Real-time local and global SAR estimation for patient safety and improved scanning performance

#The present application relates to the diagnostic imaging arts. It finds particular application in the context of patient safety and associated improved scanning performance (in terms of radio frequency (RF) duty cycle) and will be described with particular reference thereto. Furthermore, the estimation and suppression of local specific energy absorption rate (SAR) hot spots in connection with high field magnetic resonance imaging (MRI). It is to be appreciated, however, that it is also applicable to optimization and processing of other information, and is not necessarily limited to the aforementioned applications.

#For many MR applications at higher field strengths, the local SAR is a limiting factor. The SAR deposition increases with higher field strength and limits the RF power, duty cycle, and flip angles usable, leading to a lengthening of scan acquisition time to meet designated SAR limits. For a single transmitter system, the SAR was relatively easy to calculate, as all antenna elements transmitted with the same amplitude, and a fixed phase shift between them. Furthermore, the RF pulse shapes required for the experiments are known and are stored in a shape library, associated with their SAR. With the advent of multi-transmit systems where each coil element has the potential to independently transmit its own amplitude and phase, SAR must be calculated on a per channel basis also considering the parallel RF transmission pulses, which can only be calculated based on additional information, for example, B1 maps, and are thus experiment/patient specific.

#RF safety is a prerequisite for in vivo parallel transmission MRI scans, that is, scanning within SAR limits using multi-channel RF transmit coils must be guaranteed. Scans cannot be started unless they are “SAR safe.” In an MR system with multiple transmit channels, SAR reduced RF pulses can be calculated by incorporating electrical field information into an RF pulse design. In the past, methods have been used that construct RF pulses in consideration of known SAR hotspots that are common to every individual (e.g., the eyes). This is generally not sufficient for whole body imaging, as SAR hotspots can vary in both position and magnitude from patient to patient, and from RF pulse to RF pulse. Thus, an RF pulse sequence that limits SAR to acceptable levels in one patient may not be so limiting with respect to another patient. Moreover, RF sequences that accommodate a known static hotspot may inadvertently exacerbate unknown, patient specific hotspots at other locations.

#The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

#FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus in accordance with the present application;

#FIG. 2 includes exemplary waveforms for PUC sampling during RF excitation;

#With reference to FIG. 1, a magnetic resonance scanner 10 is illustrated as a closed bore system that includes a solenoidal main magnet assembly 12, although open and other magnet configurations are also contemplated. The main magnet assembly 12 produces a substantially constant main magnetic field B0 oriented along a horizontal axis of an imaging region. It is to be understood that other magnet arrangements, such as vertical, and other configurations are also contemplated. The main magnet 12 in a bore type system may typically have a field strength of around 0.5 T to 7.0 T or more.

#A gradient coil assembly 14 produces magnetic field gradients in the imaging region for spatially encoding the main magnetic field. Preferably, the magnetic field gradient coil assembly 14 includes coil segments configured to produce magnetic field gradient pulses in three orthogonal directions, typically longitudinal or z, transverse or x, and vertical or y directions.

#A radio frequency coil assembly 16, including n coil elements 161, 162, . . . 16n, generates radio frequency pulses for exciting resonance in dipoles of the subject. The signals that the radio frequency coil assembly 16 transmits are commonly known as the B1 field. The radio frequency coil assembly 16 also serves to detect resonance signals emanating from the imaging region. The illustrated radio frequency coil assembly 16 is a send/receive coil that images the entire imaging region, however, local send/receive coils, local dedicated receive coils, or dedicated transmit coils are also contemplated. In one embodiment, the radio frequency coil assembly 16 includes an 8 channel transmit/receive antenna.