MRI Scan Parameters and Tradeoffs

MRI scans employ specialized sequences of radio waves, gradient pulses and imaging algorithms to generate detailed images of the human body. The precision and clarity of the generated images are influenced by modifiable MRI scan parameters to achieve optimal image quality. This article explores the key MRI scan parameters that affect MRI scan procedures.

Make sure to take a screenshot of the mobile-friendly and color-coded MRI scan parameters tradeoffs chart below.

Understanding the impact of key MRI scan parameters on image quality is crucial for healthcare providers and imaging technicians. This valuable resource caters not only to professionals but also to patients and individuals eager to broaden their knowledge of MRI technology.

What Are The MRI Scan Parameters?

MRI scan parameters are the individual protocol settings that determine the characteristics and image quality of MRI scan images. These parameters include variables such as repetition time (TR), echo time (TE), scan matrix (phase-freq), field of view (FOV), and slice thickness. Each scan parameter influences the quality, contrast, and resolution of the resulting images. The careful selection of these parameters is essential to optimize the MRI examination for specific diagnostic needs.

Patient Position and Patient Entry

Select a patient position and that matches the patients orientation with respect to the magnet. Ensure Patient Position matches the actual orientation to avoid incorrectly annotated or rotated images, impacting medical treatment. Verify the patient orientation icon aligns with the chosen entry. Use shortcuts like S for supine, P for prone, L for left lateral, R for right lateral, H for head first, and F for feet first.

Coil Selection

Allows user to select the desired MRI coil configuration for the scan protocol. MRI coils capture signals emitted by the body’s tissues during the scan, with different coils designed for specific areas of the body. This option allows the MRI technologist to enable or disable specific receivers within the coil. This can help to improve image quality and reduce likelihood of image artifact.

When selecting a coil, consider the coil’s penetration depth (approximately half its diameter); and smaller coils provide better signal-to-noise ratio (SNR) but offer less coverage and depth penetration. Important: The system requires a match between the plugged-in coil and the main coil selection in the MRI scan protocol.

Now, let’s explore a list of key MRI scan parameters and their roles in shaping the imaging process.

1. Plane

Select the desired scan plane from axial, sagittal, coronal, and oblique imaging options. Axial, sagittal, and coronal scan planes, intersect at right angles in relation to each other, making them orthogonal.

Oblique planes represent scan orientations different from axial, sagittal, and coronal planes. A simple oblique is a plane tilted in just one direction from an orthogonal plane, while a complex oblique is tilted in two directions. Oblique prescriptions may increase TR, TE, FOV, and slice thickness, while also decreasing matrix selection and the number of slices.

2. Mode

Imaging mode determines the image format and type of information gathered, for example 2D mode or 3D mode. Make a selection in the Mode text box (2D, 3D, Cine, Calib, or MR Spectroscopy). Alternatively, use shortcuts (2 for 2D, 3 for 3D, C for Cine, or M for MR Spectroscopy), which are not case-sensitive.

In 2D Mode, raw image data is acquired and reconstructed into two-dimensional images, where brightness corresponds to the MRI signal intensity from protons. A simultaneous RF pulse and gradient pulse excite a specific-sized tissue slice.

However, 3D Mode excites an entire scan volume or slab with a wide RF pulse. Spatial encoding is performed in the phase, frequency, and slice axes.

Cine Mode, an optional software package for dynamic views like the heart, utilizes retrospective gating techniques and is compatible with Vascular and Gradient Echo family pulse sequences.

Calib Mode automatically fills scan parameter fields, excluding slice thickness and start and end locations. A calibration scan, essential for ASSET-acquired images, compensates for phase wrapping or aliasing within each coil in the phased array set. This scan measures each receive coil’s sensitivity, necessary for unwrapping aliasing in the ASSET scan. It’s utilized for PURE images to generate reference maps, with two reference images acquired for every scan location: a body reference image and a surface coil reference image in the calibration file.

MRS Mode, an optional software package, serves as an image-guided Proton Brain Exam, acquiring a volume-localized, water-suppressed spectrum from single or multiple voxels or a volume of interest (VOI).

3. Pulse Seq

Pulse Sequence (Pulse Seq.) allows for customized selection of gradient and RF pulse sequences. Different pulse sequences yield varying image contrasts and resolutions, some tailored for specific tissues or structures. MRI scanners provide diverse pulse sequence selections like T1-weighted, T2-weighted, and fluid-attenuated inversion recovery (FLAIR), and more. The available Pulse Sequence Displays (PSDs) in the list are dependent on the chosen Pulse Sequence Family.

Related: MRI Protocols and Imaging Options

4. Grad Mode

Gradient Mode (Grad Mode) – Select gradient mode from Whole (for a localizer scan) or Zoom (for FOVs equal to or less than 40 cm in the X and Y direction and 35 cm in the Z direction). Gradient Mode selection is exclusive to system configurations featuring inner volume excitation scanning options, eg. twinspeed, zoom, zoomit, izoom.

Considerations for Zoom Mode: When utilizing CTL coils with Zoom mode, swapping the phase to S/I for a sagittal spine image may result in an annefact artifact. Opt for Whole Body mode when swapping phase/frequency on a sagittal image acquired with the CTL coil. Zoom mode is alternatively known as FOCUS (GE), ZOOMit (Siemens), and iZOOM (Philips).

5. Imaging Options

Imaging options optimize SNR, spatial resolution, # of slices, and reduce motion artifacts. System configuration and purchased options determine the available imaging options. These applications function as single-click PSDs, automatically completing suggested scan parameters.

Notes and Considerations: When you choose a Pulse Sequence, the Imaging Options compatible with the selected Pulse Family appear in the Imaging Options list. While selecting Imaging Options compatible with the Pulse Sequence, the system displays those available with other PSDs in the family in gray. If a selected Imaging Option excludes another option on the list, it becomes grayed out. For instance, selecting Square Pixel and ZIP x 2 would gray out No Phase Wrap and ZIP x 4 because they are incompatible with the active imaging options.

6. TE

Time to Echo (TE) reflects the time it takes for the signal to be detected after the RF pulse is turned off. TE plays a critical role in determining the contrast of the resulting images. Short TE values (in milliseconds) are used for T1-weighted images, while long TE values are used for T2-weighted images. TE may also be referred to as Echo Time.

7. TR

Repetition Time (TR) is the interval between successive excitations of a slice, spanning from one pulse sequence’s start to the next. In conventional MRI, users select a fixed value for TR. However, in cardiac-gated MRI studies, TR can vary from beat to beat based on the patient’s heart rate. Repetition time (TR) corresponds to the interval between successive radiofrequency (RF) pulses and plays a crucial role in determining image contrast. T1-weighted images use short TR values, while T2-weighted images employ long TR values.

8. TI (Inversion Time)

In MRI, inversion time (TI) refers to the duration between the center of the initial (180°) inverting pulse and the start of the subsequent (90°) refocusing pulse within an inversion recovery (IR) pulse sequence. TI plays a critical role in manipulating the contrast of acquired images by selectively nullifying signals from certain tissues based on their longitudinal magnetization properties. Optimal selection of the inversion time is crucial for enhancing image contrast in specific clinical applications, such as highlighting particular tissue characteristics or pathology.

9. Flip Angle

In MRI, the Flip Angle refers to the angle between the magnetic field and the longitudinal axis of the scanned tissue. This flip angle determines the extent of magnetic resonance, influencing the resulting image contrast. MRI scanners provide a variety of flip angles, enabling the customization of image contrast to suit various imaging applications. How does flip angle affect MRI scan procedures? See our MRI scan parameters tradeoffs chart below.

10. Echo Train Length (ETL)

Echo Train Length (ETL) in MRI refers to the number of 180° refocusing pulses executed within a single repetition time (TR) period. A longer ETL, characterized by an increased number of refocusing pulses, can enhance the signal-to-noise ratio and improve image resolution. However, this comes at the cost of a lengthier acquisition time. Conversely, a shorter ETL may reduce acquisition time but could compromise image quality. MRI technologists carefully consider and select an appropriate ETL based on the specific imaging requirements, balancing the need for optimal image quality with practical considerations such as scan time.

11. Bandwidth

Bandwidth in MRI refers to the specific range of frequencies that an MRI system is calibrated to receive. The receive bandwidth influences the number of frequencies incorporated into an image and plays a crucial role in achieving optimal image quality. The choice of system bandwidth is contingent upon your selected TE (echo time), matrix (Phase/Freq), and FOV (field of view).

The bandwidth selection provides flexibility to narrow the system’s receiver bandwidth, thereby increasing the signal-to-noise ratio (SNR). When the bandwidth is narrowed, the system becomes more selective in detecting signals from a smaller frequency range, resulting in the exclusion of more random electronic noise and contributing to improved image quality. Careful consideration of bandwidth settings is essential to achieve an optimal balance between signal strength and image fidelity in MRI scans.

12. Matrix

The matrix size refers to the quantity of pixels within the image, and it directly influences image resolution. Larger matrix sizes result in higher resolution images. MRI scanners offer a versatile selection of matrix sizes, ranging from 128 x 128 to 512 x 512. This flexibility empowers healthcare professionals to tailor image resolutions according to specific imaging applications, ensuring optimal clarity and detail in diagnostic interpretations. However, higher matrix sizes result in slower reconstruction times and increased memory storage requirements, prompting MRI technologists to strike a balance for optimal performance.

Related Article: GE MRI ZIP 512

13. Freq

The frequency (Freq) corresponds to the number of data points or samples collected along the frequency encoding direction (see below). Higher frequency values generally result in higher image resolution in the frequency-encoding direction.

14. Freq Dir

The frequency direction is the specific orientation in physical space along which the frequency encoding is performed. MRI images are created by applying magnetic gradients in different directions. The frequency direction is one of these gradient directions.

The choice of frequency direction depends on the orientation of the anatomy and the imaging goals. For example, in a standard axial brain MRI, the frequency direction might be from the patient’s head to feet.

15. Phase

In MRI, phase encoding adjusts the signal’s position using a pulse before reading imaging signals. This step is crucial for improving image quality and accurately showing body structures during scans.

16. NEX

Number of Excitations (NEX) in MRI refers to how many times a pulse sequence is repeated during a single acquisition. Increasing the NEX can enhance the signal-to-noise ratio, providing clearer images. However, it also extends the scan time. MRI technologists carefully adjust the NEX to balance the need for improved image quality with practical considerations like scan duration.

17. FOV

Field of View (FOV) in MRI is the portion of the body captured in the scan. It can be tailored to focus on a specific area or encompass a broader region. Adjusting the FOV directly impacts image quality. A larger FOV captures more tissue in the scan area, while a smaller FOV provides higher resolution images.

MRI scanners offer various FOV options ranging from 10 cm to 50 cm, enabling healthcare professionals to optimize image quality based on specific imaging needs.

18. Slice Thickness

Slice Thickness in MRI is the thickness of the individual image slices generated during the scan. Thinner slices generally yield higher resolution images but may extend scan times. MRI machines provide a variety of slice thickness options ranging from 0.5mm to 10mm, enabling doctors to select the most suitable thickness for each patient and medical condition.

19. Center

Center (Center FOV) is an adaptable field that represents the center of the MRI scan image. The Magnet Isocenter is initially set at 0,0,0 but can be adjusted in three dimensions: S/I (Superior/Inferior), R/L (Right/Left), and A/P (Anterior/Posterior). Changes made to Center, Spacing, and Center in the Graphix RX Screen directly impact the imaging center. The units are measured in millimeters.

20. Spacing

Image Spacing refers to the gap between image slices. It directly affects the maximum number of allowable images within a field of view (FOV). When image spacing is increased, the number of image slices in the FOV decreases. Measurement is in millimeters.

21. No. Slices

The system automatically generates the total number of image slices for the upcoming acquisition by dividing the scan field of view (FOV) by the image spacing. This value adjusts automatically when modifications to the protocol setup are made in the Graphic Rx Screen.

MRI Scan Parameter Trade Offs

The MRI system advises on Rx Scan Time, Maximum # of Slices, Rel SNR %, # of Acqs, and Total # of slices for scan decisions. While this provides general guidance, understanding specific MRI scan parameter trade-offs is crucial. This section details the specific MRI scan parameter trade-offs affecting the procedure and image quality.

MRI Scan Parameters Trade Off Chart

It’s important to note the relative SNR% calculated by the MRI system is a value based on the current imaging parameters. Any changes made to the protocol affecting SNR are calculated, and the relative SNR% reflects their impact.

SNR changes are not computed for TR or TE adjustments. However, significant TR and TE modifications can alter SNR. Calculations exclude TR, PSD, or coil selections. For variable Spin Echo sequences, relative SNR% is provided for TE and TE2. Make adjustments as needed based on the MRI scan parameters trade off chart provided below.

Note: The magnetic field strength of an MRI scanner is measured in Tesla (T) and is one of the most important parameters in determining image quality. Higher magnetic field strengths result in improved signal-to-noise ratio, sharper images, and shorter scan times. MRI scanners offer high magnetic field strengths, ranging from 1.5T to 7T in clinical settings.

Proton Density-Weighted (PD-weighted) images display contrast primarily influenced by the number of protons in structures. Achieved by selecting scan timing parameters that minimize T1 (long TRs) and T2 (short TEs) contrast effects.

Frequently Asked Questions

What Does T1 and T2 Mean?

T1-weighted images emphasize anatomical details by exploiting differences in the time it takes for protons to align with the magnetic field. On the other hand, T2-weighted images, reliant on the time for protons’ magnetic moments to lose coherence, highlight variations in water content. The choice between T1 and T2 imaging depends on the specific diagnostic information required, with T1 MRI images providing structural insight and T2 MRI images offering heightened detection in water-related abnormalities.

Related Article: T1 vs T2 MRI

What’s The Difference Between TE and TR in MRI?

TR, the time between consecutive pulses, and TE, the time between applying the radiofrequency pulse and signal acquisition, are key parameters influencing image outcomes. Longer TR values enhance contrast, while longer TE values boost signal-to-noise ratio. Tailoring these parameters is essential for optimizing image quality in MRI scans.

MRI Scan Parameters Overview

In order to produce the highest-quality images, it is important to set the correct MRI scan parameters during protocol setup. By understanding the parameters discussed in this article, MRI technologists can optimize their GE MRI scans and ensure that they are providing the best possible care for their patients.

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