Tutorials Paraphrasing - Essay Example

ial Answer: No Part: a It will not affect the results because the frequency of the spins does not depend on position. Part: b We will observe a profile of the cube instead of three peaks. After detection of the spins across the width of the voxel, the frequency of the spins becomes dependent on the position of spins within the voxel. Therefore, we will detect a range of frequencies for every voxel. Answer: No: 2 The spins move at the frequency of 84.8 MHz rf pulse in we select red slice. When the gradient is not there, the spins move at 85 MHz rf pulse because in such cases, spins work in the presence of in-plane B. Answer: No: 3 Part: a In case of minute hand, it is 90° [79.125°], whereas in case of second hand, it is 180° [164.625°]. Part: b In case of hour hand, it is 0° [105.375°]. In case of minute hand, it is 180° [184.5°], whereas in case of second hand, it is 270°. Answer: No: 4 It would either represent the frequency of 3600° per second or 10 revolutions per second, i.e. 10 Hz. Answer: No: 5 The relationship is that frequency is a change in phase, which depends on time. Tutorial 2 Answer: No: 1 Part: a The unit is Seconds. Part: b It causes units’ inversion. Part: c Because of no spatial encoding of spins, a FID cannot be used for imaging. Answer: No: 2 Part: a Application of a magnetic field gradient is required to spatially encode the nuclear spins Part: b Data acquired with respect to time has units of m-1 and when a magnetic field gradient is applied, Fourier transformation inverts the units to m from m-1. Answer: No: 3 1-Intensity 2- Read or kx, and 3- Phase or ky. Answer: No: 4 No specific position because there is no relationship between positioning of data in bottom left corner of k-space and the image. Answer: No: 5 The k-space data needs to be acquired as echoes because when we apply gradient in any particular slice gradient, it dephases the transverse coherence very quickly eliminating any chance to acquire data. Answer: No: 6 It is because the phase gradient is zero at this stage and the centre of the echo in the read direction directly corresponds to the centre of k-space. Answer: No: 7 A 180° pulse refocuses time independent changes in transverse phase coherence. The change in gradient dependent phase only occurs if the gradient is on. It does not depend on the total time for which the gradient remains on or off. Reversing the gradient and applying it to the sample refocus the phase coherence that is lost due to gradients. Tutorial 3 Answer: No: 1 Fourier transformation determines the intensities and frequencies of the individual frequencies present in the acquired data. It is always expressed in terms of Fourier Sine transform and Fourier Cosine transform. Fourier transform analysis is also used to understand the evolvement of quantum-mechanical system with time. Answer: No: 2 Analog signals use electric currents and voltages for reproducing the data whereas digital signals use binary data strings for the same purpose. Answer: No: 3 Nyquists theorem states that the signal must be sampled equal to or greater than twice the signal’s frequency in order to define accurately the frequency of a sine curve. This theorem is used for the digitization of analogs signals. Folding or aliasing results in detecting the signal outside the FOV that is being placed over the image. This change in detection occurs because the frequency of a signal calculated outside the defined FOV is always greater than the sampling rate. Hence, this signal is placed over the signal of lower frequency. Answer: No: 4 Part: a Twice the image matrix i.e. 128*2 = 256 data points need to be collected. Part: b Dwell time is 1 / (2*SW), which is equal to 16.7 microseconds. Part: c Acquisition time = number of data points* dwell time. Part: d SW = GS*FOV. Therefore, GS = SW/FOV = 30000 / 280 = 107.1 Hz/mm. Or as w=gB, we get B = w/g = 107.1 / 42.57*10^6 = 2.5 mT/m. Answer: No: 5 Due to reduction in voxel size by reducing slice thickness, the 2 major effects on the resultant image will be: (i) Increase in the spatial resolution, and (ii) Reduction in the SNR Answer: No: 6 As an operator, in order to reduce the scan time, I can: (i) Reduce the number of phase encoding (ii) Reduce the NEX, and (iii) Reduce the TR Tutorial 4 Answer: No: 1 TE is the time between the pulse, which generates a net transverse magnetization and the acquired echo formation. The echo is generated by a 180° pulse for spin echo sequences, which results in refocusing the all time independent loss of transverse spin coherence. The T2 relaxation curve determines the maximum signal intensity of the echo. An increase in the TE time increases the T2 decay, and decreases the signal intensity. Answer: No: 2 Figures 6.6, 6.7, and 6.8 show the effect of increase in the TE time with a constant long TR time. - Figure 6.6 is a proton density weighted image and was acquired with a very short TE (20 ms), giving low contrast and a high signal for all T2 values. Figure 6.7 was acquired with a mid range TE (100 ms), which gives high signal for areas that have long T2 values. For example, CSF fluid and low signal for areas with short T2 values, such as, grey matter. This is a typical T2 weighted image and gives good contrast. Figure 6.8 was acquired with a long TE (500 ms). In this figure, signal is only obtained from area with long T2 values, such as, the CSF fluid. The signals from all other regions with shorter T2 values were reduced to zero. Answer: No: 3 The longitudinal magnetization cannot recover completely before the next pulse if the TR value is less than 5*T1. This also decreases the transverse magnetization, which is generated by the next pulse and in the end, results in obtaining a signal. Answer: No: 4 These three images are acquired with varying TR values and short TE values. - Figure 6.11 was acquired with a very short TR (0.2 s), which not only results in low image contrast but also reduces the signal that is being obtained from all sample T1 values. - Figure 6.12 is a typical T1 weighted image was acquired with a mid range TR (1 s). This results in generating low signal from regions, which have long T1 values, such as, CSF fluid. Similarly it generates brighter signal from regions with shorter T1 values, such as, brain tissue. - Figure 6.13 is a proton density image and shows little image contrast and was acquired with a long TR (5 s). This results in bright signal from all T1 values because of very litle saturation of the signals. Answer: No: 5 The 90 and 180 pulses are considered slice selective pulses that rotate only a narrow range of spin frequencies. These pulses are applied in the presence of slice selection gradients that result in forming only distinct slices of the sample that are being rotated into the transverse plane. Answer: No: 6 If we apply gradients to a sample, it results in dephasing of the transverse spins and loss of the signals. Whereas, if we apply a large phase encoding gradient to the first echo in an echo train, it would decrease the transverse spin coherence and the signals not only for the first but also for all subsequent echoes. A rewind gradient refocuses the spin coherence that is de-phased by the phase gradient. This maximizes the signal that becomes free to be acquired with the encoded echo of the next phase. Tutorial 5 Answer: No: 1 If we acquire a signal, which follows 90 pulses from a fully relaxed sample, it will not give the greatest signal per unit time. The longitudinal magnetization recovers very quickly following an excitation pulse. The magnetization slows with the passage of time. All of this process results in 60 percent longitudinal relaxation, which recovers in the first T1 time period and in the further four T1 time periods that are required for the remaining 40 percent of the longitudinal relaxation to be recovered. Answer: No: 2 When we use the pulse angle of less than 45°, the increase, which occurs in transverse magnetization, is always less than the decrease in the longitudinal magnetization. Answer: No: 3 To maximize the signal for the sample, I would use 18° pulse angle. Answer: No: 4 In case of a fixed pulse angle, the magnitude of the decrease in longitudinal magnetization decreases with a decrease in the initial longitudinal magnetization. If the longitudinal magnetization is not able to recover completely between each pulse, the initial longitudinal magnetization along with the change in longitudinal magnetization decreases with successive pulses. However, there occurs an increase in the recovery of longitudinal magnetization by T1 relaxation per unit time with a decrease in the longitudinal magnetization. The spins are saturated by a sequence with rapid pulsing, which results in decreasing the longitudinal magnetization with every successive pulse. Therefore, the process of reduction of the longitudinal magnetization due to the pulse becomes slow, but quickens the increase in longitudinal magnetization by T1 recovery. Answer: No: 5 With a gradient echo pulse sequence, proton density images are acquired with small pulse angles, whereas T1 weighted images are acquired with medium to large pulse angles. Answer: No: 6 A crusher gradient is a strong gradient, which destroys the coherence of the transverse magnetization. This also opens up the likelihood that transverse magnetization, which is dephased by the read gradient for one phase of k-space, can be refocused by the gradients that are required for another phase of k-space. Therefore, the signal becomes a mix of different phase encoding gradients instead of each line of k-space causing artefacts in the images. Answer: No: 7 There are two components, which include impure pulse angles and Hahn echo. Any two small angles for an echo formation, which is originally described by Hahn. The fact is that the rf pulses, which rotate the nuclear spins never rotate all of the spins by the exact quoted amount. Therefore, some spins experience a 180 pulse even with a small pulse angle. This results in a spin echo that corresponds to the signal that is generated by the second following pulse. The echo has a very long TE that plays its part in increasing the effective TE of the combined signals. This also sets up the partial T2 weighting for the gradient echo images. Tutorial 6 Answer: No: 1 Part: a Pixel shift = (42.58 x 10 6 x 3.5 x 4x 256) / 10 6 x BW 10 = 152606.7 / BW BW=15260.7 Hz Part: b 2 pixels = 152606.7 / BW BW = 76303.3 Hz Answer: No: 2 Chemical misregistration is in the phase direction. It appears as a black line o as the absence of a signal in that voxel. On the other hand, chemical shift is in the read direction. The appearance is such that a black border appears on one side and a bright border on the other side. Answer: No: 3 Aliasing occurs when an area is under sampled with respect to the Nyquist theroem. In other words, aliasing occurs, if excited spins do not lie inside the chosen field of view. To solve the situation, we should either off centre the slices, swap phase/read directions (ONLY if rectangular and alias appears in phase direction), increase the field of view, or over sample using filtering/decimation in order to make it go back to the desired field of view. Answer: No: 4 The change in material from the body to the prosthesis results either in B0 inhomogeniety or in susceptibility artefact, which is perceived as signal voids and distortions. If the prosthesis is made of ferromagnetic metal, the distortions will be largest. In order to minimize the effect, we should use shorter echo times and a spin echo sequence instead of gradient echo. We need to be aware about the fact that the field inhomogeniety will interfere with suppression pulses and will not be effective if fat suppression is required. In some cases, if we change the direction of the phase encoding direction, it may help us improve the appearance of our region of interest. Read More