Magnetic Resonance Imaging - Essay Example

MAGNETIC RESONANCE IMAGING by of the school/ of the Describe how MRI sequences spatially en nuclei in the read and phase direction. Spatial encoding in read direction Readout or frequency encoding is achieved by encoding the signals along a direction in space with varying frequencies. Here, the spin echo signal is sampled n number of times where n is the number of projections along the frequency-encoded axis. To rebuild the distribution of the sources along the encoded direction, it is generally essential to obtain a set of signals with a suitable set of frequencies. When there are no other position encoding components present, the FT of the signal that results is simply a one-dimensional projection profile of the subject. A readout gradient produces the magnetic gradient field to encode signals into different frequencies. Thus, the gradient causes a range of larmor frequencies to exist in the direction in which it is applied. After measuring an MR signal, these frequencies can be separated using a Fourier transform. Spatial encoding in phase direction Spatial encoding in phase direction involves the application of a pulsed magnetic field gradient to change the spin phase in one direction and locate the MR signal. When a gradient field is switched off for a brief period and then again switched on at the starting of the pulse sequence immediately following the RF pulse, it will cause the external voxels to precess either slower or faster in relation to the central voxels. When the x or y component is plotted as a function of the number of phase encoding steps and time, it varies sinusoidally, slower at the centre of the image, and faster along the right and left edges. Thus, the phase of the xy-magnetisation vector will differ systematically during readout. Each component in the signal experiences a different phase encoding gradient pulse and an FT analysis can be used to determine the precise spatial reconstruction. The phase encoding gradient enables the spatial signal location to be encoded along a second dimension through different spins. It is applied after selection of the slice and before excitation of the remaining two gradients. The spatial resolution is determined by the number of phase encoding gradients used. Reference: Brown, Mark A and Semelika, Richard C (2010) MRI: Basic Principles and Application, 4th Edn. New Jersey: Wiley-Blackwell. 2) Why are two Fourier transforms required to convert k-space data to an image? Because the two encoding directions, namely the read and phase, are independent, two Fourier transforms are used to convert k-space data to an image. The image is first Fourier transformed in the read direction followed by transformation in the phase direction. During read Fourier transformation, each echo in the read direction is converted into an intensity profile. The units are converted from m-1 to m and the signal intensity is indicated by the number of location of the samples along the read axis. However, the unit of the phase direction remains to be m-1. A second Fourier transform in the phase direction is therefore essential. Phase FT can separate the different rates of change of phase that are present in the various levels of a slice. Phase encoding is carried out by introducing different rates of change of phase over many signal measurements. The position of the nuclei in both the phases is determined after the second FT. References: Brown, Mark A and Semelika, Richard C (2010) MRI: Basic Principles and Application, 4th Edn. New Jersey: Wiley-Blackwell. Mac Robbie, Donald W (2003) MRI from Picture to Proton, Cambridge: Cambridge University Press. 3) Draw a pulse program diagram for a spin echo pulse sequence. Discuss how each component of the MRI experiment affects transverse spin coherence and how this affects the lines of k-space data. The spin echo sequence involves at least two RF pulses, a slice selective 90° pulse and a re-phasing pulse. A spin-echo signal is generated when a 90° pulse and a 180° pulse are sequentially applied. This series is repeated in intervals of time and with each repetition, a k-space line is filled. Fig. Spin echo pulse diagram. Brown and Semelika (2010) MR images are generated by the application of slice, read, phase, and read magnetic field gradients. However, when any of the three gradients are applied, it causes the nuclear spins in the transverse plane to rapidly dephase. The dephasing effect is so strong that at the end of the slice selection gradient, no effective signal remains. By generating a spin echo, it is possible to recover the signal that was lost due to loss of phase coherence. The spin echo sequence consists of a slice selective 90° pulse followed by a rephasing 180° pulse. The 180° pulse is applied at the centre of the period to refocus T₂* relaxation. Each echo contains the information for one phase encoding increment for one slice, that is, for one line of k-space. The spin echo pulse realigns the time independent changes in nuclei resonating frequency. The first 90° pulse tips the longitudinal magnetization vector z into the xy plane. The magnetization is then dephased over the transverse xy plane. A 180° pulse is now generated to rephrase the signal in the xy plane. Slice gradient When the slice selection pulse is applied, the spins in the transverse plane acquire phase differences that are dependent on their position in the exited slice. The transverse spins are rapidly dephased and the signal reduces to zero. This loss of phase coherence and hence the lost signal, can be recovered by reversing the gradient for a time period that is equal to the time between the 90° pulse and the end of the slice gradient. One slice is denoted by one K-space with the slice gradient is located at its centre. Phase gradient The phase gradient is applied before the 180° pulse and it causes the signal to dephase. The Time of Reparation (TR) is represented by the time duration between the 90° pulse and the succeeding 180° pulse. By applying a gradient during the pulse sequence for a short period of time, k-space is vertically filled or spatially encoded through phase encoding. The strength of the phase gradient determines the position of the line of k-space in the y axis or phase encoding direction. At the end of the phase recovery gradient, the lost signal is recovered. Read gradient The read gradient is applied during data acquisition time when the intensity of the signal is at the highest and it causes the signal to dephase and reduce to zero. By applying a preparation gradient for the time period between the start of the read gradient and the centre of the spin echo, the loss of phase coherence due to the addition of a read gradient can be minimized. The preparation gradient is the reverse of the read gradient. The read gradient then refocuses the phase of the spin whose alignment will correspond with the centre of the spin echo. When a read gradient is applied during echo acquisition, one line of k-space is filled. The horizontal lines of data in k-space represent the read direction. The centre of the data space contains maximum signal. When the 180° refocusing pulse is applied, the dephased signal begins to rephase and reaches maximum amplitude when the nuclei are completely rephased. As the nuclei dephase once more, the signal decreases in amplitude. The maximum amplitude occurs in the centre because the line is obtained as a result of additional dephasing due to the phase encoding gradients. Reference: Brown, Mark A and Semelika, Richard C (2010) MRI: Basic Principles and Application, 4th Edn. New Jersey: Wiley-Blackwell. References Brown, Mark A and Semelika, Richard C (2010) MRI: Basic Principles and Application, 4th Edn. New Jersey: Wiley-Blackwell. Mac Robbie, Donald W (2003) MRI from Picture to Proton, Cambridge: Cambridge University Press. Read More