Magnetic resonance 3DTOF angiography method based on variable echo time

Abstract

The invention discloses a magnetic resonance 3DTOF angiography method based on variable echo time. The method comprises the following steps: step 1, respectively applying a magnetization transmission pulse and a saturation pulse to an imaging area; step 2, calculating the coding gradient lobe area of the imaging region to obtain the time of flow compensation, and determining the specific change condition of the echo time of the imaging region; 3, adopting a 3DTOF gradient echo sequence to obtain a final scanning sequence time sequence, and scanning to obtain magnetic resonance angiography data; and S4, performing filtering processing on the obtained magnetic resonance angiography data, performing reconstruction, generating a DICOM image, performing MIP three-dimensional reconstruction, and finally obtaining a three-dimensional image of the blood vessel. A variable echo time method is adopted, the closer to the center of a k space, the shorter the echo time is, the fewer signal errors are caused by blood flow, meanwhile, more sufficient time is provided for gradient compensation, artifacts caused by blood flow are reduced, and a higher-quality magnetic resonance blood vessel image is obtained.

Description

Technical Field

[0001] The present invention relates to a magnetic resonance 3DTOF angiography method based on variable echo time. Background Art

[0002] Magnetic resonance imaging has currently become one of the important imaging means in medical examinations and is increasingly widely used in medical imaging examinations. Magnetic resonance angiography (MRA) is one of the important branches of magnetic resonance imaging. Time-of-flight (TOF) has become the most widely used MRA method in clinical practice. Compared with contrast-enhanced magnetic resonance angiography (CE-MRA) that requires injection of gadolinium contrast agent, the TOF method is safer and has lower cost. The 3DTOF method is often used for cranial arterial angiography examinations and can diagnose symptoms such as aneurysms and vascular stenosis.

[0003] The principle of TOF is based on the inflow enhancement effect of blood flow. The fast GRE sequence is used to repeatedly excite the stationary tissue in the imaging area to be in a saturated state, while the blood flowing into the imaging area is not saturated and shows a high signal when excited, forming a contrast with the stationary tissue. However, due to reasons such as blood eddy current, turbulence, and slow blood flow in distal or small blood vessels, the imaging signal is very weak in some places, and the obtained image is difficult to distinguish from the lesion, easily causing misdiagnosis.

[0004] Therefore, there is a market demand for providing a magnetic resonance 3DTOF angiography method based on variable echo time that can improve the vascular display quality and contrast. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a magnetic resonance 3DTOF angiography method based on variable echo time that can improve the display effect of small blood vessels.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a magnetic resonance 3D TOF angiography method based on variable echo time, comprising the following steps: Step 1, applying a magnetization transfer pulse and a saturation pulse to the imaging region respectively; Step 2, calculating the encoded gradient lobe area of the imaging region to obtain the flow compensation time and determining the specific change of the echo time of the imaging region; Step 3, using a 3D TOF gradient echo sequence to obtain the final scan sequence timing and scanning to obtain magnetic resonance angiography data; Step S4, filtering and reconstructing the obtained magnetic resonance angiography data, generating DICOM images and performing MIP three-dimensional reconstruction to finally obtain three-dimensional images of blood vessels. Using an imaging method based on variable echo time improves the display effect of small blood vessels to a certain extent, and the final MIP result obtained can meet clinical requirements.

[0007] Preferably, Step 1 is specifically to apply a first group of magnetization transfer pulses and a second group of flow saturation pulses to the imaging region. The first group of magnetization transfer pulses is used to suppress the signals of stationary tissues in the imaging region and increase the contrast between flowing blood and stationary tissues; the second group of flow saturation pulses is applied above the imaging slice to suppress the blood flow of non-target blood vessels. Using magnetization transfer pulses and flow saturation pulses can effectively enhance the contrast of blood vessels and improve the display effect of small blood vessels.

[0008] Preferably, Step 2 is specifically the gradient lobe area ), where is the gradient plateau time, is the gradient ramp time, and the flow compensation times in the phase direction and in the slice selection direction are calculated respectively. Among them, the calculation formula for the flow compensation time in the phase direction is , is the first gradient lobe, is the second gradient lobe, is the gradient ramp time, is the gradient lobe area in the phase direction, is the maximum applied gradient field strength, is the time from the end of the second gradient lobe to the echo center, and the first gradient and the second gradient are applied in sequence; the calculation formula for the flow compensation time in the slice selection direction is , where is the first gradient lobe, is the second gradient lobe, is the gradient ramp time, is the gradient lobe area in the slice selection direction, is the maximum applied gradient field strength, is the time from the end of the second gradient lobe to the echo center. and are the zero-order moment and the first-order moment of the slice selection gradient respectively. Their calculation time range starts from the moment when the spin phase is zero at the center of the radiofrequency pulse and ends at the end of the gradient waveform. Their expressions are where represents the magnetic field gradient at time , , n represents the current order. According to the flow compensation time, the specific change of the echo time is calculated. TE = max( is the given readout direction gradient lobe time. Using flow compensation can reduce the influence of blood flow on the phase and improve the quality of vascular images.

[0009] Preferably, step 3 is specifically to use the previously calculated gradient lobes, adopt a 3DTOF gradient echo sequence to obtain the timing of the final scan sequence, and scan to obtain magnetic resonance angiography data according to the timing of the final scan sequence.

[0010] Preferably, the 3DTOF gradient echo sequence includes a slice selection radiofrequency pulse and a slice selection gradient, a flow compensation gradient, and a spoiler gradient.

[0011] Preferably, step 4 is specifically to fill the scanned magnetic resonance angiography data into the k-space, perform high-pass filtering after obtaining the k-space data. The high-pass filter formula is , represents the filter response value at the coordinate (u, v) in the frequency domain plane, and the range is between [0, 1]. is the cut-off frequency starting from the origin of the frequency domain plane. represents the radial distance from the coordinate point (u, v) in the frequency domain plane to the origin of the frequency domain plane. is the order of the high-pass filter. Reconstruct the filtered k-space data to obtain DICOM format image data, and then perform MIP three-dimensional reconstruction to finally obtain the three-dimensional image of the blood vessel.

[0012] The scope of the present invention is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover other technical solutions formed by any combination of the above technical features or their equivalent features. For example, the technical solutions formed by mutually replacing the above features with the (but not limited to) technical features with similar functions disclosed in the present application, etc.

[0013] Due to the application of the above technical solutions, the present invention has the following advantages compared with the prior art: 1. The present invention uses a vascular imaging method based on variable echo time to solve, to a certain extent, the problem of poor display of small blood vessels in TOF. The final MIP three-dimensional reconstruction image obtained can meet clinical requirements; 2. By using variable echo time, the echo time closer to the center of k-space is shorter, reducing signal errors caused by blood flow and effectively improving the display effect of small blood vessels. Flow compensation can reduce the influence of blood flow on the phase and improve the quality of vascular images; 3. Filtering the acquired k-space data using high-pass filtering can further enhance the contrast and display effect of blood vessels. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a schematic flowchart of the method of the present invention; Figure 2 is a graph showing the relationship between the flow compensation time and the variable echo time in the slice selection and phase directions; Figure 3 is a sequence timing diagram of the maximum echo time of the present invention; Figure 4 is a sequence timing diagram of the minimum echo time of the present invention; Figure 5 is a k-space diagram before and after high-pass filtering of the present invention; Figure 6 is a MIP comparison diagram between the method of the present invention and the traditional TOF method; DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below with reference to the accompanying drawings. First, magnetization transfer pulses and saturation pulses are used to suppress the background and blood vessels of the part to be imaged, improving the contrast of vascular tissues. Aiming at the problem that the traditional TOF method has poor display of distal blood vessels and small blood vessels, a method of variable echo time is adopted to reduce the echo time near the center of k-space, and at the same time, flow compensation is added to reduce the influence of blood flow on the phase, improving the quality of vascular images. The acquired data is first subjected to high-pass filtering and then reconstructed to further enhance the imaging effect of blood vessels and improve the quality of the three-dimensional image after MIP. The method of the present invention has been applied to a magnetic resonance imaging system, and the imaging effect has been improved compared with the previous method.

[0016] As Figure 1 shown in the magnetic resonance 3D TOF vascular imaging method based on variable echo time, the final magnetic resonance vascular image can be obtained according to this process, which includes the following steps.

[0017] Step 1: By applying magnetization transfer pulses and saturation pulses, background and vessel suppression are performed on the part to be imaged. The first group of magnetization transfer pulses and the second group of flow saturation pulses are applied. The first group of magnetization transfer pulses are used to suppress the signals of stationary tissues in the imaging area, increasing the contrast between flowing blood and stationary tissues. The second group of flow saturation pulses are applied several centimeters above the imaging slice to suppress the blood flow of non-target vessels such as veins. In this embodiment, by utilizing the characteristic that the precession frequencies of bound water and free water are inconsistent, a narrow-bandwidth magnetization transfer pulse that deviates from the center frequency of free water is used to excite the bound water, and a magnetization transfer effect is generated through the transfer between bound water and free water to suppress the background. The flow saturation pulse is used to flip and dephase the magnetization vectors of hydrogen protons in venous blood to pre-saturate the venous blood, so that no signal is generated during the acquisition in the target area, suppressing venous imaging and avoiding interfering with the imaging results. By using magnetization transfer pulses and saturation pulses, effective background and vessel suppression are performed on the target area.

[0018] Step 2: Calculate the encoded gradient lobe areas in the slice selection and frequency directions according to the set parameters to obtain the time for flow compensation, thereby determining the specific variation of the echo time. The current encoded gradient lobe area is jointly determined by the fields of view, resolutions, and fixed number of steps in the slice selection and frequency directions. The gradient lobe area ), where is the gradient plateau time, is the gradient ramp time. Calculate the flow compensation times in the phase direction and in the slice selection direction respectively. The calculation formula for the flow compensation time in the phase direction is , where is the first gradient lobe, is the second gradient lobe, is the gradient ramp time, is the gradient lobe area in the phase direction, is the maximum applied gradient field strength, is the time from the end of the second gradient lobe to the peak of the echo. Apply the first gradient and the second gradient in sequence. The calculation formula for the flow compensation time in the slice selection direction is , where is the first gradient lobe, is the second gradient lobe, is the gradient ramp time, is the gradient lobe area in the slice selection direction, is the maximum applied gradient field strength, is the time from the end of the second gradient lobe to the peak of the echo, and are the zero - order moment and the first - order moment of the slice - selection gradient respectively. Their calculation time range starts from the moment when the spin phase is zero at the center of the RF pulse and ends at the end of the gradient waveform. Their expressions are , where represents the magnetic field gradient at time, n represents the current order. According to the flow compensation time, the specific change of the echo time is calculated as TE = max( , , ), where is the given lobe time of the read - out direction gradient. Using flow compensation can reduce the influence of blood flow on the phase and improve the quality of vascular images. In this embodiment, the size of the target image is 512×256, the slice thickness is 1mm, the FOV is 22cm, is 250μs, is 800, TR is 32ms. TR is the repetition time, which is a core parameter directly affecting image contrast, signal - to - noise ratio and scan time. TR represents the time interval between two consecutive radio - frequency (RF) excitation pulses in the same imaging slice. Using a short TR reduces motion artifacts. According to the formula, the values of , are calculated. The relationship between , and the echo time is as shown in Figure 2 . The echo time is determined by the longest lobe time of the gradients applied in the slice - selection, read - out and phase directions. It can be calculated that the maximum echo time at the periphery of k - space is 4.8ms, and the minimum echo time at the center of k - space is 3.0ms.

[0019] Step 3, determine the scanning sequence timing according to the previous calculations and perform scanning to obtain magnetic resonance angiography data. Using the previously calculated gradient lobes, a 3DTOF gradient - echo sequence is adopted, including but not limited to slice - selection RF pulses and slice - selection gradients, flow - compensation gradients, spoiling gradients, etc., and scan the target area according to the final scanning sequence timing to obtain magnetic resonance angiography data. The final scanning sequence timing is as shown in Figure 3 and Figure 4 . The flow compensation and the pre - dispersive phase gradient are combined. Figure 3 is the sequence timing diagram of the maximum echo time. Gs, Gp, Gr represent the slice - selection, phase and read - out directions respectively. At this time, the scanned data will be filled into the outermost area of k - space. Figure 4 is the sequence timing diagram of the minimum echo time. Gs, Gp, Gr represent the slice - selection, phase and read - out directions respectively. At this time, the scanned data will be filled into the central area of k - space.

[0020] ​​Step 4: First, perform filtering on the previously scanned data and then reconstruct it to generate DICOM format images and perform MIP three-dimensional reconstruction. Fill the magnetic resonance angiography data obtained by scanning in Step 3 into the k-space. After obtaining the k-space data, perform high-pass filtering. The high-pass filter formula is , represents the filter response value at the coordinate (u, v) in the frequency domain plane, and the range is between [0, 1]. is the cut-off frequency starting from the origin of the frequency domain plane. represents the radial distance from the coordinate point (u, v) in the frequency domain plane to the origin of the frequency domain plane. is the order of the high-pass filter. Reconstruct the filtered k-space data to obtain DICOM format image data, and then perform MIP three-dimensional reconstruction to finally obtain the three-dimensional image of the blood vessels. In this embodiment, the cut-off frequency is selected as 1 and the order is 6. The obtained k-space comparison diagram is as Figure 5 shown. It can be found that applying high-pass filtering with a very small cut-off frequency enhances the display of high-frequency signals. Figure 6 This is the comparison diagram of the MIP three-dimensional reconstruction of the final result of this method and the traditional TOF method. The image generated by this method is on the left side, and the traditional TOF method is on the right side. By comparison, it can be found that the display effects of small blood vessels and distal blood vessels are both enhanced.

[0021] The present invention adopts a magnetic resonance 3D TOF angiography method based on variable echo time, which improves the display effect of small blood vessels to a certain extent. The obtained final MIP result can meet the clinical needs; by using the method of variable echo time, the closer to the center of the k-space, the shorter the echo time, the less signal error caused by blood flow, and at the same time, there is more sufficient time for gradient compensation to reduce the artifacts caused by blood flow and obtain higher-quality magnetic resonance angiography images. The final magnetic resonance angiography images obtained by the entire angiography method can meet the clinical needs.

[0022] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and the purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly. It should not be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A magnetic resonance 3DTOF vascular imaging method based on variable echo time, characterized in that: The method comprises the following steps: step 1, applying a magnetization transfer pulse and a saturation pulse to an imaging region respectively; step 2, calculating the coded gradient lobe area of ​​the imaging region, obtaining the flow compensation time, and determining the specific change of the echo time of the imaging region; step 3, using a 3DTOF gradient echo sequence to obtain a final scanning sequence timing, and scanning to obtain magnetic resonance angiography data; step S4, filtering and reconstructing the obtained magnetic resonance angiography data, generating a DICOM image, and performing MIP three-dimensional reconstruction, and finally obtaining a three-dimensional image of the blood vessel.

2. The method for magnetic resonance 3D TOF vascular imaging based on variable echo time according to claim 1, characterized in that: Specifically, step 1 includes applying a first group of magnetization transfer pulses and a second group of flow saturation pulses to the imaging area, wherein the first group of magnetization transfer pulses is used to suppress the signal of static tissue in the imaging area and increase the contrast between flowing blood and static tissue; and the second group of flow saturation pulses is applied above the imaging slice to suppress blood flow in non-target blood vessels.

3. The method for magnetic resonance 3D TOF vascular imaging based on variable echo time according to claim 1, characterized in that: The step 2 is specifically as follows: the gradient lobe area ),in, is the gradient plateau time, is the gradient climbing time, and the phase direction flow compensation time is calculated separately and level selection direction flow compensation time , where the calculation formula for the phase direction flow compensation time is , is the first gradient lobe, is the second gradient lobe, is the gradient climbing time, is the phase gradient lobe area, is the maximum applied gradient field strength, is the time from the end of the second gradient lobe to the echo center. The first gradient and the second gradient are applied in sequence. The calculation formula for the flow compensation time in the slice selection direction is: ,in is the first gradient lobe, is the second gradient lobe, is the gradient climbing time, Select the directional gradient lobe area for the slice, is the maximum applied gradient field strength, is the time from the end of the second gradient lobe to the echo center, and are the zero-order moment and first-order moment of the layer selection gradient, respectively. The calculation time range is from the moment when the spin phase at the center of the RF pulse is zero to the moment when the gradient waveform ends. Its expression is: ,in express The magnetic field gradient at the moment, n represents the current order, and the specific change of the echo time is calculated according to the flow compensation time, TE=max( , , ),in is the gradient lobe time for a given readout direction.

4. The method for magnetic resonance 3D TOF vascular imaging based on variable echo time according to claim 1, characterized in that: The step 3 specifically uses the previously calculated gradient lobe and a 3DTOF gradient echo sequence to obtain a final scanning sequence timing, and obtains magnetic resonance angiography data according to the final scanning sequence timing.

5. The method for magnetic resonance 3D TOF vascular imaging based on variable echo time according to claim 4, characterized in that: The 3DTOF gradient echo sequence includes slice selection radio frequency pulses and slice selection gradients, flow compensation gradients, and damage gradients.

6. The method for magnetic resonance 3D TOF vascular imaging based on variable echo time according to claim 1, characterized in that: The step 4 specifically involves filling the scanned magnetic resonance angiography data into the k-space, and performing high-pass filtering after obtaining the k-space data. The high-pass filter formula is: , Represents the filter response value at the coordinate (u,v) in the frequency domain plane, ranging between [0,1], is the cutoff frequency measured from the origin of the frequency domain plane, Represents the radial distance from the coordinate point (u, v) in the frequency domain plane to the origin of the frequency domain plane. is the order of the high-pass filter. The k-space data after filtering is reconstructed to obtain DICOM format image data, and then MIP three-dimensional reconstruction is performed to finally obtain a three-dimensional image of the blood vessel.

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