The effect of contrast agents on MRI signals
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The effect of contrast agents on MRI signals
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[post_date] => 2025-01-09 08:12:48
[post_date_gmt] => 2025-01-09 13:12:48
[post_content] => Practice Passage (Question 1-4)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Protons have an intrinsic magnetic moment coupled to the direction of their spins. During MRI, a magnetic field is used to align the spins of protons, and, as a result of this, the sample acquires a net, measurable magnetization parallel to the static applied field. Once the protons are aligned with the static field, radio-frequency (RF) photons are then applied to the protons, which cause them to enter a higher energy state, with their spins aligned antiparallel to the field. The steps are outlined in Figure 1.
Figure 1. Alignment of proton spins before the measurement begins, after application of a static magnetic field, and after application of a radio-frequency (RF) field.
The applied RF field is then removed, and the protons gradually return to the lower-energy, aligned state, emitting photons with the same energy as the exciting pulse. This relaxation occurs gradually, with the number of spins aligned opposite the static field decreasing exponentially in time. The time constant, known as T1, depends on composition and structure of the tissue.
In addition, neighboring spins in the sample tend to become aligned with one another due to pairwise magnetic interactions, which further improves the MRI signal. After the removal of the RF, this spin-spin alignment exponentially decays with a different time constant, T2, which also depends on material properties. The dependence of T1 and T2 on structural properties gives rise to contrast between different tissues in MRI.
In many applications, “contrast agents” consisting of fluids with particularly short T1 times are used in order to differentiate otherwise similar tissues. Experimental results indicate a relationship between the observed relaxation timescale and the concentration of contrast agent used, as shown in Figure 2.
Figure 2. The strength of the MRI signal along two, perpendicular measurement axes allows T1 and T2 to be separately observed for different concentrations of contrast agent. A: Measurement of the signal along one direction allows T1 to be observed for two different concentrations of contrast agent X. B: Measurement along another, orthogonal direction allows T2 to be observed for different concentrations of contrast agent Y.
Data adapted from: Yoo, B., & Pagel, M. D. (2007). An overview of responsive MRI contrast agents for molecular imaging. Frontiers in Bioscience: a Journal and Virtual Library, 13, 1733-1752.
[post_title] => The effect of contrast agents on MRI signals
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[question] => Based on the description of MRI found in the passage, which of the following is most likely proportional to the “signal” graphed against time in Figure 2A?
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[answer] => 1
[description] => Reason for the Correct Answer:
T1 affects the time over which spins return to their equilibrium state, aligned with the static field.
As described in the text, aligned magnetic dipoles give rise to a non-zero net magnetization, representing the asymptote in the figure.
The number of RF photons emitted by the sample should decrease after the cessation of applied RF
The only macroscopic quantity readily measured in this simple MRI is the total magnetization of the material.
T1 sets the timescale for alignment with the static field.
The graph must show the net magnetization along the direction of the applied, static field. This is the only quantity described in the passage that would have a non-zero asymptote with time, and that makes sense physically.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The net magnetization of the sample along the direction of the static magnetic field.
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[each_answer] => B. The net magnetization of the sample along the direction of the applied RF field.
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[2] => Array
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[each_answer] => C. The number of RF photons emitted by different parts of the sample.
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[each_answer] => D. The number of RF photons absorbed by the sample during excitation.
)
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Suppose that the applied RF field also heats the sample slightly. What effect would this have on the apparent T1 observed after the RF is removed?
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[answer] => 2
[description] => Reason for the Correct Answer:
Recall that T1 is a timescale for relaxation to a low-energy state.
Thermal effects tend to drive systems towards more disordered states
The applied RF field tends to cause spins to become misaligned with the static field. Heating would further contribute to this effect.
An increasing temperature should thus result in a larger apparent re-alignment time, T1
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[answers] => Array
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[0] => Array
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[each_answer] => A. It would depend on the chemical composition of the contrast agent
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[1] => Array
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[each_answer] => B. T1 increases
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[2] => Array
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[each_answer] => C. T1 would remain the same
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[3] => Array
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[each_answer] => D. T1 decreases
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[2] => Array
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[quiz_unique_key] => 2261298308
[question] => Which of the following explanations provides the best physical reasoning for why increased concentration of a contrast agent would change the effective T2 of the contrast agent?
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[answer] => 1
[description] => Reason for the Correct Answer:
While stimulated emission is involved in T1, it should not affect spin-spin coupling timescale T2. Stimulated emission would not strongly increase with density increases.
It is apparent in both graphs that higher concentrations have smaller relaxation times, eliminating two answer choices.
The force between two magnetic dipoles decreases with increasing distance.
The most physically likely explanation for the effect is that higher concentrations of contrast agent encourage electromagnetic interactions by decreasing the mean distance between spins.
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[answers] => Array
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[0] => Array
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[each_answer] => A. A reduced mean distance between spins increases their electromagnetic coupling.
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[1] => Array
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[each_answer] => B. A higher density of photons emitted during relaxation provokes stimulated emission of photons from nearby protons.
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[2] => Array
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[each_answer] => C. A higher density of emitted photons results in greater re-absorption of photons by neighboring spins.
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[each_answer] => D. Higher concentrations of contrast agents shield particles from the applied magnetic field.
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[quiz_unique_key] => 2377279144
[question] => In the Figure, which of the tested agents displays the largest T1, and which displays the largest T2?
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[answer] => 3
[description] => Reason for the Correct Answer:
A sample with a large T1 timescale should take a long time to reach the asymptote, corresponding to all spins aligned with the external magnetic field.
In a sample with a large spin-spin (T2) relaxation time, it should take a long time for the signal from transversely-magnetized spins to decay to zero.
T2 is a signal decay timescale, and so a smaller time constant would indicate faster decay.
T1 is a signal recovery timescale, and so a smaller time constant would indicate faster return to the asymptotic value.
Low X and Low Y represent the largest T1 and T2, respectively
)
[answers] => Array
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[0] => Array
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[each_answer] => A. Largest T1: Low X and Largest T2: High Y
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[each_answer] => B. Largest T1: High X and Largest T2: High Y
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[2] => Array
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[each_answer] => C. Largest T1: Low X and Largest T2: Low Y
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[3] => Array
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[each_answer] => D. Largest T1: High X and Largest T2: Low Y
)
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[4] => Array
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[quiz_unique_key] => 83407773
[question] => The frequency of the applied RF signal used to excite spins is directly proportional to the magnitude of the static magnetic field used to align the spins, with proportionality constant 5 hz/T. If the strength of the applied field is known to be 20 T plus or minus 3 T, which of the following correctly describes the uncertainty in the INVERSE frequency (1/frequency)?
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[answer] => 2
[description] => Reason for the Correct Answer:
Error propagation for inverse proportionalities works differently than it does for direct proportionalities
The error of a reciprocal relation,
, is given by 
Alternately, the error must be a smaller quantity than 1/15 s, which would be the error in the case where the inverse frequency and the magnetic field were directly proportional.
The result of substituting the given values for 𝚫B, B, and k is 3/2000 s
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[each_answer] => A. 4 s
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[each_answer] => B. 3/2000 s
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[each_answer] => C. 1/15 s
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[each_answer] => D. 3 / 5 s
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