Encoding and image formation
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- MRI encodes spatial information using magnetic field gradients to locate signal in three dimensions within the body. Slice selection, phase encoding, and frequency encoding are used to position the signal. - Gradients induce small linear variations in the magnetic field to alter the precessional frequency of nuclei along each gradient axis, allowing their location to be identified. Slice selection selects the imaging plane using the slice select gradient. Phase encoding further locates signal within that plane using the phase encoding gradient before signal readout. Frequency encoding then reads out the signal and samples it along the frequency encoding gradient axis. This spatial encoding stores data in k-space which is then transformed to an image using an FFT.
Encoding and image formation
- 2. Content o Encoding o Gradients o Slice selection o Frequency encoding o Phase encoding o Sampling o Nyquist Theorem o Data collection and image formation o K space description o K space filling o Fast Fourier transform (FFT)
- 3. Encoding • Locating the signal in 3 dimensions so that it can positioned each signals at correct points on the image. • First locate a slice. • Once a slice is selected, the signal is located or encoded along both axis of the image. • These are performed by gradients.
- 4. Select a slice Locate/encode signal along both axis of the image Locating slice & encoding signal is performed by gradients
- 5. Gradients • Gradients are alteration to the main magnetic field and generated by cell of wire located with in the bore of magnet through which current is passed. • The passage of current through a gradient coil induces a gradient coil induces a gradient magnetic field around it. • which either subtracts from, or adds to the main static magnetic field Bo. • Bo is altered in a linear fashion by the gradient coils. • so, the magnetic strength and therefore precessional frequency experienced by nuclei situated along the axis of the gradient can be predicted called spatial encoding. • 3 gradients coils situated within the bore of the magnet.
- 6. • These are named A/c to the axis along which they act when they are switched on. • These directions in a super conducting magnet. o Z- gradient alters the magnetic field strength along the z- (long) axis of the magnet. o Y – gradient alters the magnetic field strength along the Y- (vertical) axis of the magnet. o X- gradient alters the magnetic field strength along the X- (horizontal) axis of the magnet. • Gradients can be used to either dephase or rephase the magnetic moments of nuclei. • Gradients also perform the 3 main tasks in encoding- 1. Slice selection 2. Phase encoding 3. Frequency encoding
- 8. Slice selection • Locating a slice within the scan plane selected. • When a gradient coil is switched on. The magnetic field strength and therefore precessional frequency of nuclei located along its axis. • Nuclei situated within a slice have particular precessional frequency. • A slice can be selectively excited, by transmitting RF with a band of frequencies coinciding with the larmour frequencies of spins in a particular slice as defined by the slice select gradient.
- 10. Slice thickness • A band of nuclei must be excited b y the excited by the excitation pulse. • The slope of the slice select gradient determined the differences in precessional frequency between the two point on the gradients. • Steep gradient slopes result in a large difference in precessional frequency between two points on the gradient. • While shallow gradient slopes result in a small difference in precessional frequency between the same two points. • The RF pulse transmitted to excite the slice must contain a range of frequencies to match the difference in precessional frequency b/w two points.
- 11. • This frequency range is called the bandwidth and the RF is being transmitted at this point it is specifically called transmit bandwidth. o To achieve thin slices, a step slices select slope or narrow transmit bandwidth is applied. o To achieve thick slices, a shallow slices select slope or broad transmit bandwidth is applied. Note:- To achieve the axial slices then cut the z-gradient. To achieve the coronal slices then cut the Y-gradient. To achieve the sagittal slices then cut the X-gradient.
- 12. A B Thick slice- shallow gradient Thin slice- steep gradient To achieve thin slices, a steep slice select slope & narrow bandwidth is applied. To achieve thick slice, a shallow slice select slope & broad bandwidth is applied. Bandwidth Bandwidth
- 13. Phase encoding • The signal must now be located along the remaining short axis of the image and this localization of signal is called phase encoding. • When the phase encoding gradient is switched on, the magnetic field strength and therefore precessional frequency of nuclei along the axis of the gradient is altered. • The phase encoding gradient is usually switched on just before the application of the 180* rephasing pulse.
- 14. Fig showing phase change of nuclei when phase encoding gradient is switched on
- 16. Frequency encoding In this process signal is located along the long axis (z- axis) of the anatomy. When frequency encoding gradient is switched on- Now signal can be located along the axis according to its frequency. Magnetic field strength & precessional frequency of signal along the axis of the gradient is altered in a linear fashion. Thus, this gradient produces a frequency difference or shift of signal along the axis.
- 18. When gradients are switched on Spin echo sequence Gradient echo spin sequence Slice select 90 & 180 RF pulse Excitation pulse Phase encoding Before 180 RF pulse B/t excitation & signal collection Frequency encoding Collection of the signal Collection of the signal
- 19. Gradient switch on timing in spin echo sequence
- 20. Plane Slice selection Phase encoding Frequency encoding Sagittal X Y Z Axial body Z Y X Axial head Z X Y coronal Y X Z Gradient axis in imaging
- 21. The frequency encoding gradient is switched on while the system reads frequencies present in the signal, & samples or digitizes them. This gradient is sometimes called Readout gradient. Duration of readout gradient is called sampling time/ acquisition window. Sampling rate- rate at which frequencies are sampled or digitized during readout. system samples frequencies up to 1024 different times. Each sample is stored as a data point. Sampling
- 22. Collected data or information from each signal is stored as data points K-space is rectangular in shape & has two axis perpendicular to each other. Unit of k-space is radians per cm. K-space is a special frequency domain where information about the frequency of a signal & where it comes from in the patient is stored.
- 23. Frequency axes- horizontal Phase axes- vertical. Each slice has its own k-space. Data space– matrix of processed image data.(analog signal)
- 24. An MR image consists of a matrix of pixels, the number of which is determined by the no. of lines filled in K- space and no. of data points in each line. Each data point contains phase and frequency information from the whole slice at a particular moment in time during read out. The FFT process mathematically converts this to frequency amplitudes in the frequency domain. It is necessary because gradients spatially locate signal according to their frequency, not their time