MRI PHYSICS- Armrit Test Questions and Answers (Graded A+)

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.