Image Basics | MRIQUIZ

A comprehensive set of practice flashcards covering MRI physics and imaging parameters: T1/T2 relaxation, tissue contrast, TR/TE weighting, SNR/NEX/NSA, voxel/pixel calculations, FOV, k-space, ETL, Half-Fourier, Rectangular FOV, and scan-time formulas.

What is a pulse sequence in MRI?

A set of specifically timed instructions to the magnet telling it how images should look with regards to the tissue being sampled.

After the initial RF excitation in an MR pulse sequence, which two simultaneous relaxation processes occur?

T1 relaxation and T2 relaxation.

How do the typical time scales of T1 and T2 relaxation compare?

T1 relaxation takes a few seconds, while T2 relaxation occurs in milliseconds.

What is T1 relaxation also known as, and what does it involve?

Spin-lattice relaxation; regrowth along the longitudinal (Z) axis.

What does the 63% longitudinal magnetization recovery represent in T1?

The recovery level used to define T1 relaxation (63% of the original longitudinal magnetization).

How is T2 relaxation defined in terms of transverse magnetization?

Time for 63% of the transverse magnetization to decay (to 37% of the original value).

Which tissues typically have short T1 and short T2, and which have long T1 and long T2?

Fat has short T1 and short T2; Water has long T1 and long T2.

Which hydrogen species has a higher Larmor frequency, water or fat?

Water.

On T1-weighted images, which tissues tend to appear bright?

Fat and gadolinium-enhancing tissues.

What TR/TE pattern yields a T1-weighted image?

Short TR and short TE.

On T2-weighted images, which tissues tend to appear bright?

Long T2 tissues such as edema or cerebrospinal fluid (CSF).

Which parameter primarily controls T2 weighting in MRI?

Echo Time (TE).

Which parameter primarily controls T1 weighting in MRI?

Repetition Time (TR).

What are the typical effects of increasing TR on SNR, scan time, and T1 weighting?

Increases SNR and scan time; reduces T1 weighting.

What are the typical effects of decreasing TR on SNR, scan time, and T1 weighting?

Reduces SNR and scan time; increases T1 weighting.

Provide typical TR ranges for T1-weighted, PD-weighted, and T2-weighted sequences.

T1-weighted: 350–700 ms; PD-weighted: 1500–3000 ms; T2-weighted: 2000–6000 ms.

How is overall scan time related to TR, Phase Matrix, and NEX?

Scan time is directly proportional to TR multiplied by Phase Matrix and NEX (TR x Phase Matrix x NEX).

Name several methods to reduce MRI scan time mentioned in the notes.

Reduce TR; reduce NSA/NEX; increase parallel imaging factor; utilize RecFOV; use Half-Fourier/Halfscan; use the coarsest matrix.

How does slice thickness affect SNR, resolution, and anatomical coverage?

Thicker slices increase SNR and coverage but reduce spatial resolution; thinner slices improve resolution but reduce SNR.

How does increasing or decreasing NEX/NSA affect SNR and scan time?

Increasing NEX/NSA increases SNR by the square root of the increase and increases scan time proportionally.

How does changing the FOV affect SNR and voxel size?

Increasing FOV increases SNR and voxel size (coverage); decreasing FOV reduces SNR and voxel size.

How do you calculate pixel size from FOV and matrix?

Pixel size = Field of View ÷ Matrix (per dimension).

What is voxel size and how is it calculated?

Voxel size = Pixel area × Slice thickness; Pixel area = (FOV/Phase matrix) × (FOV/Frequency matrix).

What is an isotropic voxel and why is it desirable?

A voxel with equal dimensions in phase, frequency, and slice directions; facilitates accurate reformats.

What happens to voxel size and SNR when FOV is halved while keeping the matrix fixed?

Voxel volume decreases by a factor of 4; SNR reduces to about 25% of the original.

What happens to voxel size and SNR when FOV is doubled while keeping the matrix fixed?

Voxel volume increases by a factor of 4; SNR increases about 4 times.

What is k-space in MRI?

The storage space for MR signal data; each line corresponds to a phase-encoded line.

Where is the center of k-space and what information does it contain?

The center contains the contrast information and has the highest signal amplitude.

What is elliptic centric k-space filling and why is it used?

Low-frequency data (center) are filled first to maximize contrast and signal efficiency.

What is ETL (Echo Train Length) and how does it affect scan time and image quality?

ETL is the number of echoes per TR in a fast spin echo sequence; higher ETL reduces scan time but can increase blurring and SAR and reduce SNR.

What are the typical ETL ranges for T1-weighted, PD-weighted, and T2-weighted sequences?

T1: ~2–7; PD: ~3–12; T2: ~10–40.

What is Half-Fourier (Half Fourier) imaging in MRI?

Acquiring slightly more than half of phase-encoding data and filling the rest by symmetry; reduces scan time with an SNR penalty.

What is Halfscan (Partial Echo) imaging in MRI?

Shortening or reducing data in the frequency-encoding direction (partial echoes); reduces TE and scan time with an SNR penalty.

What is Rectangular FOV and how does it affect scan time and pixel shape?

A method to reduce scan time by not filling as many phase-encoding lines; with 50% Rectangular FOV, phase matrix must be half the frequency matrix to maintain square pixels.

Why is the center of k-space crucial for dynamic imaging and contrast?

Center k-space data carry the majority of signal amplitude and contrast; arranging data (centric filling) optimizes contrast during dynamics.

How is a 2D Spin Echo sequence scan time calculated?

Scan time = TR × Phase Matrix × NEX (in seconds, divide by 1000).

How is a Fast Spin Echo (FSE) scan time calculated?

Scan time = TR × Phase Matrix × NEX ÷ ETL.

How is a 3D Gradient Echo scan time calculated?

Scan time = TR × Phase Matrix × NEX × number of slices.

What is the relationship between ETL and the number of slices in a Fast Spin Echo sequence?

Increasing ETL reduces the number of available slices.

What is meant by the 'effective TE' in fast spin echo imaging?

The TE value at which the center of k-space (or the bulls-eye of the FSE) is centered, yielding the strongest signal and contrast.