MR IMAGING FOR RADIATION THERAPY PLANNING

Abstract

The invention relates to a method of MR imaging of a body (10) of a patient positioned in an examination volume of an MR device (1). It is an object of the invention to provide a method that enables geometrically correct MR-only radiation therapy planning at minimum scan times. The method of the invention comprises the following steps: acquiring first MR imaging data representative of at least one region of the body (10); analyzing said first MR imaging data to delineate at least one anatomical structure within said body region; acquiring second MR imaging data of said body region using a multi-point Dixon sequence; deriving a B0 map from said second MR imaging data; analyzing said B0 map to determine at least one low fidelity region of said B0 map; performing B0 mapping to refine the B0 map using a multi-acquisition gradient echo sequence restricted to at least one region where said delineated anatomical structure and said low fidelity region overlap completely or partially; and correcting geometrical distortions in said first and/or second MR imaging data using the refined B0 map. Moreover, the invention relates to a MR device (1) for carrying out the method, and to a computer program to be executed on a MR device (1).

Claims

1. A method of magnetic resonance (MR) imaging of a body of a patient positioned in an examination volume of an MR device, the method comprising: acquiring first MR imaging data representative of at least one region of the body; analyzing said first MR imaging data to delineate at least one anatomical structure within said body region; acquiring second MR imaging data of said body region using a multi-point Dixon sequence; deriving a B.sub.0 map from said second MR imaging data; analyzing said B.sub.0 map to determine at least one low fidelity region of said B.sub.0 map; performing B.sub.0 mapping to refine the B.sub.0 map using a multi-acquisition gradient echo sequence, in particular a dual acquisition gradient echo sequence restricted to at least one region where said delineated anatomical structure and said low fidelity region overlap completely or partially; and correcting geometrical distortions in said first and/or second MR imaging data using the refined B.sub.0 map.

2. The method of claim 1, wherein the B.sub.0 map is determined to be of low fidelity at positions where both the magnitude of the B.sub.0 gradient and the magnitude of the geometrical distortion associated with B.sub.0 are above respective predetermined thresholds.

3. The method of claim 1, wherein a simulated computed tomography (CT) image is computed from at least one of the corrected first or second MR imaging data by assigning a Hounsfield Unit value to each pixel or voxel of the second MR imaging data.

4. The method of claim 3, wherein a radiation therapy plan is generated using the simulated CT image.

5. The method of claim 1, wherein said analyzing of said first MR imaging data involves automatic segmenting of said anatomical structure.

6. The method of claim 1, wherein an overlay of said delineated anatomical structure with said low fidelity region is displayed.

7. A method of magnetic resonance (MR) imaging of a body of a patient positioned in an examination volume of an MR device, the method comprising: acquiring first MR imaging data representative of at least one region of the body; analyzing said first MR imaging data to delineate at least one anatomical structure within said body region; acquiring second MR imaging data of said body region using a single-point or two-point Dixon sequence; deriving a B.sub.0 map from said second MR imaging data; analyzing said B.sub.0 map to determine at least one low fidelity region of said B.sub.0 map; performing B.sub.0 mapping to refine the B.sub.0 map using an at least three-point Dixon sequence restricted to at least one region where said delineated anatomical structure and said low fidelity region overlap completely or partially; and correcting geometrical distortions in said first and/or second MR imaging data using the refined B.sub.0 map.

8. A magnetic resonance (MR) device including at least one main magnet coil to generate a uniform, steady magnetic field B.sub.0 within an examination volume, a number of gradient coils configured to generate switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil configured to generate radio frequency (RF) pulses within the examination volume, one or more receiving coils configured to receive MR signals from a body of a patient positioned in the examination volume, a control unit configured to control the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit, wherein the MR device is configured to perform the following steps: acquiring first MR imaging data representative of at least one region of the body; analyzing said first MR imaging data to delineate at least one anatomical structure within said body region; acquiring second MR imaging data of said body region using a multi-point Dixon sequence; deriving a B.sub.0 map from said second MR imaging data; analyzing said B.sub.0 map to determine at least one low fidelity region of said B.sub.0 map; performing B.sub.0 mapping to refine the B.sub.0 map using a multi-acquisition gradient echo sequence or an at least three-point Dixon sequence restricted to at least one region in which said delineated anatomical structure and said low fidelity region overlap; and correcting geometrical distortions in said first and/or second MR imaging data using the refined B.sub.0 map.

9. A computer program comprising computer readable instructions stored on a non-transitory computer readable medium for causing a MR device to carry out the steps of the method of claim 1 when said computer program is executed on a computer by which the MR device is controlled.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

[0031] FIG. 1 shows a MR device for carrying out the method of the invention;

[0032] FIG. 2 schematically shows the method of the invention as a flow chart;

[0033] FIG. 3 schematically shows the process of analyzing the B.sub.0 map to determine the low fidelity region as a flow chart;

[0034] FIG. 4 shows MR image data illustrating the background of the approach of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] With reference to FIG. 1, a MR device 1 is shown. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field B.sub.0 is created along a z-axis through an examination volume. The device further comprises a set of shimming coils 2′, wherein the current flow through the individual shimming coils of the set 2′ is controllable for the purpose of minimizing B.sub.0 deviations within the examination volume.

[0036] A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

[0037] More specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a body RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

[0038] For generation of MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used as receiving coils to receive MR signals induced by body-coil RF transmissions.

[0039] The resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 11, 12 and 13 and demodulated by a receiver 14 preferably including a pre-amplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.

[0040] A host computer 15 controls the shimming coils 2′ as well as the gradient pulse amplifier 3 and the transmitter 7 to generate any of a plurality of MR imaging sequences, such as echo planar imaging (EPI). For the selected sequence, the receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modem MR devices, the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

[0041] Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such like SENSE. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

[0042] An embodiment of the method of the invention is described with reference to FIGS. 2-4 and with further reference to FIG. 1 as follows:

[0043] After positioning the body 10 in the examination volume of the main magnet coil 2, a first MR imaging scan is started in step 21 for acquiring first MR imaging data, e.g. using a T.sub.2-weighted scan. The first MR imaging data is representative of a region of the anatomy of body 10.

[0044] In step 22, a delineation of at least one anatomical structure, such as, e.g., a tumor to be treated or an organ-at-risk that should be prevented from being irradiated in radiation therapy, is performed either manually (e.g. by a radiologist interactively analyzing the first MR imaging data displayed on video monitor 18) or by a suitable auto-segmentation technique as it is per se known in the art. The result of the delineation may be a map (referred to in the following as TRB map) that covers only the borders of the delineated anatomical structure. The width of the borders may be a preset parameter depending on the type of tumor or organ-at-risk.

[0045] In step 23, second MR imaging data covering the same region of the anatomy of body 10 is acquired. A multi-point Dixon technique is employed for this purpose. Step 23 includes the derivation of a fat map, a water map and a B.sub.0 map from the acquired mDIXON data.

[0046] In step 24, the derived B.sub.0 map is analyzed to determine one or more low fidelity regions. This involves the steps depicted in FIG. 3. In step 33, the magnitude of the spatial gradient of the B.sub.0 map is calculated for every image position. A map G is generated in step 34 covering all regions (i.e. indicating all image positions) in which this magnitude is larger than a pre-determined threshold. In step 35, a map D is calculated covering all regions in which the geometrical distortion resulting from the B.sub.0 map at the respective image positions is larger than a further pre-determined threshold. A measure of the local geometrical distortion may be, e.g., the magnitude of a pixel/voxel shift caused by the respective local B.sub.0 value. Finally, in step 36, a map E=G∩D is calculated which covers all regions that are covered by both maps G and D. E marks the image positions at which a low fidelity of the B.sub.0 map can be expected. E covers only those regions in which the mDIXON B.sub.0 map is known to be error prone. It can be expected that a geometrical correction based solely on the mDIXON B.sub.0 map in these low fidelity regions would lead to significant errors in dose planning for radiotherapeutic treatment, because the geometrical distortion itself as well as its spatial variation are large.

[0047] An overlay of the low fidelity map E with the results of the delineation of anatomical structures in step 22 is displayed on video monitor 18 in step 25. This informs the user about potential errors in dose planning. The user may react by adjusting the dose plan which may be advisable in regions in which map E coincides with map TRB. In step 26, a map R=E∩TRB is calculated indicating the image regions where the delineated anatomical structures and the low fidelity region of the B.sub.0 map overlap (completely or partially), i.e. error prone regions of the B.sub.0 map at the border of a tumor or an organ-at-risk.

[0048] A field of view is determined in step 27 that covers map R. The field of view may be subdivided into several distinct regions. In step 28, an automated dedicated B.sub.0 mapping scan using a multi-acquisition gradient echo imaging sequence is performed for the determined field of view. The B.sub.0 mapping scan may consist of several sub-scans each addressing a different region of the field of view.

[0049] The mDIXON B.sub.0 map is then updated in step 29 accordingly in the regions indicated by map R by replacing the B.sub.0 values of the mDIXON B.sub.0 map by the corresponding values obtained from the dedicated B.sub.0 mapping scan in these regions. The result is a refined, i.e. higher fidelity B.sub.0 map.

[0050] In step 30, the refined B.sub.0 map is then used to correct geometrical distortions in the first and second MR imaging data.

[0051] A simulated CT image is computed in step 31 from the distortion-corrected second MR imaging data. This involves assigning a Hounsfield Unit value to each pixel or voxel of the second MR imaging data.

[0052] In step 32, the user/radiologist performs dose planning for radiation therapy using the simulated CT image as well as the geometrically corrected first MR imaging data. In case of auto-segmentation being used for the delineation of tumors and organs-at-risk in step 22, all steps 21-31 can be performed fully automatically. In particular, this enables automatic acquisition of the B.sub.0 map update on the fly directly after the mDIXON scan. This means that the patient does not have to stay in the examination volume of the MR device 1. Patient throughput can thus be maximized.

[0053] It has further to be noted that in typical cases only very few or even no regions at all will be contained in map R. This means that the additional acquisition time required for the dedicated B.sub.0 mapping scan is either zero or at least very small. Therefore, the approach proposed by the invention combines both, high precision in distortion correction and very short acquisition time if compared to conventional B.sub.0 mapping for the full field of view.

[0054] FIG. 4 shows examples of sagittal slices of MR image data of the head/neck region. The image values of the two top images (FIGS. 4a and 4b) indicate the local geometrical image distortion (the magnitude of the B.sub.0-induced voxel shift). The top image (FIG. 4a) shows the geometrical distortion derived from the mDIXON B.sub.0 map. The image below (FIG. 4b) shows the geometrical distortion derived from conventional dedicated B.sub.0 mapping. The third image of FIG. 4c shows the difference of the two images of FIGS. 4a and 4b. The image of FIG. 4d is the mDIXON in-phase image shown as an anatomical reference. As can be seen, geometrical distortions are well estimated by mDIXON in regions with low spatial variation (e.g. region indicated by white circle 42). Differences exceeding 50% occur in regions with high spatial variation of B.sub.0 (circle 41). As can be seen in FIG. 4, the mDIXON B.sub.0 map is of low fidelity mostly around nasal, oral, and ear cavities, the sphenoid sinus and dental fillings. If a tumor to be treated by radiation therapy or an organ-at-risk to be prevented from being irradiated is within one of these regions there is a significant risk of misalignment and erroneous dose calculation if the geometrical correction is based solely on the mDIXON B.sub.0 map. To this end, the invention proposes to refine the mDIXON B.sub.0 map in a targeted fashion only in the relevant regions.