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