Ultrasound Elastography: BMI-4202 Lecture III

• Ultrasound Elastography is Lecture III of BMI-4202 Ultrasonography II.
• Presented by lecturer Raynell Gordon

Objectives

At the end of this lecture, students should be able to:

• Define ultrasound elastography and explain its principles as a technique.
• Identify and describe the various methods and techniques employed in ultrasound elastography.
• Analyze ultrasound elastography's clinical significance and diverse applications in diagnosing medical conditions.
• State the advantages and disadvantages of ultrasound elastography.

Clinical Examination: Palpation

• Palpation is a method of patient assessment.
• It includes inspection, percussion, and auscultation.
• It involves tactile examination of a patient using fingers or hands.
• Provides information on the size, shape, consistency (soft, firm, or hard), texture, location, and tenderness of an organ or body part.
• Palpation can be deep or superficial.
• Generally applied to the abdomen or chest.

Common Palpation Findings

• Rigidity presents as an involuntary contraction of the abdominal muscles that results in feeling hard or rigid.
• Guarding presents as a voluntary contraction of muscles due to fear of pain on palpation.
• Rebound tenderness indicates appendictis.

Palpation In Ultrasound Sono-palpation

• Sono-palpation is the combination of ultrasound imaging and physical examination.
• It is used to localize pain and correlate it to a specific visualized anatomic structure.
• It can achieve higher diagnostic accuracy
• Provides information on the compressibility of an image.
• Sono-palpation is also used for ductal carcinomas in situ, soft tissue ganglions, and acutely inflamed appendix.
• Additional clinical information from sono-palpation includes rebound tenderness and Murphy's sign.
• Sono-palpation is limited by the patient's BMI and image location.
• Facilitates the construction of morphological images of the organ.
• Does not provide information regarding tissue consistency (elastic properties).
• Changes in tissue consistency can be a pathological or normal physiological process.
• Examples are hepatic and renal fibrosis and breast cancer, and aging.

What Is Ultrasound Elastography?

• Ultrasound elastography uses sound waves to access tissue's mechanical properties (stiffness and elasticity).
• It assesses the tendency of tissue to resist deformation when an external force is applied or to return to its original shape if the force is removed.
• Two approaches used are shear wave/transient elastography and strain/compression/static elastography.
• Detects pathological changes based on a tissue's stiffness and elasticity.

Elastic Modulus

• Elastic Modulus measures the resistance of a material to elastic deformation.
• It is calculated using a formula.
• Low-Modulus materials are floppy and stretch a lot (ex: rubber).
• Of importance to U/S Elastography is Young's modulus.
• Elastic modulus is the stress (force applied to a cross-sectional area) for a certain material divided by the strain (deformation of the material).
• A measure of the hardness/softness of a material
• E = Young's modulus σ = uniaxial; ɛ = strain.
• Soft materials have a small Young's modulus where a given stress results in a large strain.
• Hard or stiff materials have a large Young's modulus, and a given stress results in a little strain.

Ultrasound Wave Propagation

• There are two types of wave propagation: Longitudinal and Shear
• In longitudinal waves, wave motion is parallel to the direction of the propagation.
• Longitudinal waves are used in B-mode ultrasounds.
• In shear waves, wave motion is perpendicular to the direction of wave propagation.
• Bulk's modulus defines longitudinal wave propagation.
• The speed of ultrasound in soft tissue is approximately 1540m/s with little variation between tissues.
• Due to this little variation between tissue, is not enough difference in tissue contrast to allow elastography measurements.
• Changes in tissue consistency occur as a pathological or normal physiological process.
• Examples are hepatic and renal fibrosis and breast cancer, and aging.
• Shear wave speed (Cs) is 1 – 10 m/s in soft tissue
• G is Shear/Rigidity Modulus
• The low wave speed facilitates high differences in G between tissue providing adequate contrast for elastography measurements.
• Young modulus and Shear's Modulus are related by E = 3G = 3pCs2

Non-Invasive Ultrasound Elastography

• Elastography is a non-invasive sonographic imaging technique used clinically to assess the stiffness or elasticity of soft tissues in the body.
• Enables the differentiation between normal and diseased tissue.
• Used in diagnosing conditions involving fibrosis, breast cancer, thyroid nodules, and certain musculoskeletal disorders, and monitoring the treatment response.
• Elastography provides additional information beyond traditional ultrasound imaging to improve sonographic diagnosis.
• To assess tissue's Young's modulus, U/S elastography relies on the application of an external force to the study area.
• The external force is classified according to the means of excitation. It can be the static method (aka quasi-static methods) and the dynamic methods.

Quasi-Static Methods

• Quasi-Static Methods in Ultrasound Elastography are also referred to as strain imaging.
• In this approach, stress/compression is applied to a tissue, and the strain is measured.
• Measured induced strain by comparing the reference and compressed images.
• It was the first elastography technique developed.
• Provides a qualitative assessment of Young's modulus.
• There are two approaches/techniques: Strain elastography and Acoustic Radiation Force Impulse Imaging (ARFI) strain imaging.
• Strain Elastography is further subdivided according to the method of excitation

Transducer Palpation

• The operator exerts manual pressure with the transducer.
• Appropriate for superficial organs, such as breasts and thyroids.
• Challenging to assess the elasticity of deeper organs, for example, the liver.
• Ultrasound probe is held steady, and tissue displacement is generated by internal physiological motion -cardiovascular and respiratory motion.
• The induced tissue displacement in the same direction as the applied stress is then measured.
• The manual or physiologically applied stress is not quantifiable, but it provides a qualitative assessment of Young's modulus and, thus, tissue elasticity.
• The strain measurement is overlaid on a B-mode as an elastogram.

Elastogram Color Associations

• Low-strain/stiff tissue is displayed in blue.
• High-strain/soft tissue is displayed in red.
• A pseudo-quantitative measurement – strain ratio – can be used to assess tissue elasticity.
• Strain ratio is the ratio of the strain measured in adjacent tissue to the stain measured in the target lesion.
• A strain ratio of >1 means the target lesion compresses less than the tissue, indicating lower strain and greater stiffness.

Acoustic Radiation Force Impulse

• Acoustic Radiation Force Impulse is an alternative approach for measuring strain.
• A short duration = 0.1 – 0.5msec, high-intensity.
• Push pulse is used to displace tissue (approx. 10 – 20 micrometers).
• Displacement within the region is measured by same methods as in strain elastography.
• An elastogram overlaid on a B-mode image is also displayed.

Dynamic Elastography/Shear Wave Imaging

• A time-varying force is applied to the region/tissue.
• The time-varying force can be mechanical force with mixed frequencies.
• Shear waves are generated and are related to the tissue's Shear modulus.
• The Shear modulus results in quantitative and qualitative assessment of tissue elasticity.
• Produces higher resolution Young's modulus maps compared to quasi-static methods.
• Three approaches include dimensional transient, point shear wave and dimensional shear wave.

Dynamic Elastography: 1-Dimension Transient

• This is a widely used and validated technique for assessing liver fibrosis Shear waves are generated by the mechanical vibrating device.
• Probes contain both a transducer and a mechanical vibrating device.
• No direct b-mode image guidance; the operator uses time motion ultrasound to locate a portion 2.5-6.5 below the skin surface from vascular structures.
• Shear waves are generated by a mechanical vibrating device.
• The device exerts a controlled vibrating external punch on body's surface.
• The probe uses A-mode ultrasound to measure shear waves speed, and Young's modulus is calculated
• The tissue volume is approximately 1cm wide x 4cm long, 100 times larger than the average biopsy sample volume.
• Repeated measurements are taken for validation purposes meet the following criteria.
  • A minimum of 10 valid measurements
  • The ratio of valid measurement to the total number of measurements is >60%.
  • Interquartile range reflects the variability of measurements, <30% of the median value of the liver stiffness measurement.

Dynamic Elastography: Point Shear Wave

• In this technique, ARFI induces tissue displacement in the normal direction in a single focal location.
• Shear waves are generated is directly reported and converted to Young's modulus.
• This provides with quantitative estimate of tissue elasticity.
• Can be performed using a conventional ultrasound machine and a standard probe.
• Has two advantages. The operator uses the b-mode to visualise the organ and their generated waves are often unaffected with obesity problems.

Dynamic Elastography: 2D Shear Wave

• Dynamic Shear Wave Elastography uses ARFI, in multiple location instead of focal.
• This generates cylindrical shear which allows real-monitoring to measure wave speed.
• This creates quantitative elastogram for b-mode image that tissue stiffness.

Limitations of Ultrasound Elastography

• Technical confounders are the limitations of ultrasound: reverberation, acoustic shadowing, clutter artifacts, and operator dependency.
• The reliance of theories assumes elasticity by tissue, and Linearity resulting linear strain as incremental function with non-deformed quilibrium non-deformity. It is also assumed the Isotropic symmetry that responds similar to any direction by tissue.
• These assumptions do not hold in all clinical scenarios.
• These assumptions also violate conventional models.
• Models describe soft properties as heterogenous materials/viscous responses when probed.
• Is often effected by attenuation, fluids or sub-fat affecting measurements.
• It is also susceptible to internal respiratory motion of the cardiacs.