Patent Application
20150094562
The present application is associated with US Publication No. 2008/0081986 entitled “Method and Apparatus for Generating a Magnetic Resonance Image” and published on Apr. 3, 2008. The entire contents of that publication are incorporated herein by reference.
Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water and fat become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis”, by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio Frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MR images may be created by applying currents to the gradient and RF coils according to known algorithms called “pulse sequences”. A pulse sequence diagram may be used to show the amplitude, phase and timing of the various current pulses applied to the gradient and RF coils for a given pulse sequence. The selection of a pulse sequence determines the relative appearance of different tissue types in the resultant images, emphasizing or suppressing tissue types as desired. The inherent MR properties of tissue, most commonly the relaxation times T1 and T2, may be exploited to create images with a desirable contrast between different tissues. For example, in an MR image of a brain, gray matter may be caused to appear lighter or darker than white matter, according to the MRI system operator's choice of pulse sequence.
A pulse sequence may include a “spin preparation”, which is comprised of RF and gradient pulses that are played out (i.e., performed or applied) prior to the acquisition of MR data. A spin preparation may be used to control the appearance of a specific tissue type in an image, or to suppress signal from a certain tissue. Tissue suppression techniques are most commonly used for suppressing signal from fat. Multiple spin preparations are known that are able to suppress signal from fat, including CHESS (Chemical Shift Selective) pulses and Inversion Recovery preparations.
In certain clinical imaging applications, it is desirable to suppress the signal not only from fat tissue but also from another type of tissue in the same set of images. In cardiac MRI, for example, a paramagnetic contrast agent is used to visualize injured myocardial tissue. After a bolus of contrast agent is delivered intravenously, infarcted tissue retains a higher concentration of contrast agent for a longer period. This contrast agent shortens the T1 in the infarcted tissue, causing it to appear bright relative to healthy myocardium on T1-weighted images. Imaging the heart after a delay post injection of a contrast agent is called “myocardial delayed enhancement imaging”. Tissues that have a delayed hyper-enhancement are considered non-viable. In this type of imaging, it is desirable to choose a pulse sequence that can suppress the signal from healthy myocardium, so that the borders of the bright contrast-media-enhancing infarcted tissue may be clearly depicted. However, the presence of adjacent pericardial fat, which is also bright on a T1-weighted sequence, can negatively impact the identification of the infarct's borders.
In addition, MR image contrast changes can occur as a result of a subject's heart rate. Moreover, these contrast changes can make the MR image difficult to read. For example, a subject's heart rate might increase because he or she is nervous during an MRI procedure. Other subject's may suffer from arrhythmia which can result in unpredictable heart rate changes. Note that heart rate changes of +1-20% have been measured during a breath-hold scan in volunteers and can be even larger in cardiac patients. In some cases, an operator may manually attempt to adjust MR image timing to account for heart rate changes, but such an approach is prone to errors. Another approach is to use a myocardial delayed enhancement application (and IR prep pulse sequence) that triggers at every second heart beat. This may result in an improved IR prep contrast, but may also result in a decreased sensitivity to heart rate changes and longer total scan times.
It would therefore be desirable to provide systems and methods to facilitate an acquisition of MR images in an automated, accurate, and consistent manner.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of embodiments. However it will be understood by those of ordinary skill in the art that the embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the embodiments.
To suppress signals from a particular type of tissue in a subject, a spin preparation may include a combination of a non-selective inversion RF pulse and two tissue-selective inversion RF pulses. According to some embodiments, the tissue-selective inversion RF pulses are inserted in the spin preparation between another non-selective inversion RF pulse (used to suppress another type of tissue in the subject) and an acquisition window of the pulse sequence. For example, a combination of fat-selective inversion RF pulses may suppress the fat signal without disturbing the desired T1 contrast that develops between the other (non-fatty) tissues of interest. The resultant spin preparation is comprised of: an inversion RF pulse configured to invert the longitudinal magnetization from all tissues including the fat tissue and a second tissue, followed by a first fat-selective inversion RF pulse, then a delay, followed by a second fat-selective inversion RF pulse such that fat is also nulled when the magnetization from the first tissue is nulled. In this application, “nulled” is used to mean that the longitudinal magnetization of a tissue is significantly reduced, such that it no longer detracts from a reader's ability to visualize the surrounding tissue. This does not require that data is acquired at exactly the null point of the tissue, but holds for a window of time around the null point. As will be described, according to some embodiments the timing of pulses within the pulse sequence are dynamically adjusted based on the subject's current ECG rate.
The system control computer 32 includes a set of modules in communication with each other via electrical and/or data connections 32a. Data connections 32a may be direct wired links, or may be fiber-optic connections or wireless communication links or the like. In alternative embodiments, the modules of computer system 20 and system control computer 32 may be implemented on the same computer systems or a plurality of computer systems. The modules of system control computer 32 include a CPU module 36 and a pulse generator module 38 that connects to the operator console 12 through a communications link 40. It is through link 40 that the system control computer 32 receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module 38 operates the system components that play out (i.e., perform) the desired pulse sequence and produces data called RF waveforms which control the timing, strength and shape of the RF pulses to be used, and the timing and length of the data acquisition window. The pulse generator module 38 connects to a gradient amplifier system 42 and produces data called gradient waveforms which control the timing and shape of the gradient pulses that are to be used during the scan. The pulse generator module 38 may also receive patient data from a physiological acquisition controller 44 that receives signals from a number of different sensors connected to the patient, such as Electro Cardio Gram (ECG) signals from electrodes attached to the patient (e.g., to determine a subject's current ECG rate). The pulse generator module 38 connects to a scan room interface circuit 46 that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit 46 that a patient positioning system 48 receives commands to move the patient table to the desired position for the scan.
The gradient waveforms produced by the pulse generator module 38 are applied to gradient amplifier system 42 which is comprised of Gx, Gy and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated 50 to produce the magnetic field gradient pulses used for spatially encoding acquired signals. The gradient coil assembly 50 forms part of a magnet assembly 52 that includes a polarizing magnet 54 and a whole-body RF coil 56. A patient or imaging subject 70 may be positioned within a cylindrical imaging volume 72 of the magnet assembly 52. A transceiver module 58 in the system control computer 32 produces pulses that are amplified by an RF amplifier 60 and coupled to the RF coils 56 by a transmit/receive switch 62. The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil 56 and coupled through the transmit/receive switch 62 to a preamplifier 64. The amplified MR signals are demodulated, filtered and digitized in the receiver section of the transceiver 58. The transmit/receive switch 62 is controlled by a signal from the pulse generator module 38 to electrically connect the RF amplifier 60 to the RF coil 56 during the transmit mode and to connect the preamplifier 64 to the coil during the receive mode. The transmit/receive switch 62 can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode.
The MR signals sensed by the RF coil 56 are digitized by the transceiver module 58 and transferred to a memory module 66 in the system control computer 32. Typically, frames of data corresponding to MR signals are stored temporarily in the memory module 66 until they are subsequently transformed to create images. Most commonly, a Fourier transform is used to create images from the MR data. These images are communicated through the high speed link 34 to the computer system 20 where it is stored in memory, such as disk storage 28. In response to commands received from the operator console 12, this image data may be archived in long term storage, such as on the tape drive 30, or it may be further processed by the image processor 22 and conveyed to the operator console 12 and presented on display 16.
Note that the recovery of longitudinal magnetization in an inversion preparation (IR prep) experiment may depend on T1 relaxation. Therefore, the amount of longitudinal magnetization which is recovered when the next IR prep pulse is applied may depend on the time that has passed before the previous IR prep pulse. In gated MR scans, the time between IR prep pulses may depend on a trigger signal (e.g., ECG). If the amount of signal recovery changes, then the timing of the signal may need to be changed in order to achieve the same IR prep contrast. According to some embodiments described herein, a measurement of the current heart rate is added to the scan and the current heart rate may be used to adapt the timing of IR prep pulses.
For example,
At S210, a magnetic field may be applied to a subject having a first tissue and a second tissue, and the magnetic field may cause a net longitudinal magnetization in the tissues. According to some embodiments, the first tissue may be normal myocardial tissue (which should preferably be nulled in the acquired MR image) and the second tissue may be myocardial infarct tissue (which should preferably be easily readable in the acquired MR image). At S220, an inversion RF pulse may be generated to invert the longitudinal magnetization from the first and second tissues.
At S230, heart-rate timing information associated with a current ECG of the subject may be measured. The heart-rate timing information may, for example, comprise “tau” representing a length of time between two heartbeats. At S240, an inversion time TI may be dynamically calculated based at least in part on the heart-rate timing information. According to some embodiments, this calculation may be described as:
TI=−T1*log [0.5+0.5*exp(−tau/T1)]
where T1 represents a tissue relaxation time and the log is the natural logarithm. At S250, an excitation RF pulse is generated a period of time after the generation of the inversion RF pulse, the period of time being based on the dynamically calculated inversion time TI.
At S260, magnetic resonance imaging data may be acquired. According to some embodiments, the generation of the excitation pulse and the acquisition of magnetic resonance imaging data are repeated to acquire multiple k-space lines of magnetic resonance imaging data. Moreover, the period of time may be such that k-space lines of magnetic resonance imaging data corresponding to a central aspect of k-space are acquired when a longitudinal magnetization of the first tissue is at or near a null point. Note that the generation of the excitation pulse and acquisition of magnetic resonance imaging data may be performed in accordance with: a fast gradient recalled acquisition, a balanced steady-state free precession acquisition, a spoiled gradient recalled acquisition, and/or any other type of readout.
According to some embodiments, the subject also includes fat tissue (which should preferably be nulled in the acquired MR image). In this case, two fat inversion RF pulses may be generated to invert the longitudinal magnetization from the fat tissue. Moreover, a fat inversion time TIfat may be dynamically calculated based at least in part on the heart-rate timing information. In this case, the generation of the excitation RF pulse may occur a period of time after the generation of the second fat inversion RF pulse, and the period of time may be based on the dynamically calculated fat inversion time TIfat. For example, TIfat may be calculated as follows:
TI
fat
=−T1fat*log [0.5+exp(−(TI−td)/T1fat)−exp(−TI/T1fat)+0.5*exp(−tau/T1fat)]
where T1fat represents a fat tissue relaxation time, the log is the natural logarithm, and td represents the time between the non-selective inversion pulse and the first fat inversion RF pulse (as will be illustrated as element 453 in
An inversion RF pulse 356 (rf0) may be generated prior to an image acquisition 358 that occurs a trigger delay 330 after the subject's first QRS 314. Moreover, the image acquisition 358 may occur an inversion time TI 340 after the peak of this inversion RF pulse 356. The time between the subject's first QRS 314 and the beginning of the inversion RF pulse 356 is referred to as “extrapre” 352 and the time between the end of the inversion RF pulse 356 and the start of image acquisition 358 is referred to as “extrapost” 354. According to some embodiments, the inversion RF pulse 356 is timed relative to the center of ky-readout or image acquisition 358 (inversion time TI 340) according to the following formula:
TI=−T1*log [0.5+0.5*exp(−tau/T1)]
with T1 representing the T1 relaxation time and the log being the natural logarithm. As a result, the MZ of a first tissue 362 (e.g., normal myocardial tissue) may be near a null point 366 during the center of the image acquisition 358 (and thus be suppressed in the resulting MR image) while the MZ of a second tissue 364 (e.g., myocardial infarct tissue) may not be near the null point 366 during the center of the image acquisition 358 (and thus not be suppressed in the resulting MZ image).
In addition to suppressing normal myocardial tissue in a resulting MR image, some embodiments described herein may be used to suppress fat tissue in the image. For example,
In addition to the rf0 inversion RF pulse, two fat-inversion RF pulses may be generated prior to an image acquisition 458: an rf_tipup RF pulse 456 and an rf_cssat RF pulse 457. The center of the image acquisition 458 may occur a trigger delay 430 after the subject's first QRS. Moreover, the image acquisition 458 may occur an inversion time TI 440 after the peak of the rf0 inversion RF pulse. Moreover, center of the image acquisition 458 may occur a TI_tipup inversion time 442 after the peak of the rf_tipup RF pulse 546 and a TI_fat inversion time 444 after the peak of the rf_cssat RF pulse 457. The time between the end of the rf_tipup RF pulse 456 and the beginning of the rf_cssat RF pulse 457 is referred to as “extrapre” 452 and the time between the end of the rf_cssat RF pulse 457 and the start of image acquisition 458 is referred to as “extrapost” 454. According to some embodiments, the fat tipup and fat inversion RF pulses may be timed according to:
TI_fat=−T1_fat*log [0.5+exp(−(TI−td)/T1_fat)−exp(−TI/T1_fat)+0.5*exp(−tau/T1_fat)]
where “td” is the delay of the fat rf tipup RF pulse 546 relative to the rf0 inversion RF pulse. Note that both times TI and TI_fat may depend on the subject's current heart rate (HR=60/tau). As a result, the MZ of fat tissue 470 may be near a null point 466 during the center of the image acquisition 458 (and thus be suppressed in the resulting MR image).
Thus, spin preparation may be used to suppress signal from fat and/or other types of tissue with the above-described MR system 10, or any similar or equivalent system for obtaining MR images. In the example of
The inversion RF pulse rf0 may be a non-selective 180 degree inversion pulse that inverts the longitudinal magnetization for all tissues. The inversion RF pulse rf0 is followed by fat-selective inversion RF pulse 456 and fat-selective inversion RF pulse 457 to impact the longitudinal magnetization of fat 470 (MZfat) (e.g., driving the spin population of the fat tissue into a state which has an equal number of spins aligned with and against the positive z axis (+z), so that there is no net fat magnetization along the z axis). The rate of recovery of fat magnetization is known. The timing of the fat-selective inversion RF pulse 457 may be chosen such that fat achieves its null at approximately the same time as the healthy heart tissue (the center of image acquisition 458). Preferably, an acquisition scheme will be used that acquires the central lines of k-space when both the fat tissue and the first tissue are at or near their null points. Examples of acquisition schemes that are compatible with this spin preparation are a “centric encoding scheme”, in which the central lines of k-space are acquired early in the base sequence or a “sequential encoding scheme”, in which the central lines of k-space are acquired near the middle of the base sequence. The null points of fat and the first tissue may be timed to coincide with the acquisition of the central lines of k-space by appropriate modification of the TI, and the timing of the fat-selective inversion pulse 457 TI_fat. Note that the dynamic timing of either TI or TI_fat might be performed or, according to some embodiments, the dynamic timing of both TI and TI_fat may be performed together.
Computer-executable instructions for performing a spin preparation according to the above-described method may be stored on a form of computer readable media. Computer readable media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer readable media includes, but is not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technology, compact disk ROM (CD-ROM), digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired instructions and which may be accessed by MRI system 10 (shown in
As mentioned above, the spin preparation described with respect to
Thus, some embodiments described herein may help make scans less sensitive to variations in a subject's heart rate which are commonly seen during MRI examinations. This may lead to a more wide use of IR prep for MR imaging.
The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.
