CH 2: IMAGE WEIGHTING AND CONTRAST Comprehensive Exam Actual Questions And Verified Answers

What are the 2 factors that affect image contrast in diagnostic imaging? - correct answer Intrinsic contrast parameters: are those that cannot be changed because they are inherent to the body's tissues. Ex) Bone density Extrinsic contrast parameters: are those that can be changed because they are under our control. Ex) TR What are examples of intrinsic factors? - correct answer T1 recovery time T2 decay time Proton density (PD) Flow Apparent diffusion coefficient (ADC) All these are inherent to the body's tissues and cannot be changed. What are examples of extrinsic factors? - correct answer TR TE Flip angle TI Turbo factor/echo train length b value. These are all selected in the scan protocol. Describe relaxation. - correct answer - When the RF excitation pulse is switched off, and therefore hydrogen nuclei return to their low-energy state and their magnetic moments dephase - During relaxation, hydrogen nuclei give up absorbed RF energy, and the net magnetic vector (NMV) returns to B0. - At the same time but independently, magnetic moments of hydrogen nuclei lose phase coherence. - Relaxation therefore results in recovery of magnetization in the longitudinal plane and decay of coherent magnetization in the transverse plane. What are T1 recovery and T2 decay? - correct answer - The recovery of longitudinal magnetization is caused by a process termed T1 recovery. - The decay of coherent transverse magnetization is caused by a process termed T2 decay. - T1 recovery and T2 decay occur at two different rates. T2 decay occurs 5-10 times faster than T1 recovery. An important learning step is to understand the timing of different components of a pulse sequence Explain T1 Recovery. - correct answer - T1 recovery is caused by hydrogen nuclei giving up their energy to the surrounding environment or molecular lattice (spin-lattice energy transfer). - The term recovery refers to the recovery of longitudinal magnetization, and T1 relates to the fact that it is the primary relaxation process. What is Spin-lattice energy transfer? - correct answer - Hydrogen nuclei giving up their energy to the surrounding environment or molecular lattice. - Energy released by spins to the surrounding molecular lattice causes magnetic moments of hydrogen nuclei to recover their longitudinal magnetization. Differentiate between the quantum theory and classical theory descriptions of relaxation. - correct answer According to quantum theory, the number of high-energy spins decreases, and the number of low-energy spins increases as energy is lost by high-energy spins during the relaxation process. According to classical theory, the NMV gradually realigns itself in the longitudinal plane as the proportion of spin-up and spin-down hydrogen nuclei changes. What is the rate of T1 recovery? - correct answer The rate of T1 recovery is an exponential process and occurs at different rates in different tissues. Longitudinal magnetization is related exponentially to recovery time. This means that most longitudinal recovery happens at the beginning of the time frame. As time progresses, gradually less and less longitudinal recovery occurs until the longitudinal magnetization is fully recovered. What is the T1 recovery time? - correct answer A time constant associated with the exponential relationship of T1 recovery. Is the time it takes for 63% of the longitudinal magnetization to recover in a tissue. The T1 recovery time of a tissue is an intrinsic contrast parameter that is inherent to the tissue. What is the time during which T1 recovery occurs? - correct answer The time during which T1 recovery occurs is the time between one RF excitation pulse and the next. This is the repetition time (TR). The TR therefore determines how much T1 recovery occurs in a tissue. It is therefore the variable shown on the horizontal axis. How can you calculate the amount of longitudinal magnetization at time t (ms) after the removal of the excitation pulse? - correct answer Mzₜ = Mz (1 − e⁻ᵗ/ᵀ¹) Therefore SI = (1 − e⁻ᵗ/ᵀ¹) - Mzt is the amount of longitudinal magnetization at time t (ms) after the removal of the excitation pulse - Mz is full longitudinal magnetization - T1 is the T1 recovery time (ms) and is the time taken to increase the longitudinal magnetization by a factor of e. - SI is the signal intensity in a tissue What happens when t = T1? - correct answer When t = T1, 63% of the longitudinal magnetization recovers. When t = 2 × T1, 86% recovers and when t = 3 × T1, 95% recovers. It usually takes between 3 and 5 T1 recovery times for full recovery to occur. What is the T2 decay? - correct answer - T2 decay is caused by the magnetic fields of neighboring hydrogen nuclei interacting with each other. - The term decay refers to the loss of coherent transverse magnetization, and T2 relates to the fact that it is the secondary relaxation process. - This type of relaxation is termed spin-spin relaxation and causes dephasing of magnetic moments of the spins. Explain spin-spin relaxation. - correct answer - Spin-spin relaxation is caused by one spin transferring energy to another spin rather than into the lattice. - It occurs because hydrogen nuclei are in the same environment and experiencing the same B0 field. - Magnetic moments of all the hydrogen nuclei (spin-up and spin-down) lose phase coherence in this way. - Spin-spin interaction is inherent to the tissue Besides spin-spin interactions, what else is dephasing caused by? - correct answer - Dephasing is also caused by inhomogeneities in the B0 field. - Inhomogeneities are areas within the magnetic field that do not exactly match the external magnetic field strength. - Some areas have a magnetic field strength slightly less than the main magnetic field, while other areas have a magnetic field strength slightly higher than the main magnetic field. What happens to hydrogen nuclei in areas of inhomogeneity? - correct answer - If a hydrogen nucleus lies in an area of inhomogeneity with higher field strength, the precessional frequency of its magnetic moment increases, i.e. it speeds up. -If a hydrogen nucleus lies in an area of inhomogeneity with lower field strength, the precessional frequency of its magnetic moment decreases, i.e. it slows down. - This relative acceleration and deceleration of magnetic moments due to magnetic field inhomogeneities, and differences in the precessional frequency in certain tissues, causes immediate dephasing of the magnetic moments of the spins and produces a free induction decay (FID) What is the rate of T2 decay? - correct answer - The rate of T2 decay is an exponential process and occurs at different rates in different tissues - Coherent transverse magnetization is related exponentially to decay time. This means that there is more coherent transverse magnetization at the beginning of the time-frame and, as time progresses, there is less coherent transverse magnetization until all the magnetic moments dephase. What is T2 decay time? - correct answer - A time constant associated with the exponential relationship between coherent transverse magnetization and decay time. - It is called the T2 decay time and is the time it takes for 63% of the transverse magnetization to dephase (37% is left in phase) in a tissue. How can you calculate the amount of transverse magnetization at time t (ms) after the removal of the excitation pulse? - correct answer Mxyₜ = Mxy e⁻ᵗ/ᵀ² therefore SI = e⁻ᵗ/ᵀ² - Mxyt is the amount of transverse magnetization at time t (ms) after the removal of the excitation pulse - Mxy is full transverse magnetization - T2 is the T2 decay time (in ms) and is the time taken to reduce the transverse magnetization by a factor of e - SI is the signal intensity in a tissue What happens when t = T2? - correct answer When t = T2, 63% of the coherent transverse magnetization has decayed and 37% remains