Week 11 Lecture 2 Notes: Additional Modes of Ultrasound Imaging

Additional Modes of Ultrasound Imaging

Real Time Adaptive Filtering (RTAF)

• RTAF is a post-processing technique that uses an algorithm to:

  • Reduce speckle

  • Enhance borders

• Improves contrast resolution.

• Improves imaging of scatterer reflectors.

• Looks at adjacent tissue within a frame and also tissue across several frames.

• Best used as an adjunct to compound imaging.

• RTAF and Spatial Compounding Together:

  • Reduce acoustic noise

  • Improve specular reflector continuity

  • Enhances borders and represses speckle

• Examples of RTAF implementations:

  • X-Res on Philips Epiq

  • Dynamic TCE - Siemens

  • SRI-HD GE

  • AplioPure Plus - Toshiba (Canon)

  • XRES Philips

Elastography

• Elastography measures the hardness (stiffness) of tissue.

• It helps differentiate between soft, benign lesions and hard, sinister lesions.

• It can qualify and quantify the elastic properties of a lesion.

• Young’s modulus measures the ratio of applied stress to the “strain” (original length vs change in length of tissue when stress is applied).

  • Soft tissue: small Young’s modulus (small stress results in large strain).

  • Hard tissue: large Young’s modulus (large stress results in small strain).

• Bulk Modulus = {Stress \over Strain} = {\Delta Pressure \over {\Delta Volume \div Volume}}

Types of Sonoelastography

• Strain Elastography

  • Static Compression

  • Qualitative

• Shear wave Elastography

  • Acoustic radiation force Impulse (ARFI)

  • Qualitative and Quantitative

Strain Elastography

• Direct stress is exerted by applying pressure with the transducer or tissue motion such as respiration and cardiac movement.

• STRAIN = Calculation \space of \space the \space displacement \space of \space tissues \space when \space stress \space is \space applied

• Strain elastography displays stiffness information over a grayscale image.

• Used in breast and cardiac elastography.

• Qualitative only.

• Color Coding:

  • Blue = stiff tissue

  • Green = average tissue

  • Red = soft tissue

• Strain ratio calculation with ROIs (regions of interest) to compare lesion strain to normal soft tissue.

Strain Elastography - Applications

• Breast

• Thyroid

• Prostate

• Lymph node

• Liver

• Skin

• Echocardiography

Strain Elastography - Limitations

• Provides information about relative stiffness only - more suited to focal lesions such as in breast.

• Compression sonoelastography is operator dependent - Not reproducible.

• May be subject to the eggshell effect - harder tissues on the periphery of a lesion cannot be deformed, limiting the determination of the internal strain.

• Cannot provide information about stiffness if the disease is diffused through an organ, e.g., diffuse liver disease, as no normal tissue is available for comparison.

Shear Wave Elastography

• Shear waves are generated using Acoustic Radiation Force Impulse (ARFI).

• Shear waves propagate perpendicular to the main beam at much lower speeds.

• Speckle tracking algorithm tracks displacements (strain) in tissues due to shear wave propagation.

• Shear wave Elastography is qualitative and quantitative.

• Types:

  • Point shear wave elastography

  • 2D or real-time shear wave elastography

Shear Wave Elastography - Applications

• Breast ultrasound

• Liver ultrasound:

  • Detection of small lesions

  • Evaluation of diffuse liver disease

• Prostate ultrasound

• Thyroid nodule ultrasound

• Musculoskeletal ultrasound

• Vascular

Cardiac Strain Elastography

• Uses strain produced by the myocardium instead of an external force.

• Strain and strain rate indicators of myocardial function for myocardial ischemia.

• Use 2D speckle tracking to track movements of the myocardium.

• It tracks the change in length at the reference point (end-diastole) and the current length (usually end-systole).

• It is displayed as a color map.