CN107219484B - Excitation turnover angle determining method and device for multilayer parallel excitation diffusion imaging - Google Patents
Excitation turnover angle determining method and device for multilayer parallel excitation diffusion imaging Download PDFInfo
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Abstract
The embodiment of the invention discloses a method and a device for determining an excitation turnover angle of multilayer parallel excitation diffusion imaging. The method comprises the following steps: determining a first region to be imaged and a second region to be imaged, wherein the first region to be imaged and the second region to be imaged have different longitudinal relaxation times; determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging; an excitation flip angle of the diffusion weighted sequence is determined based on the image contrast function. By applying the embodiment of the invention, the imaging tissue contrast which is the same as or similar to that of single-layer excitation diffusion imaging can be obtained when the multi-layer parallel excitation diffusion imaging is performed.
Description
Technical Field
The invention relates to the technical field of diffusion imaging, in particular to an excitation turnover angle determining method and device for multi-layer parallel excitation diffusion imaging.
Background
Magnetic resonance (Magnetic Resonance, MR) imaging is a biomagnetic nuclear spin imaging technique that has rapidly evolved with the development of computer technology, electronic circuit technology, superconductor technology. The magnetic field and the radio frequency pulse are utilized to make the hydrogen nucleus (H+) precessing in the human tissue vibrate to generate radio frequency signals, and the radio frequency signals are processed by a computer to be imaged. When an object is placed in a magnetic field, it is irradiated with an appropriate electromagnetic wave to resonate, and then the electromagnetic wave released by it is analyzed, the position and kind of nuclei constituting the object can be known, whereby an accurate stereoscopic image of the interior of the object can be drawn.
MR marks a molecule by magnetization of H+ without interfering with its diffusion process. The diffusion gradient is added into any conventional MR imaging sequence to highlight the diffusion effect, so that Diffusion Weighted Imaging (DWI) can be performed, and the diffusion behavior of water molecules in tissues can be directly detected.
Recently, multi-layer parallel excitation techniques have been successfully introduced into single-shot planar echo imaging (ss-EPI) and read-direction segmented planar echo imaging (rs-EPI), thereby reducing the scan time of diffusion-weighted imaging. The multi-layer parallel excitation technique accelerates scanning by simultaneously exciting n layers (n is a layer acceleration factor) to reduce the repetition Time (TR) of scanning all layers, theoretically the scanning time can be reduced to one-nth. Typically, a 90 degree excitation flip angle is used in diffusion weighted imaging to maximize signal strength, so the repetition Time (TR) needs to be much greater than the longitudinal relaxation time (T1) of the imaged tissue in order for the longitudinal magnetization vector to return to an equilibrium state sufficiently after excitation.
However, if the longitudinal magnetization vector of some imaged tissue does not fully return, then the signal of these imaged tissue will decay during the subsequent scan, affecting the final image contrast.
Disclosure of Invention
The embodiment of the invention provides a method and a device for determining an excitation angle of multilayer parallel excitation diffusion imaging, so that the image contrast is improved.
The technical scheme of the embodiment of the invention is as follows:
according to an aspect of the embodiment of the invention, a method for determining an excitation flip angle of multi-layer parallel excitation diffusion imaging includes:
determining a first region to be imaged and a second region to be imaged, wherein the first region to be imaged and the second region to be imaged have different longitudinal relaxation times;
determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging;
and determining the excitation flip angle of the diffusion weighted sequence based on the image contrast function.
In one embodiment, the determining the excitation flip angle of the diffusion weighted sequence based on the image contrast function comprises:
determining an image contrast value between the first region to be imaged and the second region to be imaged when single-layer excitation diffusion imaging is performed;
an excitation flip angle is determined when the function value of the image contrast function is equal to the image contrast value.
In one embodiment, the diffusion weighted sequence is a read-direction segmented planar echo imaging sequence;
the excitation turnover angle is theta 1opt Wherein:
e is a natural base number; TR (TR) short Repetition time for the multi-layer parallel excitation diffusion imaging; TR (TR) long Exciting a repetition time of diffusion imaging for the monolayer; t (T) 1A A longitudinal relaxation time for a first region to be imaged; t (T) 1B Is the longitudinal relaxation time of the second region to be imaged.
In one embodiment, the determining the excitation flip angle of the diffusion weighted sequence based on the image contrast function comprises:
determining an excitation flip angle when the function value of the image contrast function is a maximum value; or (b)
And determining an excitation flip angle when the function value of the image contrast function is a preset value.
In one embodiment, the determining the image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging comprises:
determining a signal strength function of the first region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the first region to be imaged and a transverse relaxation time of the first region to be imaged;
determining a signal intensity function of a second region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the second region to be imaged and a transverse relaxation time of the second region to be imaged;
and determining the image contrast function based on the signal intensity function of the first region to be imaged and the signal intensity function of the second region to be imaged.
According to an aspect of the embodiment of the present invention, an excitation flip angle determining apparatus for multi-layer parallel excitation diffusion imaging includes:
a region to be imaged determining module configured to determine a first region to be imaged and a second region to be imaged, where the first region to be imaged and the second region to be imaged have different longitudinal relaxation times;
an image contrast function determining module for determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging;
and the excitation flip angle determining module is used for determining the excitation flip angle of the dispersion weighting sequence based on the image contrast function.
In one embodiment, the excitation flip angle determining module is configured to determine an image contrast value between the first region to be imaged and the second region to be imaged when performing single-layer excitation diffusion imaging; an excitation flip angle is determined when the function value of the image contrast function is equal to the image contrast value.
In one embodiment, the diffusion weighted sequence is a read-direction segmented planar echo imaging sequence;
the excitation turnover angle is theta 1opt Wherein:
e is a natural base number; TR (TR) short Repetition time for the multi-layer parallel excitation diffusion imaging; TR (TR) long Exciting a repetition time of diffusion imaging for the monolayer; t (T) 1A A longitudinal relaxation time for a first region to be imaged; t (T) 1B Is the longitudinal relaxation time of the second region to be imaged.
In one embodiment, the excitation flip angle determining module is configured to determine an excitation flip angle when the function value of the image contrast function is a maximum value; or determining an excitation flip angle when the function value of the image contrast function is a predetermined value.
In one embodiment, the image contrast function determining module is configured to determine a signal intensity function of the first region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the first region to be imaged, and a transverse relaxation time of the first region to be imaged; determining a signal intensity function of a second region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the second region to be imaged and a transverse relaxation time of the second region to be imaged; and determining the image contrast function based on the signal intensity function of the first region to be imaged and the signal intensity function of the second region to be imaged.
As can be seen from the above technical solution, in an embodiment of the present invention, a first region to be imaged and a second region to be imaged are determined, wherein the first region to be imaged and the second region to be imaged have different longitudinal relaxation times; determining an image contrast function between a first region to be imaged and a second region to be imaged when performing multi-layer parallel excitation diffusion imaging; the excitation flip angle of the diffusion weighted sequence is determined based on the image contrast function. It can be seen that the embodiment of the invention does not limit the excitation flip angle to 90 degrees, but determines the excitation flip angle of the diffusion weighting sequence through an image contrast function, so that the same or similar imaging tissue contrast as that of single-layer excitation diffusion imaging can be obtained when multi-layer parallel excitation diffusion imaging is performed.
Moreover, the embodiment of the invention can be applied to not only the rs-EPI sequence, but also other dispersion weighted sequences such as the ss-EPI sequence and the like. In addition, the embodiment of the invention can be widely applied to diffusion weighted scanning of different parts.
Drawings
FIG. 1 is a flow chart of an excitation angle determination method for multi-layer parallel excitation diffusion-weighted imaging according to an embodiment of the present invention;
FIG. 2 is a sequence diagram of the rs-EPI sequence of the unipolar dispersion pattern;
FIG. 3 is an exemplary comparison of an acquisition profile according to an embodiment of the present invention with an acquisition profile of the prior art;
fig. 4 is a block diagram of an excitation angle determining apparatus of multi-layer parallel excitation diffusion imaging according to an embodiment of the present invention.
Detailed Description
In order to make the technical scheme and advantages of the present invention more apparent, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the detailed description is intended to illustrate the invention and is not intended to limit the scope of the invention.
For simplicity and clarity of description, the following description sets forth aspects of the invention by describing several exemplary embodiments. Numerous details in the embodiments are provided solely to aid in the understanding of the invention. It will be apparent, however, that the embodiments of the invention may be practiced without limitation to these specific details. Some embodiments are not described in detail in order to avoid unnecessarily obscuring aspects of the present invention, but rather only to present a framework. Hereinafter, "comprising" means "including but not limited to", "according to … …" means "according to at least … …, but not limited to only … …". The term "a" or "an" is used herein to refer to a number of components, either one or more, or at least one, unless otherwise specified.
In prior art multi-layer parallel excitation diffusion imaging, an excitation flip angle of 90 degrees is typically employed to maximize signal intensity. In order for the longitudinal magnetization vector to return to its equilibrium state sufficiently after excitation, the repetition Time (TR) needs to be much longer than T1 of the imaged tissue. If the longitudinal magnetization vector of some imaged tissue does not return completely, then the signal of these imaged tissue will decay during the next scan, affecting the final image contrast. It can be seen that in an actual scan, the decrease in TR is related not only to the layer acceleration factor, but also to the reversion of the longitudinal magnetization vector.
For example, in head monolayer excitation diffusion imaging, TR requires at least more than 3 seconds in order to obtain a suitable tissue contrast. However, with multi-layer parallel excitation diffusion imaging, the TR required to excite all layers may be much less than 3 seconds. To ensure T2 contrast at b0 or low b values in diffusion weighted imaging, the minimum TR of multi-layer parallel excitation diffusion imaging cannot be exploited, and TR in actual scanning still depends on the T1 value of the tissue of interest.
The applicant found that: in multilayer parallel excitation diffusion imaging (i.e., short TR diffusion imaging), the recovery of longitudinal magnetization vectors can be accelerated by decreasing the excitation flip angle, thereby increasing signal strength or maximizing contrast between specific imaged tissues.
In fact, in current multi-layer parallel excitation diffusion imaging, there is no technical solution to maintain the tissue contrast of interest based on excitation flip angle optimization.
Applicants have also found that: by optimizing the excitation flip angle, the same or similar imaging tissue contrast as that of single-layer excitation diffusion imaging (i.e., long TR diffusion imaging) can be obtained at the time of multi-layer parallel excitation diffusion imaging (i.e., short TR).
Fig. 1 is a flowchart of an excitation angle determination method of multi-layer parallel excitation diffusion imaging according to an embodiment of the present invention.
As shown in fig. 1, the method includes:
step 101: a first region to be imaged and a second region to be imaged are determined, wherein the first region to be imaged and the second region to be imaged have different longitudinal relaxation times.
Here, the first region to be imaged and the second region to be imaged may be respective imaged tissues having different longitudinal relaxation times. For example, the first region to be imaged and the second region to be imaged may be grey matter and cerebrospinal fluid, respectively, and so on.
Step 102: an image contrast function between a first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging is determined.
And the image contrast function between the first region to be imaged and the second region to be imaged is used for reflecting the contrast degree between the signal intensity of the first region to be imaged and the signal intensity of the second region to be imaged.
In one embodiment, step 102 specifically includes:
firstly, determining the signal intensity of a first region to be imaged based on the repetition time of a diffusion weighting sequence, the echo time of the diffusion weighting sequence, the longitudinal relaxation time of the first region to be imaged and the transverse relaxation time of the first region to be imaged; the signal strength of the second region to be imaged is determined based on the repetition time of the diffusion-weighted sequence, the echo time of the diffusion-weighted sequence, the longitudinal relaxation time of the second region to be imaged and the transverse relaxation time of the second region to be imaged.
Then, an image contrast function between the first region to be imaged and the second region to be imaged is determined based on the signal intensity of the first region to be imaged and the signal intensity of the second region to be imaged. For example, the signal intensity of the first region to be imaged is compared with the signal intensity of the second region to be imaged, so that an image contrast function between the first region to be imaged and the second region to be imaged is obtained.
Step 103: the excitation flip angle of the diffusion weighted sequence is determined based on the image contrast function.
In one embodiment, step 103 includes: determining an image contrast value between a first region to be imaged and a second region to be imaged when single-layer excitation diffusion imaging is performed; an excitation flip angle is determined when the function value of the image contrast function is equal to the image contrast value.
Specifically, it is possible to first determine an image contrast value between a first region to be imaged and a second region to be imaged when single-layer excitation diffusion imaging is performed, then equal the function value of the image contrast function to the image contrast value, and determine the excitation flip angle at this time. In this way, the determined excitation turnover angle can ensure the image contrast value of the first region to be imaged and the second region to be imaged in the multi-layer parallel excitation diffusion imaging process to be equal to the image contrast value between the first region to be imaged and the second region to be imaged in the single-layer excitation diffusion imaging process.
Therefore, after the embodiment of the invention is applied, the imaging tissue image contrast equivalent or similar to the imaging tissue image contrast of the long TR scanning can be obtained during the short TR scanning, so that the staff can conveniently analyze the short TR scanning image based on the existing image analysis experience of the long TR scanning.
In one embodiment, step 102 specifically includes: the excitation flip angle when the function value of the image contrast function is a maximum value is determined.
Here, by calculating the excitation flip angle when the function value of the image contrast function is the maximum value, the highest imaging tissue image contrast can be ensured at the time of short TR scanning, thereby facilitating the better observation and analysis of the scanned image by the staff.
In one embodiment, step 102 specifically includes: an excitation flip angle is determined when the function value of the image contrast function is a predetermined value.
Here, by calculating the excitation flip angle when the function value of the image contrast function is a predetermined value, a corresponding image contrast value can be scanned based on the predetermined value set by the staff, so that the staff can analyze the short TR scanned image based on respective image contrast sensing habits.
In one embodiment, the diffusion weighted sequence is a read-direction segmented planar echo imaging (rs-EPI) sequence;
excitation turning angle is theta 1opt Wherein:
wherein e is a natural base; TR (TR) short Repetition time for multi-layer parallel excitation diffusion imaging; TR (TR) long Repetition time for single layer excitation diffusion imaging; t (T) 1A A longitudinal relaxation time for a first region to be imaged; t (T) 1B Is the longitudinal relaxation time of the second region to be imaged.
In the embodiment of the invention, the diffusion weighting sequence can be implemented as an rs-EPI sequence, and can be also implemented as other diffusion weighting sequences such as a single excitation plane echo imaging sequence.
After determining the excitation flip angle based on the method shown in fig. 1, a specific multi-layer parallel excitation diffusion imaging scan can be performed according to the excitation flip angle. The multi-layer parallel excitation diffusion imaging scanning process is a mature technology in the field, and the embodiment of the invention is not repeated.
The diffusion-weighted acquired signal can be derived from the longitudinal magnetization vectors at different times of the sequence. The specific implementation of the embodiment of the invention is described in detail below by taking an rs-EPI sequence adopting a monopole dispersion mode as an example.
FIG. 2 is a sequence diagram of the rs-EPI sequence of the unipolar dispersion pattern. As shown in FIG. 2, in the rs-EPI sequence acquisition, steady-state longitudinal magnetization vectors at different time points can be labeled with A-G. It is assumed that the transverse magnetization vector at the G-point is negligible because of the existence of the spoiling gradient and the negligible T1 relaxation upon excitation by a Radio Frequency (RF) pulse.
Let the longitudinal magnetization vector at steady state be M 0 The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal direction at point AMagnetization vector of M A The method comprises the steps of carrying out a first treatment on the surface of the The longitudinal magnetization vector at point B is M B The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal magnetization vector at point C is M c The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal magnetization vector at point D is M D The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal magnetization vector at E point is M E The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal magnetization vector at point F is M F The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal magnetization vector at point G is M G ；θ 1 The overturning angle is excited; θ 2 To refocus the pulse angle. Then:
M B ＝M A cosθ 1
M C ＝M B E 1 +M 0 (1-E 1 )
M D ＝M C cosθ 2
M E ＝M D E 2 +M 0 (1-E 2 )
M F ＝M E cosθ 2
M G ＝M F E 3 +M 0 (1-E 3 )
wherein:
E 1 ＝e -TE1/2T1 ；E 2 ＝ e-TE2/2T1 ；e is natural logarithm; TE1 and TE2 are echo times of the rs-EPI sequence respectively; t1 is the longitudinal relaxation time.
Using steady-state condition M A ＝M G And set θ 2 180 degrees, longitudinal magnetization vector M at point G G And the signal intensity S at that time diffusion Can be represented by the following formula:
S diffusion ＝M G Dinθ 1 sin 2 (θ 2 /2)e -TE1/T2 e -bD 。
wherein T2 is the transverse relaxation time; b is a dispersion sensitive factor and D is a dispersion coefficient in the direction of the applied dispersion gradient. Here, the second refocusing pulse is used to obtain navigation data for phase correction, so that only the recovery of the longitudinal magnetization vector is affected, and no effect is exerted on the signal strength.
Thus, the contrast between a tissue pair of interest comprising tissue a (i.e., region a to be imaged) and tissue B (i.e., region B to be imaged) can be expressed by:
wherein M is GA (T 1A ,TR,TE1,TE2,θ 1 ) Is the longitudinal magnetization vector of tissue A at point G, and the independent variables include T 1A ,TR,TE1,TE2,θ 1 ；M GB (T 1B ,TR,TE1,TE2,θ 1 ) Is the longitudinal magnetization vector of tissue B at point G, and the independent variables include T 1B ,TR,TE1,TE2,θ 1 ；T 1A T1 value for organization A; t (T) 1B T1 value for organization B; t (T) 2A T2 value for organization a; t (T) 2B T2 value for organization B; d (D) A Is the dispersion coefficient of the tissue A in the direction of the applied dispersion gradient; d (D) B Is the diffusion coefficient of tissue B in the direction of the applied diffusion gradient. Can be based onCalculate M GA (T 1A ,TR,TE1,TE2,θ 1 ) And M GB (T 1B ,TR,TE1,TE2,θ 1 )。
Setting θ 1 The optimization conditions of (2) are as follows: long TR (TR) long ) And theta is theta 1 A contrast equal to 90 degrees, a contrast equal to a short TR (TR short ) And theta is theta 1 Is the contrast at the optimum.
If TE1, TE2<<TR, the following optimum flip angle θ can be obtained 1opt ：
Wherein:e is a natural base number; TR (TR) short Repetition time for multi-layer parallel excitation diffusion imaging; TR (TR) long Repetition time for single layer excitation diffusion imaging; t (T) 1A A T1 value for a first region to be imaged; t (T) 1B Is the T1 value of the second region to be imaged.
The calculation of the optimal flip angle θ is described in detail above using the rs-EPI sequence as an example 1opt Is a specific formula of (a). Those skilled in the art will recognize that: the method for deducing the optimal excitation flip angle based on the rs-EPI sequence can be popularized to the ss-EPI dispersion sequence by simply adapting the number of the refocusing pulses. In general, embodiments of the present invention are applicable to all sequences in which the transverse magnetization vector is negligible prior to re-excitation.
The excitation flip angle optimization algorithm provided by the invention can be widely applied to multi-layer parallel excitation diffusion imaging, and can obtain the same or similar image contrast ratio under the short TR as that under the traditional long TR, so that the minimum TR under the multi-layer parallel excitation can be applied to actual scanning.
In practicing the present invention, the optimal excitation flip angle can be automatically calculated based on the above-described derivation process by adjusting the corresponding parameters (e.g., adjusting the T1 value, long TR value, and the equivalent contrast desired to be optimized for a pair of tissues of interest in an optimization option box) in the parameter card of the diffusion weighted sequence.
Such as: the invention is implemented by using an rs-EPI sequence, wherein parameters in head scanning are as follows:
te=79 ms; the visual field is 252mm multiplied by 252mm; the bandwidth is 530Hz/Pixel; the layer thickness is 5mm; the acquisition matrix is 224×224; the number of scanning layers is 20; the number of segments is 11; the echo spacing is 0.36ms; the dispersion Mode is 4-scan-trace; the dispersion Scheme (dispersion Scheme) is Monopolar (Monopolar); b=0, 1000s/mm 2 The method comprises the steps of carrying out a first treatment on the surface of the The intra-layer general auto-calibration partial parallel acquisition factor (GRAPPA factor) is 2; the layer acceleration factor is 2.
Using the prior art dual layer parallel excitation acquisition scheme (tr=4800) as a reference, the total scan time was 4 minutes 12 seconds; in the double-layer parallel excitation acquisition with tr=2000, the 90-degree excitation flip angle of the prior art and the optimal excitation flip angle of the present invention are adopted, respectively, and the total scan time is 1 minute 52 seconds.
Fig. 3 is an exemplary comparison of an acquisition profile according to an embodiment of the present invention with an acquisition profile of the prior art. Fig. 3 includes sub-graphs 3 (a) -3 (H), for a total of eight sub-graphs. In fig. 3, the double-layer parallel excitation acquisition sample graph when the excitation flip angle is 90 degrees under different TRs of the rs-EPI sequence is compared with the sample graph acquired when the excitation flip angle is optimal.
Fig. 3 (a) is an image acquired using prior art dual layer parallel excitation diffusion imaging, where tr=4800 ms, scan time 4 minutes 12 seconds, b=0 s/mm 2 The excitation flip angle is 90 degrees; fig. 3 (B) is an image acquired using prior art dual layer parallel excitation diffusion imaging, where tr=2000 ms, scan time=1 min 52 seconds, b=0 s/mm 2 The excitation flip angle is 90 degrees; FIG. 3 (C) is an image acquired by dual-layer parallel excitation diffusion imaging with an optimal excitation flip angle calculated by the present invention, wherein TR is 2000ms, scan time is 1 min 52 seconds, b=0 s/mm 2 ，T 1A 1500ms, T 1B 4000ms, a length TR of 4800ms, and an optimal excitation flip angle of 49 degrees; fig. 3 (D) is an image acquired by a corresponding ss-EPI, acquisition matrix=192×192, tr=5200 ms, scan time 1 minute 14 seconds. FIGS. 3 (E) -3 (F) are high b-value images corresponding to FIGS. 3 (A) -3 (D), respectively, where b is equal to 1000s/mm 2 . All sub-images in fig. 3 are displayed with an automatic window width level.
As shown in fig. 3 (B), as TR decreases, the contrast between gray matter and cerebrospinal fluid correspondingly decreases. After the optimal excitation flip angle is adopted, the double-layer parallel excitation rs-EPI with short TR can obtain image contrast similar to that with long TR, so that the scanning time can be further reduced.
Notably, are: at high b values, the improvement of the invention for image contrast is also evident (marked with white circles in fig. 3). By acquiring corresponding ss-EPI diffusion weighted images, the short TR double-layer parallel excitation rs-EPI after optimizing the excitation flip angle can be found to obtain the image quality obviously superior to that of ss-EPI in similar scanning time.
Based on the analysis, the invention also provides an excitation angle determining device for multi-layer parallel excitation diffusion imaging.
Fig. 4 is a block diagram of an excitation angle determining apparatus of multi-layer parallel excitation diffusion imaging according to an embodiment of the present invention.
As shown in fig. 4, the apparatus 400 includes:
a region to be imaged determining module 401, configured to determine a first region to be imaged and a second region to be imaged, where the first region to be imaged and the second region to be imaged have different longitudinal relaxation times;
an image contrast function determination module 402 for determining an image contrast function between a first region to be imaged and a second region to be imaged when performing multi-layer parallel excitation diffusion imaging;
the excitation flip angle determination module 403 is configured to determine an excitation flip angle of the diffusion weighted sequence based on the image contrast function.
In one embodiment, the excitation flip angle determining module 403 is configured to determine an image contrast value between the first region to be imaged and the second region to be imaged when performing single-layer excitation diffusion imaging; an excitation flip angle is determined when the function value of the image contrast function is equal to the image contrast value.
In one embodiment, the diffusion weighted sequence is a read-direction segmented planar echo imaging sequence;
excitation turning angle is theta 1opt Wherein:
e is a natural base number; TR (TR) short Is multi-layer parallel excitationRepeating time of diffusion imaging; TR (TR) long Repetition time for single layer excitation diffusion imaging; t (T) 1A For the T1 value, T of the first region to be imaged 1B Is the T1 value of the second region to be imaged.
In one embodiment, the excitation flip angle determining module 403 is configured to determine the excitation flip angle when the function value of the image contrast function is a maximum value; or, determining an excitation flip angle when the function value of the image contrast function is a predetermined value.
In one embodiment, the image contrast function determining module 402 is configured to determine a signal intensity function of the first region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the first region to be imaged, and a transverse relaxation time of the first region to be imaged; determining a signal intensity function of the second region to be imaged based on the repetition time of the diffusion weighted sequence, the echo time of the diffusion weighted sequence, the longitudinal relaxation time of the second region to be imaged and the transverse relaxation time of the second region to be imaged; an image contrast function is determined based on the signal intensity function of the first region to be imaged and the signal intensity function of the second region to be imaged.
In one embodiment, the diffusion weighted sequence comprises an rs-EPI or ss-EPI sequence.
The excitation angle determining method for multi-layer parallel excitation diffusion imaging provided by the embodiment of the invention can be written as a plug-in program installed in a magnetic resonance diffusion imaging control host, a personal computer, a mobile terminal and the like according to an application program interface of a certain specification, and can also be packaged as an application program for users to download and use.
The excitation angle determining method for multi-layer parallel excitation diffusion imaging provided by the embodiment of the invention can be stored on various storage media in a storage mode of instruction or instruction set storage. These storage media include, but are not limited to: floppy disk, optical disk, DVD, hard disk, flash memory, etc.
In addition, the excitation angle determining method of multi-layer parallel excitation diffusion imaging provided by the embodiment of the invention can be applied to storage media based on flash memory (Nand flash), such as a USB flash disk, a CF card, an SD card, an SDHC card, an MMC card, an SM card, a memory stick, an xD card and the like.
In summary, in an embodiment of the present invention, a first region to be imaged and a second region to be imaged are determined, wherein the first region to be imaged and the second region to be imaged have different longitudinal relaxation times; determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging; an excitation flip angle of the diffusion weighted sequence is determined based on an image contrast function. It can be seen that the embodiment of the invention can obtain the same or similar imaging tissue contrast as the single-layer excitation diffusion imaging when the multi-layer parallel excitation diffusion imaging is performed.
Moreover, the invention can be applied not only to rs-EPI sequences, but also to other dispersion weighted sequences. Moreover, the invention can be widely applied to diffusion weighted scanning of different parts, as long as the T1 value of the tissue pair of interest can be obtained.
The foregoing is merely a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (4)
1. The excitation turnover angle determining method for multi-layer parallel excitation diffusion imaging is characterized by comprising the following steps of:
determining a first region to be imaged and a second region to be imaged, wherein the first region to be imaged and the second region to be imaged have different longitudinal relaxation times;
determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging;
determining an excitation flip angle of a diffusion weighted sequence based on the image contrast function,
the determining the excitation flip angle of the diffusion weighted sequence based on the image contrast function comprises:
determining an image contrast value between the first region to be imaged and the second region to be imaged when single-layer excitation diffusion imaging is performed and an excitation angle is 90 degrees;
determining an excitation flip angle when a function value of the image contrast function when performing multi-layer parallel excitation diffusion imaging is equal to the image contrast value when performing single-layer excitation diffusion imaging and an excitation angle is 90 degrees,
the diffusion weighting sequence is a reading direction segmented plane echo imaging sequence;
the excitation turnover angle is theta 1opt Wherein:
e is a natural base number; TR (TR) short Repetition time for the multi-layer parallel excitation diffusion imaging; TR (TR) long Exciting a repetition time of diffusion imaging for the monolayer; t (T) 1A A longitudinal relaxation time for a first region to be imaged; t (T) 1B Is the longitudinal relaxation time of the second region to be imaged.
2. The excitation flip angle determining method of multi-layer parallel excitation diffusion imaging according to claim 1, wherein,
the determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging comprises:
determining a signal strength function of the first region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the first region to be imaged and a transverse relaxation time of the first region to be imaged;
determining a signal intensity function of a second region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the second region to be imaged and a transverse relaxation time of the second region to be imaged;
and determining the image contrast function based on the signal intensity function of the first region to be imaged and the signal intensity function of the second region to be imaged.
3. An excitation flip angle determining device for multi-layer parallel excitation diffusion imaging, comprising:
a region to be imaged determining module configured to determine a first region to be imaged and a second region to be imaged, where the first region to be imaged and the second region to be imaged have different longitudinal relaxation times;
an image contrast function determining module for determining an image contrast function between the first region to be imaged and the second region to be imaged when performing multi-layer parallel excitation diffusion imaging;
an excitation flip angle determination module for determining an excitation flip angle of a diffusion weighted sequence based on the image contrast function,
the excitation flip angle determining module is used for determining an image contrast value between the first region to be imaged and the second region to be imaged when single-layer excitation diffusion imaging is performed and the excitation angle is 90 degrees; determining an excitation flip angle when a function value of the image contrast function when performing multi-layer parallel excitation diffusion imaging is equal to the image contrast value when performing single-layer excitation diffusion imaging and an excitation angle is 90 degrees,
the diffusion weighting sequence is a reading direction segmented plane echo imaging sequence;
the excitation turnover angle is theta 1opt Wherein:
e is a natural base number; TR (TR) short Repetition time for the multi-layer parallel excitation diffusion imaging; TR (TR) long Exciting a repetition time of diffusion imaging for the monolayer; t (T) 1A A longitudinal relaxation time for a first region to be imaged; t (T) 1B Is the longitudinal relaxation time of the second region to be imaged.
4. The excitation flip angle determining apparatus of multi-layer parallel excitation diffusion imaging according to claim 3,
an image contrast function determining module, configured to determine a signal intensity function of a first region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the first region to be imaged, and a transverse relaxation time of the first region to be imaged; determining a signal intensity function of a second region to be imaged based on a repetition time of the diffusion weighted sequence, an echo time of the diffusion weighted sequence, a longitudinal relaxation time of the second region to be imaged and a transverse relaxation time of the second region to be imaged; and determining the image contrast function based on the signal intensity function of the first region to be imaged and the signal intensity function of the second region to be imaged.
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