Each signal of magnetic resonance contains the information of the whole layer, so it is necessary to perform spatial localization encoding on the magnetic resonance signal, that is, frequency encoding and phase encoding. The MR signal collected by the receiving coil is actually a radio wave with spatially encoded information, which is an analog signal rather than digital information. It needs to be converted into digital information through analog-to-digital conversion (ADC), and the latter is filled into k-space, which is called digital information. lattice. k-space is closely related to the spatial localization of magnetic resonance signals.
The k-space is also called the Fourier space, which is the filling space of the original digital data of the MR signal with the spatial positioning coding information. Each MR image has its corresponding k-space data lattice. Performing Fourier transform on k-space data can decode the spatial positioning coding information in the original digital data, and decompose MR signals of different frequencies, phases and amplitudes. Different frequencies and phases represent different spatial positions, while The amplitude represents the strength of the MR signal. By assigning MR digital signals of different frequencies, phases and signal intensities to corresponding pixels, we obtain MR image data, that is, reconstruct an MR image. Fourier transform is the process of transforming the original data lattice in k-space into a magnetic resonance image lattice.
Phase encoding gradient field-----RF pulse + frequency encoding gradient field----- MR analog signal acquired by coil acquisition-----analog-to-digital conversion to digital signal-----filling in k space to form Digital lattice—Fourier transform decomposes signals of different frequencies, phases, and intensities—and distributes them to each pixel to form an image lattice to obtain an MR image.
During the MR signal acquisition of the two-dimensional image, the magnitude and direction of the frequency-encoding gradient field of each MR signal remain unchanged, while the direction and intensity of the phase-encoding gradient field change in certain steps. The phase encoding of the MR signal changes once, and the collected MR signal fills a line in the Ky direction of the k-space. Therefore, the MR signal with spatial information is called the phase-encoding line, also called the k-space line or the Fourier line.
Viewed from the phase encoding direction, the phase encoding gradient field of the MR signal filled in the center of k-space is zero, which is the lowest degree of dephasing of the proton group caused by phase encoding and cannot provide spatial information in the phase encoding direction (because there is almost no phase difference ), but the MR signal intensity is the largest, and its MR signal mainly determines the contrast of the images. We call this k-space line the zero Fourier line. However, the phase-encoding gradient field of the MR signal filling the outermost periphery of the k-space is the largest, and the phase difference of each voxel in the obtained MR signal is the largest, and the spatial information of the anatomical details in the phase-encoding direction is the most abundant. Since the applied gradient field has the largest intensity , the proton group has the highest degree of phase, and the amplitude of its MR signal is very small, so its MR signal mainly reflects the anatomical details of the image and contributes little to the contrast of the image.
To put it simply: the phase-encoding lines filling the central area of k-space mainly determine the contrast of the image, while the phase-encoding lines in the peripheral area mainly determine the anatomical details of the image. The phase-encoding lines on either side of the zero Fourier line are mirror-symmetrical.
The k-space is also mirror-symmetrical in the frequency encoding direction, and the information in the central area has an absolute influence on the contrast of images.
K-space characteristics: ①The lattice in k-space is not in one-to-one correspondence with the lattice of the image, and each point in the k-space contains the full-layer information of the scanning level. ②k space exhibits mirror symmetry in both the Kx and Ky directions. ③ The MR signal filling the central area of k-space mainly determines the contrast of images, and the MR signal filling the peripheral area of k-space mainly determines the anatomical details of the image.
The acquisition and filling of k-space data is directly related to the spatial resolution of the magnetic resonance image, and will also directly determine the acquisition time of the image. The number of pixels in the phase encoding direction of the magnetic resonance image is directly determined by the number of phase encoding steps, that is, the number of different phase encoding magnetic resonance echo signals. The same FOV, the more pixels in the phase encoding direction, the smaller the pixel diameter of the image in the phase encoding direction, and the higher the spatial resolution; however, the more steps required for phase encoding, that is, the magnetic resonance that needs to be acquired. The higher the number of signals, the longer the acquisition time required for one image.
The number of pixels in the frequency coding direction of the magnetic resonance image depends on the number of sampling points in the magnetic resonance echo signal acquisition process. The higher the spatial resolution, the longer the time required to acquire a complete echo signal due to the increase of sampling points.
In conventional MRI sequences, k-space is generally filled in a sequential symmetrical manner. Very important. For example, gradient echo T1WI sequence for liver dynamic enhanced scan (NEX=1), if the acquisition time of the whole sequence is 20s, the acquisition of MR signals for image comparison should be 10s after the start of the scan, so if you want to obtain the start bolus contrast In the hepatic arterial phase at the 25s after the dose, the start time of the scan needs to be 10s earlier, that is, the scan will be started at the 15s after the bolus injection of the contrast agent.
In fact, the filling order of the phase-encoding lines in k-space can be changed, and the k-space center-priority acquisition technique can be used. That is, at the beginning of the scan, a part of the phase encoding line near Ky=0 is encoded and collected to determine the contrast of the image, and then the phase encoding line around the k-space that determines the anatomical details of the image is collected. Three-dimensional dynamic contrast-enhanced scanning and contrast-enhanced magnetic resonance angiography (ce-MRA) have many applications.
GE equipment: k-space center priority acquisition technology applied to 3D fast spoiler gradient echo T1WI sequences including dynamic enhancement or ce-MRA sequences. In this type of sequence, the filling order of k-space data can be selected in the User CVs Sreen card of the parameter adjustment interface. If Centric is selected, the k-space center priority acquisition only occurs in the phase encoding direction within the slice. If Elliptical Centric is selected, it is The k-space center preferentially acquires phase-encoding directions that occur simultaneously within slices and phase-encoding directions between slices.
Other k-space filling methods: circuitous trajectory of EPI; helical trajectory of helical imaging; radial trajectory of propeller imaging technology.
Propeller imaging technology GE is called Propeller (propeller) technology, which is mainly used for FSE T2WI and IR-FSE FLAIR sequences with long echo chains. (Siemens called the blade technology) k-space uses a radial filling trajectory, and there is a lot of information repetition in the central area, so motion artifacts can be greatly reduced.