Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like reference numerals indicate corresponding, analogous or similar elements, and in which:
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 fat tissue in a subject, a spin preparation may include a combination of a fat-selective saturation RF pulse and a fat-selective inversion RF pulse. Preferably, the fat-selective saturation and inversion RF pulses are inserted in the spin preparation between an inversion RF pulse (used to suppress at least a second tissue in the subject) and an acquisition window of the pulse sequence. Such a combination of fat-selective saturation and inversion RF pulses suppresses the fat signal without disturbing the desired Ti 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 the second tissue, followed by a fat-selective saturation pulse (e.g. a 90° frequency-selective RF pulse), then a delay, followed by a fat-selective inversion RF pulse with flip angle tuned for the acquisition scheme such that fat is also nulled when the magnetization from the second 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. The purpose of the saturation RF pulse is to “set” the fat magnetization to a known value so that the evolution of the fat magnetization throughout the rest of the sequence may be reliably predicted, and an appropriate flip angle for the fat-selective inversion RF pulse may be determined. Without this pulse, it is usually not possible to accurately predict the longitudinal magnetization from fat during the pulse sequence. The second tissue need only have a water resonance with a longer T1 than fat for this spin preparation to be effective.
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 fiberoptic 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 ECG signals from electrodes attached to the patient. 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.
A spin preparation may be used to suppress signal from both fat and a second tissue with the above-described MR system, or any similar or equivalent system for obtaining MR images.
The inversion RF pulse 220 is preferably a non-selective 180° inversion pulse that inverts the longitudinal magnetization for all tissues including fat and the second tissue. The inversion RF pulse 220 is played out at time t1. The starting edge of the base sequence 230 at time t4 is offset from the inversion RF pulse 220 by a time delay, TI. Time delay TI is set (e.g., prescribed by a user) such that the magnetization from the second tissue achieves its null at time t5 while the base sequence is being played out. It is preferable that the central lines of k-space are acquired at or near time t5 while the second tissue is nulled. TIeff is the name given to the time delay between the inversion RF pulse 220 and the acquisition of the central lines of k-space.
The inversion RF pulse 220 is followed at time t2 by fat-selective saturation RF pulse 222 which may be a fat-selective 90° saturation RF pulse. The longitudinal magnetization of fat (Mzfat) is forced to zero by the fat-selective saturation RF pulse 222, while the longitudinal magnetization of the second tissue (Mzsecond tissue) is minimally affected. The fat-selective saturation RF pulse 222 saturates only the fat magnetization, i.e., it drives 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 purpose of the fat-selective saturation RF pulse 222 is to cause the longitudinal magnetization from fat tissue to be in a known state at a known time point in the pulse sequence 200. The rate of recovery of fat magnetization is known. Thus, the state of the fat magnetization may be determined at any point during the pulse sequence 200 after the reference time point, t2. The value of Mzfat can be determined at the time of a third RF pulse, a fat-selective inversion RF pulse 224, i.e., at time t3. An inversion flip angle for the fat-selective inversion RF pulse 224 may be chosen such that fat achieves its null at approximately the same time as the second tissue. The inversion flip angle for the fat-selective inversion RF pulse 224 may be determined using equations and methods generally known in the art. Preferably, an acquisition scheme will be used that acquires the central lines of k-space when both the fat tissue and the second 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 second tissue may be timed to coincide with the acquisition of the central lines of k-space by appropriate modification of the TI, and the flip angle of the fat-selective inversion pulse 224.
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
The longitudinal magnetization from infarcted tissue is not affected by the fat-selective RF pulses 422 and 424, and recovers much faster than the healthy myocardium, such that at the time of the first excitation pulse, there is significant longitudinal magnetization from the infarcted tissue available for excitation. Thus, the infarcted tissue will appear enhanced in brightness relative to the healthy myocardial tissue, improving visualization of the infarcted tissue's boundaries.
In various embodiments, k-space may be acquired in a sequential manner, a segmented manner, or any other ordering not described herein may be used. In each case, it is preferable that the flip angle of the fat-selective inversion pulse 424 be determined such that the null point of fat occurs whenever the central lines of k-space are acquired. Without the saturation pulse, the magnetization of fat at the time of inversion pulse 424 would not be known accurately, due to its dependence on the starting magnetization at the ECG trigger 410, which, in turn, depends on the patient's heart rate.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. The order and sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.
Many other changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of these and other changes will become apparent from the appended claims.