Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to correcting image pasting in diffusion-weighted echo planar imaging (EPI).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Multistation whole body diffusion-weighted imaging (WB-DWI) is a known imaging technique that is based on EPI, which is often the method of choice due to its fast imaging sequence. However, EPI is prone to image artifacts and suffers from diffusion encoding direction dependent distortions due to residual eddy current fields and B0 inhomogeneity. These distortions, if not corrected, can lead to mis-registration among DW images of different directions and inaccuracies in post processing operations involving DW image combination.
Overall, EPI has been significantly improved in recent years with a number of pre-processing techniques that include applying high order eddy current (HOEC) generated magnetic field error correction during application of the WB-DWI pulse and also applying HOEC-generated magnetic field corrections during image reconstruction, as examples. However, despite the improvements, technical challenges still remain, which include 1) geometric discontinuities at station boundaries due to the different B0 offset field that different stations face, and the eddy current and B0 related image distortion, 2) intensity discontinuities between stations due to the sensitivity of RF pulses relative to B0 field offset and/or different transmit gain, and 3) image blurring or ghosting due to eddy current induced mis-registration.
HOEC and slice-dependent B0 offset compensation have reduced these problems to a degree. However, slight geometric and intensity discontinuities can still exist due to the residual eddy current and B0 inhomogeneity. Conventional, generic post processing software tends to handle these problems poorly because it does not build its model based upon the WB-DWI sequence (e.g., single shot echo planar imaging). For instance, known post-processing techniques can either miss modeling some of the image degradation (such as intensity variation) or miss geometric discontinuities (which can manifest itself as an image shift in the phase encoding direction).
It would therefore be desirable to have a system and method capable of correcting geometric and intensity discontinuities due to residual eddy current and B0 inhomogeneity.
According to an aspect of the invention, an MRI apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images, and a computer programmed to execute a diffusion-weighted imaging pulse sequence to acquire MR data from a subject over two or more stations, acquire imaging data of the subject over the two or more stations, reconstruct images that correspond to each of the two or more stations, calculate an average intensity signal per slice within each of the reconstructed images, adjust intensity within at least one of the reconstructed images based on the calculated average intensity signal within each of the reconstructed images, and form a pasted image using the at least one of the reconstructed images having its intensity adjusted and another reconstructed image, having a boundary formed therebetween.
According to another aspect of the invention, a method of MR imaging includes obtaining diffusion-weighted MR imaging data of a subject from at least two stations, reconstructing images of the subject using the MR imaging data that correspond to the at least two stations, the images having a border formed therebetween, calculating an average intensity signal per slice within the reconstructed images to determine a net intensity offset between the reconstructed images, offsetting intensity of pixels within at least one of the reconstructed images based on the determined net intensity offset, and forming a pasted image using the reconstructed images, wherein at least one of the reconstructed images includes the at least one reconstructed image having had its pixel intensity offset.
According to yet another aspect of the invention, a computer readable storage medium having stored thereon a computer program comprising instructions, which, when executed by a computer, cause the computer to execute a diffusion-weighted imaging pulse sequence to acquire MR data from a subject over two or more stations, acquire imaging data of the subject over the two or more stations, reconstruct images of each of the two or more stations using the acquired imaging data, calculate an average intensity signal per slice within each of the two or more stations using the reconstructed images, determine an amount of pixel intensity offset based on the calculated average intensity per slice, adjust intensity within at least one of the reconstructed images based on the pixel intensity offset, and form a pasted image using the at least one of the reconstructed images having its intensity adjusted and another reconstructed image, having a boundary formed therebetween.
Various other features and advantages will be made apparent from the following detailed description and the drawings.
The drawings illustrate embodiments presently contemplated for carrying out embodiments of the invention.
In the drawings:
Referring to
The system control 32 includes a set of modules connected together by a backplane 32a. These include a CPU module 36 and a pulse generator module 38 which connects to the operator console 12 through a serial link 40. It is through link 40 that the system control 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 to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module 38 connects to a set of gradient amplifiers 42, to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module 38 can 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. And finally, the pulse generator module 38 connects to a scan room interface circuit 46 which 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 to the desired position for the scan.
The gradient waveforms produced by the pulse generator module 38 are applied to the gradient amplifier system 42 having 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 gradients used for spatially encoding acquired signals. The gradient coil assembly 50 forms part of a resonance assembly 52 which includes a polarizing magnet 54 and a whole-body RF coil 56. A transceiver module 58 in the system control 32 produces pulses which are amplified by an RF amplifier 60 and coupled to the RF coil 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 coil 56 during the transmit mode and to connect the preamplifier 64 to the coil 56 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 picked up by the RF coil 56 are digitized by the transceiver module 58 and transferred to a memory module 66 in the system control 32. A scan is complete when an array of raw k-space data has been acquired in the memory module 66. This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor 68 which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link 34 to the computer system 20 where it is stored in memory. In response to commands received from the operator console 12 or as otherwise directed by the system software, this image data may be archived in long term storage or it may be further processed by the image processor 22 and conveyed to the operator console 12 and presented on the display 16.
Referring to
Still referring to
As known in the art, WB-DWI images may be obtained using the exemplary diffusion-weighted EPI pulse sequence as illustrated in
As stated, such images may include 1) geometric discontinuities at station boundaries 104 due to the different B0 offset field that different stations face, and the eddy current and B0 related image distortion, 2) intensity discontinuities between stations 102 due to the sensitivity of RF pulses relative to B0 field offset and/or different transmit gain, and 3) image blurring or ghosting due to eddy current induced mis-registration. The discontinuities and blurring may be distinct and may occur in either the sagittal plane or the coronal plane, or both.
Thus, according to embodiments of the invention, the original multistation images (usually in axial planes) may be post-processed according to the flowchart illustrated in
Image intensity correction is performed at step 204 on the multistation images in a number of substeps as illustrated therein. At a high level and as will be further illustrated, step 204 includes first calculating an average per slice 206, applying a station-wise intensity correction 208, and applying a slice-wise intensity correction 210.
Data within each station is first averaged at step 206. As can be seen, a number of average intensity discontinuities can occur, which manifest themselves as distinct intensity differences between stages, as illustrated in
As can be seen in
Referring still to
Step 208 of
Referring first to
As can be seen, images 402, 404 have a border 408 formed therebetween (corresponding to one of borders 104 of
Similarly,
That described with respect to
Referring back to
After completion of all borders between stations, station-wise correction 208 is complete and slice-wise correction is performed at step 210 of
Thus, in summary and referring back to step 204 of
Referring still to
As known in the art, non-phase-encoded reference data is in general available before an actual EPI data acquisition to estimate phase correction coefficients. By inspecting the overall phase angle across different echoes, a B0 offset is derived that is experienced by the EPI echo train from the same reference data. Details are as follows: Denote X as the readout axis (assumed to be a horizontal direction), and Y as the phase-encoding (i.e., echo index) axis (assumed to be vertical). EPI reference data is first converted to the image domain by performing an inverse Fourier transform along X. Phase angles all the even (or all the odd) echoes are taken and unwrapped along Y independently for each X, which is known in the art as the Ahn and Cho method. A linear fit along the Y direction is done on the phase angles of even echoes to obtain the phase slope sn(z), wherein n is the X index and Z is the slice direction.
The X dependent, B0 induced frequency offset fn(z) is readily available via: fn(z)=sn(z)/(2π·Tesp) where Tesp is the echo spacing. To increase robustness to noise (especially for slices with little tissue), a projection is obtained of the magnitude data along Y, and then threshold the resulting magnitude (e.g., setting the threshold to 5% of the maximum value, as an example) to provide a mask on X. The fn(z)'s that are included in the mask are averaged to provide a mean frequency offset estimate for a given slice location. Finally, polynomial or other known fitting is performed on the frequency offset along the slice direction to ensure smooth slice-to-slice intensity transition. The frequency offset estimates are obtained after the reference scan but before the actual EPI scan, and the EPI pulse sequence reads in the frequency offsets and adjusts the center frequency for each slice.
As such, above is described a known pre-processing method that may be used to correct B0 induced signal loss using EPI reference data. According to the invention, if such a step is performed, then a reference scan may already be existent and available for performing step 220 of
As step 228, image 100 is assessed to determine whether an inter-station registration will be performed. Mis-registration between image stations 102 can occur in the coronal view (such as that illustrated in
A) Mis-registration correction—correlation-based. According to this embodiment, as illustrated in
As such, an amount Δt may be numerically determined in order to shift s2 by multiplying point-by-point, and sum, to maximize as a function of Δt.
B Mis-registration correction—mutual information-based. According to this embodiment, correction may be determined when no overlapping data has been acquired, corresponding to scenario B of
As known in the art, the mutual information of two discrete random variables X and Y can be defined as:
where p(x,y) is the joint probability distribution function of X and Y, and p(x) and p(y) are the marginal probability distribution functions of X and Y respectively. In the case of continuous random variables, the summation is matched with a definite double integral:
where p(x,y) is now the joint probability density function of X and Y, and p(x) and p(y) are the marginal probability density functions of X and Y respectively.
The aforementioned mutual information-based discussion is known in the art and forms the basis on which this step is performed. When significant station overlap is available, registration can be done in the sagittal plane. And, although registration can be done in multiple dimensions, 1D registration along the phase-encoding axis (anterior-posterior direction) may be preferred because 1D registration tends to be more robust and efficient.
Thus, according to the invention and referring back to
According to this embodiment, if boundary data is to be smoothed 238, then boundary data is smoothed at step 240 using a “linear kernel based” smoothing algorithm of overlapping or neighboring data. That is, data is smoothed using neighboring data in order to remove or reduce visual anomalies in the data in order to provide generally a better aesthetic appearance. As one example, referring to
According to the invention, any combination of corrective steps disclosed herein may be applied to a pasted image. That is, any combination of steps 204, 220, 228, and 236 may be applied, regardless of whether the other steps have been performed, consistent with the discussion of each step.
A technical contribution for the disclosed method and apparatus is that it provides for a computer implemented method of correcting image pasting in diffusion-weighted echo planar imaging (EPI).
One skilled in the art will appreciate that embodiments of the invention may be interfaced to and controlled by a computer readable storage medium having stored thereon a computer program. The computer readable storage medium includes a plurality of components such as one or more of electronic components, hardware components, and/or computer software components. These components may include one or more computer readable storage media that generally stores instructions such as software, firmware and/or assembly language for performing one or more portions of one or more implementations or embodiments of a sequence. These computer readable storage media are generally non-transitory and/or tangible. Examples of such a computer readable storage medium include a recordable data storage medium of a computer and/or storage device. The computer readable storage media may employ, for example, one or more of a magnetic, electrical, optical, biological, and/or atomic data storage medium. Further, such media may take the form of, for example, floppy disks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/or electronic memory. Other forms of non-transitory and/or tangible computer readable storage media not list may be employed with embodiments of the invention.
A number of such components can be combined or divided in an implementation of a system. Further, such components may include a set and/or series of computer instructions written in or implemented with any of a number of programming languages, as will be appreciated by those skilled in the art. In addition, other forms of computer readable media such as a carrier wave may be employed to embody a computer data signal representing a sequence of instructions that when executed by one or more computers causes the one or more computers to perform one or more portions of one or more implementations or embodiments of a sequence.
Therefore, according to an embodiment of the invention, an MRI apparatus includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet, and an RF transceiver system and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images, and a computer programmed to execute a diffusion-weighted imaging pulse sequence to acquire MR data from a subject over two or more stations, acquire imaging data of the subject over the two or more stations, reconstruct images that correspond to each of the two or more stations, calculate an average intensity signal per slice within each of the reconstructed images, adjust intensity within at least one of the reconstructed images based on the calculated average intensity signal within each of the reconstructed images, and form a pasted image using the at least one of the reconstructed images having its intensity adjusted and another reconstructed image, having a boundary formed therebetween.
According to another embodiment of the invention, a method of MR imaging includes obtaining diffusion-weighted MR imaging data of a subject from at least two stations, reconstructing images of the subject using the MR imaging data that correspond to the at least two stations, the images having a border formed therebetween, calculating an average intensity signal per slice within the reconstructed images to determine a net intensity offset between the reconstructed images, offsetting intensity of pixels within at least one of the reconstructed images based on the determined net intensity offset, and forming a pasted image using the reconstructed images, wherein at least one of the reconstructed images includes the at least one reconstructed image having had its pixel intensity offset.
According to yet another embodiment of the invention, a computer readable storage medium having stored thereon a computer program comprising instructions, which, when executed by a computer, cause the computer to execute a diffusion-weighted imaging pulse sequence to acquire MR data from a subject over two or more stations, acquire imaging data of the subject over the two or more stations, reconstruct images of each of the two or more stations using the acquired imaging data, calculate an average intensity signal per slice within each of the two or more stations using the reconstructed images, determine an amount of pixel intensity offset based on the calculated average intensity per slice, adjust intensity within at least one of the reconstructed images based on the pixel intensity offset, and form a pasted image using the at least one of the reconstructed images having its intensity adjusted and another reconstructed image, having a boundary formed therebetween.
This written description uses examples to disclose embodiments of the invention, including the best mode, and also to enable any person skilled in the art to practice the embodiments of the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of embodiments 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 languages of the claims.