Physics Flashcards

When a constant magnetic field is present, hydrogen protons

Answer: Stay basically orientated randomly while rotating around the field axis.

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.

Magnetization of hydrogen using MRI

Answer: perpendicular to the primary magnetic field

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.

Following an RF burst, protons will

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

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.

What differentiates T2 from T2* is

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

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.

One forms a spin echo through

Answer: changing the path of a proton using a radiofrequency pulse

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.

Spin echo sequence is modified by rapid spin echo sequence by

Answer: Adding 180-degree pulses in succession

The rapid spin echo (also known as Fast Spin Echo or Turbo Spin Echo) sequence modifies the conventional spin echo by applying a train of multiple 180-degree refocusing pulses after a single 90-degree excitation pulse. Each 180-degree pulse generates a separate echo, allowing multiple lines of k-space to be acquired within a single TR period. This significantly shortens scan times compared to conventional spin echo sequences.

Fast spin echo sequences, in contrast to spin echo, have

Answer: T2 weighting increased but susceptibility decreased

Fast Spin Echo (FSE) sequences utilize multiple 180-degree refocusing pulses, which effectively reduce the impact of magnetic susceptibility artifacts by repeatedly rephasing spins. This leads to decreased susceptibility artifacts compared to conventional spin echo. Additionally, FSE sequences can achieve stronger T2 weighting due to the extended echo train and multiple echoes contributing to the signal, often resulting in increased T2 weighting.

In contrast to the spin echo sequence (SE), the gradient echo sequence (GRE)

Answer: has T2* dephasing and lacks a 180-degree pulse.

Gradient Echo (GRE) sequences fundamentally differ from Spin Echo (SE) sequences because they do not use a 180-degree radiofrequency refocusing pulse. Instead, GRE sequences employ a gradient reversal to rephase the spins and create an echo. Since there is no 180-degree pulse to correct for static magnetic field inhomogeneities, GRE sequences are sensitive to T2* dephasing, making them more prone to susceptibility artifacts than SE sequences.

In contrast to SE, GRE sequences employ

Answer: reduced TR and TE lengths

Gradient Echo (GRE) sequences typically employ significantly shorter repetition times (TR) and echo times (TE) compared to Spin Echo (SE) sequences. This is feasible because GRE sequences use smaller flip angles for excitation and do not require the longer timing associated with the 180-degree refocusing pulses. These reduced TR and TE values enable much faster image acquisition, which is advantageous for dynamic imaging or when scan speed is crucial.

Traditional GRE order

Answer: Reduce the impact of residual transverse magnetization by using spoiler gradients.

In traditional (spoiled) Gradient Echo (GRE) sequences, spoiler gradients are applied at the end of each TR period. The primary purpose of these gradients is to intentionally dephase any residual transverse magnetization. This prevents the magnetization from contributing to the signal in subsequent TRs, ensuring that the signal primarily reflects T1 weighting and maintains consistent image contrast by avoiding contamination from previous excitations.

Balanced steady-state free precession (SSFP) sequences have an advantage over conventional GRE sequences.

Answer: To create spin echoes, hold onto your transverse magnetization.

Balanced steady-state free precession (bSSFP) sequences have a key advantage over conventional spoiled GRE sequences by actively preserving transverse magnetization from previous excitations. Unlike spoiled GREs that use spoiler gradients to eliminate residual transverse magnetization, bSSFP sequences are designed to maintain both longitudinal and transverse magnetization in a steady state. This allows for the formation of both FID and echo signals, leading to high signal-to-noise ratio and characteristic T2/T1 contrast.