Free MRI Physics Questions and Answers

Question 1

When a constant magnetic field is present, hydrogen protons

Accepted Answer

Stay basically orientated randomly while rotating around the field axis.

Answer

When hydrogen protons are placed in a strong constant magnetic field (B0), they align either parallel or anti-parallel to the field, creating a slight net magnetization. However, individual protons do not simply align; they also precess (rotate) around the axis of the main magnetic field at their characteristic Larmor frequency. While there's a net alignment, their individual spin orientations remain somewhat random relative to each other as they precess.

Question 2

Magnetization of hydrogen using MRI

Accepted Answer

perpendicular to the primary magnetic field

Answer

In MRI, the detectable signal is generated from transverse magnetization, which is created when an RF pulse tips the net magnetization of hydrogen protons away from the primary magnetic field (B0). This transverse magnetization precesses perpendicular to B0, inducing a current in the receiver coil. Therefore, the magnetization used for signal detection is perpendicular to the primary magnetic field.

Question 3

Following an RF burst, protons will

Accepted Answer

Dephase in accordance with T2*, then recover in accordance with T1 along B0.

Answer

Immediately following an RF burst, the excited protons begin to lose their phase coherence, causing the transverse magnetization to decay. This dephasing is governed by T2* relaxation, which includes both intrinsic molecular interactions and static magnetic field inhomogeneities. Simultaneously, the longitudinal magnetization, which was tipped away from B0, begins to recover along the B0 field, a process governed by T1 relaxation.

Question 4

What differentiates T2 from T2* is

Accepted Answer

The combination of changing molecular magnetic field inhomogeneity and static magnetic field inhomogeneity is represented by T2*, which equals T2.

Answer

T2 relaxation describes the decay of transverse magnetization due to intrinsic molecular interactions and microscopic magnetic field inhomogeneities. T2* relaxation, however, encompasses both these intrinsic T2 effects and additional macroscopic static magnetic field inhomogeneities inherent to the scanner and patient. Therefore, T2* is always shorter than or equal to T2, as it accounts for more dephasing factors.

Question 5

One forms a spin echo through

Accepted Answer

changing the path of a proton using a radiofrequency pulse

Answer

A spin echo is formed by applying a 180-degree radiofrequency (RF) pulse after an initial 90-degree RF excitation pulse. This 180-degree pulse effectively 'flips' the spins, causing them to rephase and produce a detectable echo signal. By reversing the effects of static magnetic field inhomogeneities, the 180-degree pulse allows for the recovery of signal that would otherwise be lost due to T2* dephasing.

MRI - Magnetic Resonance Imaging Practice Test

MRI - Magnetic Resonance Imaging Practice Test

Free MRI Physics Questions and Answers