MRI K-Space PDF

K-Space k-space is the raw data matrix in which the MR signals are stored It represents the spatial frequency distribution of the MR image A k-space of one image has two axes perpendicular to each other A. The horizontal axis is usually named as frequency encoding axis B. The vertic...

K-Space k-space is the raw data matrix in which the MR signals are stored It represents the spatial frequency distribution of the MR image A k-space of one image has two axes perpendicular to each other A. The horizontal axis is usually named as frequency encoding axis B. The vertical axis is usually named as phase encoding axis Each horizontal line is filled while signal is being measured by the frequency encoding gradient. As mentioned before, the same sequence needs to be repeated ‘N’ times, each with different phase encoding amplitude, in order to encode the spatial information of the signal along the phase encoding direction Thus, ‘N’ k-space lines, correspond to these ‘N’ amplitudes, are filled 1  Y X X: Represents the FE direction (Number of digits or pixels) Y: Represents the PE direction (Number of PE steps) E.g. 128 x 256 represents 128 PE steps (pixels) in PE direction and 256 pixels in FE direction (PE direction never larger than FE direction) E.g. 256 x 256 represents 256 PE steps (pixels) in PE direction and 256 pixels in FE direction 2  The central lines of k-space are filled with shallow (low amplitude) PE gradients (small or no dephasing). As a result, central lines contain data that have high signal amplitude and low spatial frequencies The outer lines, on the other hand, are filled with steep (positive and negative) PE gradient amplitudes and contain the high spatial resolution components of the image Whole k-space Center of k-space Periphery of k-space 3  k-Space filling K space can be filled in different ways: A- Linear (k-space lines are filled from the highest negative spatial frequency to the highest positive spatial frequency or vice versa), this is the standard way of k-space filling. B- Centric (low-high) phase encoding order is used, the zero spatial frequency is sampled first and the other K space lines are then sampled in an alternating way so that the lowest negative and lowest positive spatial frequency are filled first while the highest negative and highest positive are filled last. Centric K space filling involves filling high signal amplitudes or starting in the center of K-space and filling outward to the periphery, this is useful when performing contrast enhanced MRA imaging, because contrast media has high signal intensity data and we are required to collect all of our high signals quickly so that contrast media can be visualized in the minimal amount of time that it can be seen in the area of interest 4  C- Spiral Elliptical K space filling involves filling K-space in a spiral fashion starting in the center and working our way out to the periphery. This is beneficial in contrast enhanced MRA imaging 5  Types of Acquisition A- Sequential All lines in k space are filled for slice1 and then all the lines in k space are filled for slice 2 …..etc B- Two Dimensional (2D) Fills one line in k space for slice 1 and then go on to fill the same line of k space for slice 2 …..etc C- Three Dimensional (3D) Acquires data from an entire volume of tissue rather than separate slices. So excitation pulse is non-selective and the whole prescribed volume is excited. At the end of acquisition, the volume or slab is divided into discrete partitions by slice selected gradient. The slice selection gradient is now called slice encoding. The slice encoding step is fixed and all phase encoding steps are performed for one slice, then a different slice encoding step is fixed and all phase encoding steps are performed for another slice ….etc. This continues until all slice encoding steps are performed. The resultant slices are contiguous (no gaps) 6 3D Pulse sequence diagram RF SS PE FE Signal 7  3D Vs. 2D Imaging An entire volume of tissue is excited (None selective RF pulse) Data are saved in 3D k space rather than 2D k space The 3D data set is then re-sliced into partitions using the slice selection gradient (slice encoding) Scan time is increased by factor of SSsteps 3D is susceptible to motion artifacts It suffers higher Chemical Shift (CS) artifact with regard to 2D due to thin slices (small voxel) The SNR is improved by 3D is not affected by ‘cross talk’ SS steps or ‘slice bleeding’ artifact 8  Scan time Scan time = TR x NEX x PEsteps NEX (no.of excitations ) also called NSA (no.of signal averages). It is defined as the number of time the k-space is filled for the same slice or how many times each k-space line is repeated – NEX = 1 means that one k space for single slice – NEX >1 (e.g 3) means that same k space is repeated 3 times (or each line in k space is repeated 3 times) for single slice and the 3 repeats are then averaged – NEX