MRI PHYSICS- Armrit Test Questions and Answers with complete

MRI PHYSICS - Armrit Test Signal - Answer -Current induced iby receiver coil. Also comes from the procession of protons in the patient. Chemical pre saturation - Answer -Selectively apply 90 degree pulse at frequency of tissue to be saturated. Moves fat or HBO 90 degrees. 2nd puse wider bandwidth moves both fat and water 90 degrees. One will be saturated so will not be read. Saturating out normal anatomy, pathology is often seen more clearly. NEX number of excitations - Answer -is the number of times each line is filled with the data in K space, the number of times and echo is encoded with the same slope of phase encoding gradient. NEX (number of excitations) to increase (SNR) - Answer -signal strength is proportional to square root. To double SNR you increase __________ and scan time by factor of four. Scan time is proportional to - Answer -TR, phase matrix the number of encodings, NEX doubling all of these doubles times. Rise time - Answer -Time required for MRI machine to reach field strength for gradient. Steeper slope longer time. Receive bandwidth to increase signal (SNR) - Answer -Decrease this and you have better SNR Proton density weighed images - Answer -Long TR, short T, small flip angle highest signal to nose ratio. Magnetization transfer contrast (MTC) - Answer -Technique used to suppress bound protons by saturating them before the excitation pulse. This reduces the signal in free protons. This increases the contrast (CNR) between normal pathological tissues. Good in angiography and joint imaging. What effects voxel size - Answer -Slice thickness (slope of the gradient), FL , the matrix (#of encodings) Voxel volume to increase signal (SNR) - Answer -Lager voxel more signal (SNR), thicker slicers, larger FOV more in picture, smaller image matrix (determines amount of pixels) TR (repetition time) - Answer -Controls recovery of longitudinal magnetization. Increasing also increases scan time. TR to increase signal (SNR) - Answer -Long, lots of recovery, larger signal (SNR). Full recovery allows most signal to be rotated into transverse plane. Pixel shape is rectangular when - Answer -FOV square and scan matrix odd FOV rectangular and scan matrix is even. TE (echo time) to increase signal (SNR) - Answer -Shorter time better signal. Allows less dephasing. Spacial resolution - Answer -The ability to distinguish between two points. It is controlled by voxel size. Put structures in separate voxels, smaller voxels, thinner slices, steeper gradient, smaller FOV, fine matrix, rectangular FOV. Poor SNR = better res olution. ( 1 ) Noise - Answer -(1) Anything that interferesays with, distortso or slows down the tamale of information. (MRI machine, non signal pickup from receiver coil) Contrast to noise ratio (CNR) - Answer -Difference in SNR in adjacent regions indicate amount of contrast S/Np1 -S/Np2. Can be increased by T2 weighed image, chemical pre saturation, (MAG) magnetization transfer contrast and contrast agent. Coil to increase signal (SNR) - Answer -Type and position, best to position transverse to Bo Bound proton - Answer -Protons that are bound to large proteins, members and other macromolecules 3 volume imaging - Answer -Taking picture then slice after picture is done. Use with complicated anatomy ligaments joints. Long scan tone, no slice selection -slice encoding, finger sliceso no gaps. (1) Signal to noise ratio (SNR) - Answer -(1)Amplitude of the signal/ the amplitude of noise want the number to be large.