Background-suppressed, reduced field-of-view radial magnetic resonance imaging

Embodiments relate to a method and system to improve fat suppression and reduce motion and off-resonance artifacts in magnetic resonance imaging (MRI) by using a background-suppressed, reduced field-of-view (FOV) radial imaging. The reduction of such artifacts provides improved diagnostic image quality, higher throughput of MRI scans for the imaging center, and increased patient comfort. By using a small FOV radial acquisition that only encompasses the structures of interest, structures that cause motion artifacts, such as the anterior abdominal wall, bowel loops, or blood vessels with pulsatile flow, are excluded from the image. According to an embodiment, combining a small FOV radial acquisition with one or more background-suppression techniques minimizes the impact of artifacts caused by anatomy outside of the FOV.

TECHNOLOGY FIELD

The present invention relates generally to acquisition of magnetic resonance images, and more particularly to utilizing a background-suppressed, reduced field-of-view in radial magnetic resonance imaging.

BACKGROUND

Magnetic resonance imaging (MRI) measures tissue-specific responses to a radio frequency (RF) stimulus in a strong static main magnetic field (BO). Specifically, the magnetization of tissue is aligned with BO. An initial RF pulse tips the magnetization out of this alignment and rotates with a tissue-specific RF frequency, resulting in a signal that is picked up with a receiver coil. Additional magnetic field gradient pulses (G) are used to spatially encode the RF signal that in turn is used to construct an image.

Before acquiring images, the MR scanner goes through an adjustment step, whereby the transmitter frequency is appropriately tuned so as to stimulate only desired tissue, excluding undesired signal from fat. If fat signal dominates in the field-of-view (FOV), as might happen in obese subjects with large amounts of subcutaneous fat, the scanner may not be appropriately tuned, thereby degrading the image quality.

MRI relies on a very homogeneous static magnetic field. Unfortunately, the magnetic field homogeneity of a clinical MRI scanner is degraded in the presence of the human body. In addition, static magnetic field homogeneity is degraded by interfaces between tissues of differing magnetic susceptibility (e.g., lung and liver), with this effect being highly patient dependent. Although the static magnetic field homogeneity can be improved by the process of shimming, the need to shim over a large FOV limits the efficacy of this technique. In addition to the reliance on static magnetic field homogeneity, MRI also requires that there is negligible motion from the time of the initial RF pulse through the application of gradient pulses and the reception of signal; this is because motion disrupts the spatial encoding introduced by the gradients, resulting in artifacts in the final images.

Image quality in MRI is affected not only by motion of the organ of interest, but also the motion of other organs within the FOV. For example, while imaging the thoracic spine, which is stationary, cardiac and breathing motion can degrade image quality in the spine region. Currently, MR images are generally acquired such that the FOV is large enough to cover all tissues within the slice of interest. In some attempts to reduce the FOV, efforts have focused on specially designed RF pulses to excite a small FOV covering the region of interest.

This document describes a method and system for minimizing artifacts from features in the slice that are outside of the region of interest.

SUMMARY

Embodiments of the present invention provide a method and system for magnetic resonance image acquisition utilizing a reduced field-of-view (FOV) in radial imaging techniques.

In an embodiment, a computer-implemented magnetic resonance imaging (MRI) method for acquiring images of a patient comprises: receiving, by an input processor, an indication of a field of view (FOV) that encompasses only an anatomy of interest of the patient, where the anatomy of interest is smaller than a full dimension of the patient; shimming, by a processor configured to communicate with the input processor, on the anatomy of interest within the FOV; applying, by the processor, radial imaging techniques on the shimmed FOV to acquire images of the anatomy; and generating, at a display processor configured to communicate with the processor, data representing the acquired images of the anatomy.

In an embodiment, a magnetic resonance imaging (MM) system for acquiring images of a patient comprises: a plurality of imaging coils comprising a plurality of gradient coils and a plurality of radio-frequency (RF) coils; and one or more processors configured to perform an imaging scan using the plurality of imaging coils, comprising: receiving an indication of a field of view (FOV) that encompasses only an anatomy of interest of the patient, where the anatomy of interest is smaller than a full dimension of the patient; shimming on the anatomy of interest within the FOV; and applying radial imaging techniques on the shimmed FOV to acquire the images of the anatomy. The system further comprises a display processor configured to communicate with the one or more processors to generate data representing the acquired images of the anatomy.

According to an embodiment, the radial imaging techniques comprise one or more magnetization preparation pulses to suppress unwanted features. The one or more magnetization preparation pulses may comprise at least one of (i) a regional saturation pulse that suppresses signal from specific regions pre-selected by a user; (ii) a fat-suppression pulse that suppresses signal from fat tissue; (iii) an in-plane saturation pulse that suppresses signal from all tissues in a slice; and (iv) intersecting inversion pulses that suppress signal in regions of the FOV that lie outside of their intersection.

In an embodiment, the radial imaging techniques comprise a radial quiescent-interval slice-selective (QISS) pulse sequence including a single magnetization preparation pulse.

According to an additional embodiment, the radial imaging techniques comprise one of two-dimensional and three-dimensional radial acquisition techniques.

The FOV, in an embodiment, is a reduced FOV compared to a required FOV for Cartesian acquisition.

In an embodiment, scan time for the radial imaging techniques is a reduced scan time compared to a required scan time for Cartesian acquisition.

According to an embodiment, the FOV excludes structures that cause motion artifacts.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention relate to a method and system to improve fat suppression and reduce motion and off-resonance artifacts in magnetic resonance imaging (MRI) by using a background-suppressed, reduced field-of-view (FOV) radial imaging. The reduction of such artifacts provides improved diagnostic image quality, higher throughput of MRI scans for the imaging center, and increased patient comfort.

FIG. 1is a diagram100illustrating effects of reducing the FOV with Cartesian acquisition (102,112) and radial acquisition (104,114). Boxes102aand104a represent the FOV used for the acquisition in102and104, respectively. As seen in the reduced FOV112acquired with Cartesian k-space sampling, the features not covered within the FOV are wrapped around in the image, substantially degrading display of the imaged object. As seen in the reduced FOV114with radial k-space sampling, the features not covered within the FOV are benignly smeared throughout the image, without degrading the display of the object being imaged.

According to embodiments, off resonance artifacts are minimized by using a small field-of-view radial acquisition. In this case, shimming is only performed over the structures (i.e., the anatomy) of interest, giving a more homogeneous static magnetic field over desired structures, whereas structures that degrade static field homogeneity, such as air-containing bowel loops, lung tissue, large collections of fat, etc., are excluded.

The reduced FOV radial acquisition ensures that the frequency adjustment is correctly performed to the water peak, and not, for example, inadvertently to the fat peak in patients with large amounts of subcutaneous fat.

According to embodiments, 2D or 3D radial acquisition techniques may be utilized.

Embodiments disclosed herein may be suited to, but are not limited to, applications that entail a magnetization preparation pulse to suppress unwanted features in the image. Some of these preparations may include, but are not limited to:1. a regional saturation pulse that suppresses signal from specific regions pre-selected by the user;2. a fat-suppression pulse that suppresses signal from fat tissue;3. an in-plane saturation pulse that suppresses signal from all tissues in a slice (used, for example, in angiography applications to accentuate the signal from inflowing blood compared with suppressed background); and4. intersecting inversion pulses that suppress signal in regions of the FOV that lie outside of their intersection.

By using a small FOV radial acquisition that only encompasses the structures of interest, structures that cause motion artifacts, such as the anterior abdominal wall, bowel loops, or blood vessels with pulsatile flow, are excluded from the image. According to an embodiment, combining a small FOV radial acquisition with one or more of the above-identified background-suppression techniques minimizes the impact of artifacts caused by tissues outside of the FOV.

FIG. 2Aillustrates the resulting application of the technique, according to embodiments disclosed herein, in an angiographic application in the abdomen for renal arteries. Conventional techniques use Cartesian acquisition, requiring large FOVs and long scan times in order to compensate for respiratory motion. Background-suppressed, reduced FOV radial imaging, according to embodiments disclosed herein, is accomplished with a single breath-hold during which the entire vascular tree of interest is imaged without degradation from bowel-loop motion or static magnetic field inhomogeneity caused by bowel gas, as seen in the resulting MR image200.

FIG. 2Billustrates the resulting application of the disclosed method of background-suppressed, reduced FOV radial imaging on the heart. Resulting MR image250is a dark blood cardiac morphological image using small FOV radial imaging.

FIG. 3Ais a diagram illustrating an exemplary pulse sequence utilized with the imaging techniques described herein, according to embodiments provided herein.FIG. 3Ais a representation of an exemplary radial quiescent-interval slice-selective (QISS) pulse sequence300; andFIG. 3Bis a representation of an exemplary 2D T2-prepared balanced steady-state free-precession (bSSFP) pulse sequence350.305represents an in-plane inversion pulse (a magnetization preparation pulse);310a fat saturation pulse;315a bSSFP alpha/2 ramp pulse;320a T2 preparation pulse; and325a radial bSSFP readout. As shown inFIGS. 3A and 3B, the QISS pulse sequence300involves the application of a single magnetization preparation pulse (pulse305); whereas the T2-prepared bSSFP pulse sequence350applies four RF pulses during the magnetization preparation, resulting in a substantial increase in SAR.

The fat saturation pulse310is an example of a background suppression pulse that selectively suppresses signal from fat. The bSSFP alpha/2 ramp pulse315helps to stabilize the signal from the bSSFP readout. The T2 preparation pulse320is an effective background suppression pulse (suppresses background muscle signal so that arteries are accentuated in comparison).

Radial QISS is immune from fold over artifacts, which allows the use of much smaller FOV than is practical using Cartesian imaging. With radial QISS, the high degree of background suppression from the combination of in-plane tissue inversion and fat suppression minimizes streak artifacts, which facilitates the use of high under-sampling factors.

As an example, for Cartesian QISS, a matrix size of 256×170, FOV of 358-mm×237-mm, parallel acceleration (ipat) factor of 2 was necessary to obtain the MR anatomy of interest. In contrast, for radial QISS, the matrix of 160 and FOV of 225-mm squared is sufficient.

According to embodiments disclosed herein, scan time is reduced when utilizing small FOV radial acquisition. On most modern MRI systems, Cartesian and radial trajectories typically support under-sampling factors of 2 and 5, respectively. As an example, when imaging the coronary arteries with 1 mm spatial resolution and 80 ms temporal resolution using a cardiac-gated MRI sequence with 4 ms repetition time, a Cartesian MRI scan acquiring a 320 mm square field of view would require (320 mm/1 mm)/(80 ms/4 ms)*(½)=8 heartbeats to complete. With the method described herein (i.e., small FOV radial acquisition), assuming that a smaller 160 mm square field of view is used due to the lack of fold-over and streak artifacts, the scan only requires (160 mm/1 mm)*pi/2/(80 ms/4 ms)*(⅕)=2.5 heartbeats to complete.

For imaging the abdominal organs such as liver, kidneys, and prostate, background suppressed, reduced FOV radial imaging, according to embodiments disclosed herein, can be used to minimize the impact of: motion from such sources as bowel loops, pulsatile blood flow, and anterior abdominal wall; and off-resonance effects caused by air in bowel loop and lungs.

In the heart, the reduced radial FOV, according to embodiments disclosed herein, is beneficial in improving the quality of fat suppression and avoiding off-resonance effects when a balanced steady-state precession pulse sequence is used to collect the data.

In angiographic applications, most structures need to be suppressed, except vascular blood. Depending on the vascular branch being imaged, background-suppressed, reduced FOV radial imaging, according to embodiments disclosed herein, can be used to minimize motion artifacts from moving organs as well as undesired signal from subcutaneous fat.

For imaging of the thoracic and lumbar spine, a background-suppressed, reduced FOV radial turbo spin-echo pulse sequence can be used to image the vertebral column and spinal cord while excluding the contents of the thorax and abdomen, respectively. In the cervical spine, the FOV can be reduced to exclude swallowing artifacts from the mouth and pharynx.

FIG. 4is a flowchart400illustrating a computer-implemented MRI method for acquiring images of an anatomy of a patient, according to embodiments provided herein. At410, an indication of a FOV that encompasses only an anatomy of interest of the patient is received by an input processor. According to embodiments herein, the anatomy of interest is smaller than a full dimension of the patient. The FOV and the anatomy of interest may be determined by a user and inputted into a MRI system, such as system500described below with reference toFIG. 5.

At420, shimming on the anatomy of interest within the FOV is performed by a processor configured to communicate with the input processor. As described above, the shimming provides a more homogeneous static magnetic field over desired structures (i.e., the anatomy of interest).

At430, radial imaging techniques are applied by the processor on the shimmed FOV to acquire the images of the anatomy. Imaging can be done during free-breathing or with the patient holding their breath. As described above, various background-suppression pulses may be applied prior to radial imaging.

At440, data representing the acquired images of the anatomy is generated at a display processor configured to communicate with the processor. The data may include images such as those shown inFIGS. 2A and 2B.

Turning toFIG. 5, a system500for acquiring MRI images of an anatomy of a patient, according to embodiments provided herein, is illustrated. The system500includes a source510of the tissue, such as a patient.512,514, and516represent the coils and magnets of an MRI system and are, in an exemplary embodiment, a high field magnet512, a gradient coil514, and a radio-frequency (RF) coil516. Processors518(gradient and shim coil controller) and520(radio-frequency controller) control the MR magnets and coils. The MRI system components512,514, and516and processors518and520depicted inFIG. 5are one example of an MRI system; other components and processors may be used as known to one of skill in the art to obtain an MR image of tissue.

The system500further includes an input processor530, an image data processor540, a display processor560, and an interface570. A central control system550controls the overall operation of and data communication between each of the processors518,520,530,540, and560.

FIG. 6illustrates an exemplary computing environment600within which embodiments of the invention may be implemented. Computing environment600may include computer system610, which is one example of a computing system upon which embodiments of the invention may be implemented. Computers and computing environments, such as computer610and computing environment600, are known to those of skill in the art and thus are described briefly here.

As shown inFIG. 6, the computer system610may include a communication mechanism such as a bus621or other communication mechanism for communicating information within the computer system610. The system610further includes one or more processors620coupled with the bus621for processing the information. The processors620may include one or more central processing units (CPUs), graphical processing units (GPUs), or any other processor known in the art.

The computer system610also includes a system memory630coupled to the bus621for storing information and instructions to be executed by processors620. The system memory630may include computer readable storage media in the form of volatile and/or nonvolatile memory, such as read only memory (ROM)631and/or random access memory (RAM)632. The system memory RAM632may include other dynamic storage device(s) (e.g., dynamic RAM, static RAM, and synchronous DRAM). The system memory ROM631may include other static storage device(s) (e.g., programmable ROM, erasable PROM, and electrically erasable PROM). In addition, the system memory630may be used for storing temporary variables or other intermediate information during the execution of instructions by the processors620. A basic input/output system633(BIOS) containing the basic routines that help to transfer information between elements within computer system610, such as during start-up, may be stored in ROM631. RAM632may contain data and/or program modules that are immediately accessible to and/or presently being operated on by the processors620. System memory630may additionally include, for example, operating system634, application programs635, other program modules636and program data637.

The computer system610also includes a disk controller640coupled to the bus621to control one or more storage devices for storing information and instructions, such as a magnetic hard disk641and a removable media drive642(e.g., floppy disk drive, compact disc drive, tape drive, and/or solid state drive). The storage devices may be added to the computer system610using an appropriate device interface (e.g., a small computer system interface (SCSI), integrated device electronics (IDE), Universal Serial Bus (USB), or FireWire).

The computer system610may also include a display controller665coupled to the bus621to control a display or monitor666, such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information to a computer user. The computer system610includes an input interface660and one or more input devices, such as a keyboard662and a pointing device861, for interacting with a computer user and providing information to the processors620. The pointing device661, for example, may be a mouse, a trackball, or a pointing stick for communicating direction information and command selections to the processors620and for controlling cursor movement on the display666. The display666may provide a touch screen interface which allows input to supplement or replace the communication of direction information and command selections by the pointing device661.

The computer system610may perform a portion or all of the processing steps of embodiments of the invention in response to the processors620executing one or more sequences of one or more instructions contained in a memory, such as the system memory630. Such instructions may be read into the system memory630from another computer readable medium, such as a hard disk641or a removable media drive842. The hard disk641may contain one or more datastores and data files used by embodiments of the present invention. Datastore contents and data files may be encrypted to improve security. The processors620may also be employed in a multi-processing arrangement to execute the one or more sequences of instructions contained in system memory630. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and software.

The computing environment600may further include the computer system610operating in a networked environment using logical connections to one or more remote computers, such as remote computer680. Remote computer680may be a personal computer (laptop or desktop), a mobile device, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to computer system610. When used in a networking environment, computer system610may include modem672for establishing communications over a network671, such as the Internet. Modem672may be connected to system bus621via user network interface670, or via another appropriate mechanism.

As described herein, the various systems, subsystems, agents, managers and processes can be implemented using hardware components, software components and/or combinations thereof.

Although the present invention has been described with reference to exemplary embodiments, it is not limited thereto. Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the invention and that such changes and modifications may be made without departing from the true spirit of the invention. It is therefore intended that the appended claims be construed to cover all such equivalent variations as fall within the true spirit and scope of the invention.