3.10 Parameter Tradeoffs: Resolution and Scan Time

Parameter Tradeoffs: Resolution and Scan Time

In MRI, optimizing the balance between spatial resolution and scan time is a critical task for the technologist. High spatial resolution is required to resolve small anatomical details, but it typically demands smaller voxel sizes. Smaller voxels contain fewer spins, resulting in lower SNR, which technologists often compensate for by using parameters that increase the total acquisition time.

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Acquisition Time Calculations

The scan time for an MRI sequence depends on the pulse sequence family and the selected scan parameters. Technologists must master the mathematical formulas for calculating acquisition times for the ARRT registry exam.

1. 2D Spin Echo (SE) and Gradient Echo (GRE) Sequence

For standard 2D sequences, the acquisition time ($T_a$) is determined by:

$T_a = \text{TR} \times N_y \times \text{NEX}$

Where:

• TR (Repetition Time): The time between successive RF excitation pulses (in seconds).

• $N_y$ (Phase Matrix): The number of phase encoding steps.

• NEX (Number of Excitations): The number of times each phase line is collected.

• Worked Example: A T1-weighted spin echo sequence is programmed with a TR of 500 ms, a phase matrix of 256, and a NEX of 2. $T_a = 0.5\text{ s} \times 256 \times 2 = 256\text{ seconds} = 4\text{ minutes and 16 seconds}$

2. Fast Spin Echo (FSE) Sequence

FSE sequences collect multiple phase encoding lines during each TR interval by applying a series of 180-degree refocusing pulses. The number of echoes collected per TR is the Echo Train Length (ETL) or Turbo Factor. The formula is:

$T_a = \frac{\text{TR} \times N_y \times \text{NEX}}{\text{ETL}}$

• Worked Example: A T2-weighted FSE sequence is programmed with a TR of 4000 ms, a phase matrix of 256, a NEX of 2, and an ETL of 16. $T_a = \frac{4.0\text{ s} \times 256 \times 2}{16} = \frac{2048}{16} = 128\text{ seconds} = 2\text{ minutes and 8 seconds}$

3. 3D Volume Acquisition

3D sequences excite an entire volume of tissue rather than individual slices. Spatial localization along the slice axis requires an additional phase encoding step, called slice encoding (or partitions). The formula is:

$T_a = \text{TR} \times N_y \times N_z \times \text{NEX}$

Where $N_z$ is the number of slice-encoding partitions.

• Worked Example: A 3D gradient echo sequence has a TR of 20 ms, a phase matrix of 256, 64 slice partitions, and a NEX of 1. $T_a = 0.020\text{ s} \times 256 \times 64 \times 1 = 327.68\text{ seconds} = 5\text{ minutes and 28 seconds}$

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Spatial Resolution Factors

Spatial resolution is defined as the ability to distinguish two separate points in tissue as distinct structures. It is determined by the size of the voxels: smaller voxels result in higher spatial resolution.

Voxel Dimensions

Voxel size is determined by three parameters:

1. Pixel Width (Frequency axis): $\text{FOV}_{\text{frequency}} \div \text{Frequency Matrix}$
2. Pixel Length (Phase axis): $\text{FOV}_{\text{phase}} \div \text{Phase Matrix}$
3. Voxel Depth (Slice axis): Slice thickness

Parameter Adjustments for High Resolution:

• Reduce FOV: Decreasing the FOV reduces the pixel size, increasing spatial resolution, but decreases SNR.
• Increase Matrix Size: Increasing matrix size (e.g., from 192 to 256 phase steps) decreases pixel size, increasing spatial resolution.
• Decrease Slice Thickness: Thinner slices reduce voxel depth, increasing spatial resolution along the z-axis and reducing partial volume averaging.

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Phase Matrix vs. Frequency Matrix Tradeoffs

Understanding how the matrix dimensions affect scan time is critical:

• Frequency Encoding Matrix: Increasing the frequency matrix size (e.g., from 256 to 512) increases spatial resolution along the readout axis but has no effect on scan time. Frequency encoding occurs during readout, which takes milliseconds.
• Phase Encoding Matrix: Increasing the phase matrix size (e.g., from 256 to 512) increases spatial resolution along the phase axis but increases scan time. Each phase encoding line requires a separate TR interval.

Rectangular FOV / Asymmetric FOV

To save scan time when imaging anatomy that is longer in one direction (e.g., spine or extremities), a rectangular FOV (rFOV) is used.

• By reducing the phase FOV (e.g., to 50% of the frequency FOV), the number of phase encoding steps ($N_y$) can be cut in half while maintaining the same pixel size (spatial resolution).
• This reduces the scan time by 50% without altering spatial resolution or causing aliasing, provided the anatomy does not exceed the borders of the reduced phase FOV.

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Advanced Acceleration Techniques

When clinical needs require short scan times (e.g., for pediatric, dyspneic, or claustrophobic patients), technologists use advanced acceleration options:

1. Fractional NEX (Partial Fourier)

• The Physics: k-space has conjugate symmetry, meaning the top half is a mirror image of the bottom half.
• The Method: The scanner collects only a fraction of the phase lines (e.g., 60% or 0.75 NEX) and mathematically calculates the remaining lines.
• Tradeoff: Scan time is reduced proportionally. However, because less total signal is collected, the SNR decreases by $\sqrt{\text{fraction collected}}$.

2. Parallel Imaging (SENSE / GRAPPA)

• The Physics: Uses spatial sensitivity profiles of multi-channel phased-array receiver coils to reconstruct the spatial origin of signals.
• The Method: The scanner skips lines of phase encoding during acquisition, reducing the number of phase lines by the acceleration factor ($R$).
• Tradeoffs:
  • Scan time is reduced by a factor of $R$.
  • SNR is reduced due to the reduction in data collection, calculated as: $\text{SNR}_{\text{Parallel}} = \frac{\text{SNR}_{\text{base}}}{g \sqrt{R}}$ where $g$ is the geometry factor of the receiver coil array.
  • If the acceleration factor ($R$) is set too high, unfolding artifacts (aliasing) can occur.

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Clinical Optimization Strategies

Technologists must select parameters based on the patient's clinical situation:

1. Motion-Prone Patients (Pediatric, Claustrophobic): Prioritize scan speed. Increase parallel imaging acceleration ($R$), increase ETL on FSE sequences, use rectangular FOV, and reduce NEX.
2. High-Resolution Needs (IACs, Pituitary): Prioritize spatial resolution. Use a small FOV, high matrix, and thin slices. Compensate for the low SNR by increasing the NEX (accepting longer scan times), using dedicated local coils, or scanning at a higher field strength ($3.0\text{ T}$).

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Resolution and Scan Time Summary Table