06- Image quality parameters

Parameters and Trade-offs

  • Presented by Mr. Abdulrahman Salahudeen

Introduction

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  • Numerous parameters are available for operators when configuring a protocol.

  • Protocols should be viewed beyond just examining specific areas/pathologies (e.g., brain, tumor).

  • Definition of protocol: A ‘set of rules’ with various parameters in MRI.

  • Includes extrinsic contrast parameters, geometry parameters, and imaging options.

  • Pulse sequence choice influences image weighting and quality, impacting pathology sensitivity.

  • Timing parameters determine image weighting specifically.

Image Quality Considerations

  • Many factors determine image quality; operators must understand their interrelations for optimal quality.

  • Four main considerations affecting image quality:

    • Signal to Noise Ratio (SNR)

    • Contrast to Noise Ratio (CNR)

    • Spatial Resolution

    • Scan Time

Signal to Noise Ratio (SNR)

  • SNR measures the signal's amplitude against noise's average amplitude.

  • The signal originates from voltage induced by NMV precession in the transverse plane.

  • Noise is from random frequencies in space and generated by patient presence and background electrical noise.

  • Noise occurs randomly at all frequencies, while signal depends on factors and can be altered.

  • Increasing signal enhances SNR, while decreasing it lowers SNR.

  • Factors affecting SNR:

    • Magnetic field strength.

    • Proton density.

    • Voxel volume.

    • TR (repetition time), TE (echo time), flip angle.

    • NEX (number of excitations).

    • Receive bandwidth.

    • Coil type.

Magnetic Field Strength

  • Higher magnetic field strength increases SNR.

    • The energy gap between high- and low-energy nuclei rises, enhancing NMV size.

  • More magnetization is available for imaging, increasing SNR.

Proton Density

  • Proton density impacts signal amplitude received.

  • Low proton density areas (like lungs) yield low signal and SNR; high-density areas (like pelvis) yield high signal and SNR.

  • Proton density is inherent to tissue and remains unchanged.

Voxel Volume

  • Pixel is the building block of a digital image, representing strength of MRI signal from a voxel (volume of patient tissue).

  • Voxel determined by pixel area and slice thickness.

  • Pixel area formula: Pixel Area = FOV Dimensions / Matrix Size.

  • Coarse matrix (low freq/phase encodings) results in fewer pixels and larger voxels; fine matrix (high encodings) results in more pixels and smaller voxels.

SNR and Voxel Volume Relationship

  • SNR relates to voxel volume:

    • Smaller voxel size = lower SNR.

    • SNR affected by slice thickness, with thicker slices providing better SNR than thinner ones.

Image Matrix Impact

  • The image matrix identifies pixel count in the image (frequency & phase).

  • More pixels (fine matrix) enhance spatial resolution but reduce SNR.

Field of View (FOV) Influence

  • Larger FOV increases SNR; smaller FOV decreases spatial resolution but increases SNR.

TR, TE, and Flip Angle Effects

  • TR regulates longitudinal magnetization recovery before next excitation; longer TR = higher SNR.

  • TE controls transverse magnetization decay before echo collection; longer TE = decreased SNR.

  • Lower flip angles result in lower SNR due to reduced transverse magnetization.

Coil Type and SNR

  • Coil type significantly impacts SNR.

    • Quadrature coils enhance SNR; phased array coils further maximize it.

    • Surface coils close to the area being examined also improve SNR.

    • Coil positioning is crucial for optimizing SNR.

Optimizing Image Quality

  • Strategies for SNR enhancement:

    • Use spin echo pulse sequences where applicable.

    • Avoid very short TRs and very long TEs.

    • Apply appropriate coil positioning.

    • Utilize coarse matrices, large FOVs, thick slices, and maximize NEX.

Contrast to Noise Ratio (CNR)

  • CNR measures the signal difference between adjacent areas, influenced by SNR factors.

  • CNR critical for image quality affects visibility of high versus low signal areas.

  • CNR can be enhanced by:

    • Utilizing T2-weighted images.

    • Administering contrast agents.

    • Chemical pre-saturation techniques.

    • Magnetization transfer contrast (MTC).

Spatial Resolution

  • Spatial resolution: ability to distinguish two points as separate.

  • Affected by voxel size, influenced by:

    • Slice thickness (thinner = better resolution).

    • FOV (larger = larger pixels, smaller = better resolution).

    • Number of pixels/matrix size (larger = better resolution).

Summary of Key Points

  • To improve image quality:

    • Optimize SNR: long TR, short TE, correct coil, coarse matrix, thick slices, high NEX.

    • Optimizing CNR enhances distinguishability between tissues.

    • Improve spatial resolution through thinner slices, fine matrices, and smaller FOVs.

  • Minimize scan time to prevent motion artifacts by using shorter TRs and coarser matrices.