Method and apparatus of manual pre-scan spatial and spectral data acquisition

A method of pre-scan data acquisition includes the application of a pre-scan pulse sequence to acquire MR signals from a region-of-interest to be imaged with an imaging pulse sequence. The pre-scan pulse sequence applies a pre-scan readout gradient pulse and a pre-scan readout gradient rewinder pulse. MR signals are acquired from a region of interest during application of the pre-scan readout gradient pulse and after application of the pre-scan readout gradient rewinder pulse.

BACKGROUND OF THE INVENTION

The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and apparatus that enables a user to simultaneously view spatial and spectral data acquired from the same spatial slice of tissue an MR manual pre-scan.

When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.

Magnetic resonance imaging typically includes a patient-specific pre-scan calibration/setting of system center frequency, transmit pulse gain, fine center frequency and receive gain. When performing this multi-part procedure manually, an operator typically invokes separate pulse sequences for acquiring the center frequency data and object projection data. The operator is required to manually switch between these two modes and wait for any system software and MR physics steady state related delays before a new object projection plot or center frequency spectrum is displayed.

It would therefore be desirable to have a method and apparatus capable of reducing system software and MR physics steady state related delays caused by invoking separate pulse sequences for acquiring the center frequency data and object projection data.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a method and apparatus of acquiring MR data in a pre-scan acquisition sequence that overcome the aforementioned drawbacks. MR data is acquired in the pre-scan acquisition sequence during application of a readout gradient pulse. MR data is also acquired after application of a post readout gradient rewinder pulse.

In accordance with one aspect of the invention, the invention is embodied in a computer program stored on a computer readable storage medium and having instructions which, when executed by a computer, cause the computer to apply a pre-scan readout gradient pulse of a pulse sequence from a region-of-interest to be imaged. A first set of MR signals is acquired from the region-of-interest during application of the pre-scan readout gradient pulse. The instructions further cause the computer to apply a pre-scan readout gradient rewinder pulse of the pulse sequence after application of the pre-scan readout gradient pulse. A second set of MR signals is acquired from the region-of-interest after application of the pre-scan readout gradient rewinder pulse.

In accordance with another aspect of the invention, an MR apparatus includes an MR system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The MR apparatus also includes a computer readable storage medium having stored thereon a computer program comprising instructions which when executed by a computer cause the computer to apply an MR acquisition pulse sequence having a readout gradient pulse and a post readout gradient rewinder pulse. The computer is also caused to read out a first set of MR data from a spatial slice of a region-of-interest while applying the readout gradient pulse and read out a second set of MR data from the spatial slice of the region-of-interest after applying the post readout gradient rewinder pulse.

According to another aspect, the invention is embodied in a method of acquiring MR pre-scan image data. The method includes prescribing a pre-scan pulse sequence for a slice of a region of interest, the pre-scan pulse sequence having a first data acquisition window and a second data acquisition window. The method includes acquiring MR signals from the slice during the first data acquisition window and applying a gradient rewinder pulse between the first data acquisition window and the second data acquisition window. MR signals are acquired from the slice during the second data acquisition window.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1, the major components of a preferred MR system10incorporating the present invention are shown. The operation of the system is controlled from an operator console12which includes a keyboard or other input device13, a control panel14, and a display screen16. The console12communicates through a link18with a separate computer system20that enables an operator to control the production and display of images on the display screen16. The computer system20includes a number of modules which communicate with each other through a backplane20a. These include an image processor module22, a CPU module24and a memory module26, known in the art as a frame buffer for storing image data arrays. The computer system20is linked to disk storage28and tape drive30for storage of image data and programs, and communicates with a separate system control32through a high speed serial link34. The input device13can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription.

The system control32includes a set of modules connected together by a backplane32a. These include a CPU module36and a pulse generator module38which connects to the operator console12through a serial link40. It is through link40that the system control32receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module38operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module38connects to a set of gradient amplifiers42, to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module38can also receive patient data from a physiological acquisition controller44that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module38connects to a scan room interface circuit46which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit46that a patient positioning system48receives commands to move the patient to the desired position for the scan.

The gradient waveforms produced by the pulse generator module38are applied to the gradient amplifier system42having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated50to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly50forms part of a magnet assembly52which includes a polarizing magnet54and a whole-body RF coil56. A transceiver module58in the system control32produces pulses which are amplified by an RF amplifier60and coupled to the RF coil56by a transmit/receive switch62. The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil56and coupled through the transmit/receive switch62to a preamplifier64. The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver58. The transmit/receive switch62is controlled by a signal from the pulse generator module38to electrically connect the RF amplifier60to the coil56during the transmit mode and to connect the preamplifier64to the coil56during the receive mode. The transmit/receive switch62can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode.

The MR signals picked up by the RF coil56are digitized by the transceiver module58and transferred to a memory module66in the system control32. A scan is complete when an array of raw k-space data has been acquired in the memory module66. This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor68which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link34to the computer system20where it is stored in memory, such as disk storage28. In response to commands received from the operator console12, this image data may be archived in long term storage, such as on the tape drive30, or it may be further processed by the image processor22and conveyed to the operator console12and presented on the display16.

The present invention is directed to a method and apparatus to enable a user to interactively and/or a system to automatically determine parameters for a scan. As will be described, this interactive adjustment is through simultaneous visualization of the object projection data and the frequency spectrum. However, it is contemplated that adjustments can be made automatically without or in addition to user-visualization of the object projection and the frequency spectrum. For purposes of explanation and not limitation, the invention will be described with respect to a user-interactive technique based on simultaneous visualization of object projection data and a frequency spectrum.

The invention executes a pre-scan to acquire MR data that is used to generate an object projection plot and a frequency spectrum. The object projection plot and the frequency spectrum are displayed to the user thereby allowing the user to visualize the relative data and signals from fat and water components in a region-of-interest, such as a given acquisition slice. In one preferred example, the object projection plot may be used, for example, to visually convey information for optimizing the setting of the transmit gain, and the frequency spectrum may be used, for example, to visually convey center frequency data.

An exemplary imaging sequence for simultaneously acquiring object projection data and center frequency data in a single TR period is shown inFIG. 2. The pulse sequence70includes an RF excitation pulse80, applied with a flip angle of θ degrees that is applied in the presence of a slice selective gradient82followed by a slice rephasing gradient83, where θ is a desired angle, such as 90 degrees. Gradients82,83are followed by a zero-amplitude phase encoding gradient84and a frequency encoding gradient91and associated pre-dephaser pulse86. For purposes of illustration, the zero-amplitude phase-encoding pulse84is shown in phantom to illustrate that, in the present invention, no phase encoding is used during acquisition of pre-scan data. During the acquisition of imaging data, however, phase encoded data is acquired. A 2*θ degree RF rephasing pulse88is then applied, where 2*θ is a desired angle, such as 180 degrees. The rephasing pulse88is played out in the presence of slice selective gradient92and causes rephasing of the transverse magnetization whereupon an echo90is produced and is sampled for data collection under a frequency encoding readout gradient91during a data acquisition window94. Data sampled from echo90is used to obtain an object projection plot. Following application of the frequency encoding readout gradient91, a post readout gradient rewinder pulse96is applied whereupon a free induction decay (FID) signal98is produced and is sampled without a frequency encoding readout gradient for data collection during a bandwidth data acquisition window100. Data sampled from echo98is Fourier transformed to obtain a center frequency spectrum. Preferably, bandwidth data acquisition window100has a lower bandwidth than the bandwidth of data acquisition window94. In addition, the number of data samples sampled during data acquisition window100may be different than the number of data samples sampled during data acquisition window94.

As shown inFIG. 3, the pulse sequence70ofFIG. 2may include an optional 2*θ degree inversion pulse102that is applied after the post readout gradient rewinder pulse96and prior to the data acquisition window100. The 2*θ degree inversion pulse102is used to regain signal that has been reversibly eliminated via T2* decay mechanisms, i.e., by main magnet and susceptibility inhomogeneities. Following application of the 2*θ degree inversion pulse102, echo104is produced and is sampled without a frequency encoding readout gradient for data collection during low bandwidth data acquisition window106. Data sampled from echo104is Fourier transformed to obtain a center frequency spectrum.

To overcome the drawbacks of conventional separate pre-scan object projection data acquisition and center frequency data acquisition sequences, the present invention includes the acquisition of both the object projection pre-scan data and the center frequency pre-scan data during application of a single pre-scan pulse sequence. The object projection pre-scan data and the center frequency pre-scan data are used to simultaneously obtain and display an object projection plot and a center frequency spectrum to an MR system operator. This technique is described in greater detail with respect toFIG. 4.

As shown inFIG. 4, an interactive technique108is shown and begins at110with user prescription of scan parameters for an impending scan. These initial scan parameters may include, but are not limited to, a center frequency, an offset frequency and an amplitude of an RF saturation pulse that may be applied to suppress signal from nuclei that precess at the RF saturation pulse's saturation frequency. Based on the initial parameters for the scan, parameters for a pre-scan pulse sequence are determined at112. In accordance with the present invention, phase encoding gradients are not applied during application of the pre-scan pulse sequence at114. Spatially encoded MR echo signal data for use in generating the object projection plot is acquired in the presence of a readout gradient during a pre-scan data acquisition window at116. Following application of the readout gradient, a post readout gradient rewinder pulse is applied at118, and a 2*θ degree inversion pulse is optionally applied at120. Center frequency data is acquired at122during a pre-scan data acquisition window. Preferably, the pre-scan data acquisition window for acquiring the center frequency data has a lower bandwidth than the bandwidth of the pre-scan data acquisition window for acquiring object projection MR data. An object projection plot is generated at124, and a center frequency spectrum is generated at126. Both of these are generated by applying a Fourier transform to the respective acquired data. The object projection plot and the center frequency spectrum are simultaneously displayed to the user at128.

Technique108continues at130with the reception of one or more adjustments to the scan parameters determined by a user in real time or automatically obtained by a computer from a predetermined database of values. For instance, a user input may be received for adjusting the center frequency. Accordingly, the imaging pulse sequence parameters and/or the pre-scan pulse sequence parameters are adjusted at132based on the user adjustments. It is contemplated that one or more scan parameters may be automatically adjusted by the computer based on automatic observations of the center frequency spectrum and the object projection data of the pre-scan data as well as a result of adjustments to user-defined parameters.

The pre-scan acquisition loop is iterated until the user or computer requests it to stop. With each iteration, it is contemplated that the parameters used to acquire the pre-scan data are changed based on one or more inputs received after user visualization of the object projection plot and the center frequency spectrum or automatic changes that are made. If the calibration process does not stop134,136, the loop returns to step112with determination of the corresponding pre-scan parameters followed by execution of steps114-132heretofore described. Once calibration is terminated134,138, technique108ends at140with finalization of the imaging pulse sequence parameters and execution of an imaging pulse sequence in accordance with conventional imaging techniques. Thus, in a preferred embodiment, the pre-scan pulse sequence automatically repeats steps112-132with the user and/or automatic changes until the user or computer has signaled that the pre-scan process is complete. It is also contemplated, however, that the pre-scan acquisition loop can be pre-set to run a certain number of times and therefore not require the user or computer to “stop” the process. It is further contemplated that the object projection plot and the center frequency spectrum may be automatically analyzed with one of a number of analysis techniques and the pre-scan acquisition loop reiterated until the analysis indicates that optimal (or near-optimal) settings have been reached.

The heretofore described technique, which may be embodied in a computer program stored on a computer readable storage medium or in a computer data signal transmitted in a carrier wave, advantageously enables a user to correctly and optimally set the parameters of an imaging pulse sequence, (e.g., transmit gain, center frequency), during manual pre-scan calibration of the imaging pulse sequence. By acquiring both the object projection pre-scan data and the center frequency pre-scan data from the same spatial slice, the user may simultaneously view the object projection plot and the center frequency spectrum displays obtained from the same spatial slice without system software delays or MR physics-related delays involved with switching between two different acquisition sequences.

Therefore, the invention includes a computer program stored on a computer readable storage medium and having instructions which, when executed by a computer, cause the computer to apply a pre-scan readout gradient pulse of a pulse sequence from a region-of-interest to be imaged. A first set of MR signals is acquired from the region-of-interest during application of the pre-scan readout gradient pulse. The instructions further cause the computer to apply a pre-scan readout gradient rewinder pulse of the pulse sequence after application of the pre-scan readout gradient pulse. A second set of MR signals is acquired from the region-of-interest after application of the pre-scan readout gradient rewinder pulse.

The invention is also directed to an MR apparatus that includes an MR system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The MR apparatus also includes a computer readable storage medium having stored thereon a computer program comprising instructions which when executed by a computer cause the computer to apply an MR acquisition pulse sequence having a readout gradient pulse and a post readout gradient rewinder pulse. The computer is also caused to read out a first set of MR data from a spatial slice of a region-of-interest while applying the readout gradient pulse and read out a second set of MR data from the spatial slice of the region-of-interest after applying the post readout gradient rewinder pulse.

The invention is further embodied in a method of acquiring MR pre-scan image data. The method includes prescribing a pre-scan pulse sequence for a slice of a region of interest, the pre-scan pulse sequence having a first data acquisition window and a second data acquisition window. The method includes acquiring MR signals from the slice during the first data acquisition window and applying a gradient rewinder pulse between the first data acquisition window and the second data acquisition window. MR signals are acquired from the slice during the second data acquisition window.