnetic field during the acquisition of a complete set of image data for reconstructing an image for a time shorter than the thermal time constant of the main magnet; decreasing the amplitude of the main magnetic field outside of this acquisition time; and processing the resultant signals to provide an image of a region of the body.

Claim 15

In a method for imaging a region using nuclear magnetic resonance where the main magnet can contain the human body and has a controllable field amplitude the steps of: applying gradient fields; increasing the amplitude of the main magnetic field during the acquisition of a complete set of image data for reconstructing an image for a time shorter than the thermal time constant of the main magnet; decreasing the amplitude of the main magnetic field outside of this acquisition time; and processing the resultant signals to provide an image of a region of the body.

Claim 16

In a method for increasing the signal-to-noise ratio of an nmr system where the main magnet can contain the human body and has a controllable field amplitude the steps of: applying gradient fields; increasing the amplitude of the main magnetic field during at least the times of excitation of the body and reception of signals; decreasing the amplitude of the main magnetic field outside of the excitation and reception times; and processing the signals to provide an image of improved signal-to-noise ratio.

Claim 17

In a method for increasing the signal-to-noise ratio of an nmr system where the main magnet can contain the human body and has a controllable field amplitude the steps of: applying gradient fields; increasing the amplitude of the main magnetic field during at least the times of excitation of the body and reception of signals; decreasing the amplitude of the main magnetic field outside of the excitation and reception times; and processing the signals to provide an image of improved signal-to-noise ratio.