The subject matter disclosed herein relates to magnetic resonance pulse sequences, and more specifically, to techniques for ultra short TE (UTE) imaging.
In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) excitation field, and time varying magnetic gradient fields with gyromagnetic material having nuclear spins within the subject of interest. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils located around the subject. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions. An RF coil is employed to produce an RF excitation field. This RF field perturbs the spins of some of the gyromagnetic nuclei from their equilibrium directions, causing the spins to precess around the axis of their equilibrium magnetization. During this precession and during relaxation, RF signals are emitted by the spinning, precessing nuclei and are detected by either the same transmitting RF coil, or by a separate coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one or more algorithms to reconstruct a useful image.
One advantage of MRI is that a user (e.g., a radiologist) has the ability to image certain slices of a patient, such as sections of the abdomen, chest, head, vertebrae, and so forth from any desired angle. To obtain these slices, a gradient is superimposed on the static magnetic field, which may be considered a slice selection gradient (GSS). A desired slice may be obtained from within a plane transverse to the applied gradient by exciting the gyromagnetic nuclei within the plane. When the GSS is present, the nuclei may be selectively excited using an RF excitation pulse. To selectively excite the nuclei, the RF excitation pulse may have a frequency spectrum encompassing their Larmor frequency, or the frequency of their precessing nuclear spins.
In the medical imaging context, the nuclei selected for excitation within a desired slice may be contained within different tissues, and each tissue may have a characteristic spin relaxation parameter. For example, certain tissues within the selected slice may have differing T1 and T2 relaxation constants, which contribute to the contrast of the resultant image. In some configurations, the contrast of the image may be manipulated by the user by weighting the image in a relaxation parameter, for example either T1 or T2, which can provide valuable information for clinical diagnoses. Such manipulation may be performed by specially-designed pulse sequences that are configured to suppress or isolate a given relaxation parameter. However, the successful implementation of the pulse sequences that allow the selection of a desired slice and the generation of a weighted image may be highly sensitive to the relaxation time of the tissue (which can be on the order of the pulse sequence time) and the ability of the imaging equipment to rapidly and accurately produce the desired pulses.
While pulse sequences may be manipulated to perform various functions, the imaging equipment which produces the sequences may be limited, such as in its ability to ramp up and ramp down (slew) a pulse amplitude in a given amount of time. Such constraints generally result in the gradient and RF pulses taking on a trapezoidal shape when graphed versus time, with a ramp up period, generally constant period, and a ramp down period. These equipment-related constraints are typically unavoidable, and current techniques to image certain tissues, such as tissues having small spin-spin relaxation times (relatively short T2) are often inadequate and/or suffer from undesirable out-of-slice signal contamination (image artifacts).