NMR/MRI examination (8N080 / 3F240)

Transcription

1 NMR/MRI examination (8N080 / 3F240) Remarks: 1. This test consists of 3 problems with at total of 26 sub-questions. 2. Questions are in English. You are allowed to answer them in English or Dutch. 3. Please read the questions carefully. Give answer to the question only. 4. Be concise but accurate. Phrase precisely and avoid messy writing (If we are not able to read it, it is wrong!). 5. You are not allowed to use any books or other course material. You are allowed to use a calculator. 6. γ = γ/2π = MHz/T Examination NMR/MRI (8N080 and 3F240) page 1/5

2 Problem 1: General questions An MRI scanner with a magnetic field of 3 T contains a solenoid RF coil in which a tube with water is placed. a) Make a schematic drawing of the bore of an MRI scanner in which you indicate the position of the RF coil and the tube of water. Indicate with vectors the directions of the static magnetic field B 0 and the RF field B 1. b) Calculate the amplitude of the (circularly polarized) radio-frequency field B 1 that is required if we want to apply a hard 90 pulse (pulse with a rectangular shape) with a duration of 200 µs. c) After the 90 pulse, phase (P) and quadrature (Q) free induction decay (FID) signals are observed. Make a schematic drawing of these signals and indicate relevant parameters (time constants, frequencies, phases etc.). d) Explain how an NMR receiver derived the P and Q signals at its outputs from the RF signal picked up by the solenoid. e) During the RF pulse specified in b) a magnetic field gradient of 50 mt/m along the z axis is applied. Describe what kind of slice is selected, its orientation, and estimate the thickness of the region in which the nuclear spins are effectively excited. f) Immediately after the 90 pulse, specified in c) (so without a field gradient) a gradient of 10 mt/m along the y axis is switched on. If the tube with water has a dimension of 10 cm along the y axis, how long does it take for the nuclear spins at one end of the tube to dephase by 360 with respect to the spins at the other end? g) After a time Δ the gradient of 10 mt/m specified in f) is switched off and a gradient of g mt/m of opposite sign is switched on along the y axis (so g is negative). Describe the time evolution of the nuclear magnetization during the time interval after the 90 pulse. At what time(s) the amplitude of the signal induced in the solenoid reaches a maximum? h) In the water in the tube some small fat droplets are present. The hydrogen nuclei of the fat have a chemical shift of -7 ppm (parts-per-million) with respect to water. What is the Larmor frequency of the hydrogen nuclei of the fat? Examination NMR/MRI (8N080 and 3F240) page 2/5

3 Problem 2: Echo-planar-imaging The image below shows 4 slices from an Echo-Planar-Imaging (EPI) acquisition of a human brain at 3 Tesla. Remember: EPI is a fast imaging sequence, which generally uses a single excitation to fill the complete k-space. The slice thickness was 10 mm, and the number of pixels in phase and frequency encoding were 64 and 128, respectively. a) Make a drawing of the timeline of the EPI sequence used to acquire these images (include RF pulses and gradients). b) Draw this sequence in k-space. c) Which 2 elements in the sequence can be changed to reduce the slice thickness from 10 mm to 5 mm? Give a possible drawback for both of them? d) What should the strength of the frequency encoding gradient be, given that the acquisition time of the echo is 1 ms? e) The duration of the phase encoding gradient is 0.1 ms. What are the strengths of the phase encoding gradients needed to record the images? The EPI images of the brains display some strange distortions, especially near the frontal (air-filled) cavities in the brain, indicated by the arrows. These distortions are of course not real, but related to the EPI imaging sequence used. f) What is the cause of these image distortions? Explain your answer. g) Why are these distortions not present in regular spin-echo images of the brain? h) Give and explain 2 ways how to reduce the image distortions in the EPI images. Examination NMR/MRI (8N080 and 3F240) page 3/5

4 Problem 3: Contrast The picture below shows 2 MRI images of the neck region of a patient. The large vessel in the middle of the images is the carotid artery (Dutch: halsslagader), the large artery that supplies the brain with blood. The arrow indicates the location of an atherosclerotic plaque (Dutch: aderverkalking), which slightly occludes the artery. One of the images was recorded with a spin-echo sequence and the other one with a gradient-echo sequence. a) Which sequence was used to record image A? Explain why this sequence produces a bright blood signal. b) Which sequence was used to record image B? Explain why this sequence produces a black blood signal. c) Give 2 possible reasons why the signal to noise in image B is worse than in image A. The physician evaluating the images is not certain that the atherosclerotic plaque is large enough to have any significant effect on the blood supply to the brain. To know for sure, he would like to measure the absolute blood velocities in the carotid artery. This can be done, using the so-called pulsed-field-gradient technique. d) Explain how you can measure absolute blood flow velocities using a pulsed-field-gradient technique. e) Give a first order expression for the velocity in terms of the relevant parameters of the pulsed-field-gradients. f) The pulsed-field-gradients can also be used to measure the diffusion of water inside tissues. Explain these measurements and the difference with flow quantification. It turns out that the blood flow is not seriously obstructed by the atherosclerotic plaque. However, the physician is not convinced that the plaque is harmless, because the atherosclerotic plaque could rupture and cause a blood cloth (Dutch: bloedprop), that might get stuck in the brain causing a brain infarction (Dutch: herseninfarct). He therefore decides to use a contrast agent to get more specific information on the plaque. For this he has 2 options: A Gadolinium-based contrast agent or an iron-oxide-based contrast agent. Examination NMR/MRI (8N080 and 3F240) page 4/5

5 g) What is the effect of Gadolinium on the relaxation times? h) When you use a spin-echo sequence. How should the repetition time (TR) and the echo time (TE) be set (with respect to the relaxation times) to observe the effect of the Gadolinium in the MR image. i) What is the effect of iron-oxide on the relaxation times? j) Give a sequence that can be used to see the effect of the iron-oxide. Explain how you should set the sequence parameters in order to observe the effect of the iron-oxide in the MR image. --- END --- Examination NMR/MRI (8N080 and 3F240) page 5/5