Acoustic radiation force impulse imaging

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Acoustic Radiation Force Impulse (ARFI) Imaging is a type of ultrasound elastography used in medicine, particularly for the diagnosis and monitoring of cancers. ARFI imaging uses acoustic radiation force to generate images of the mechanical properties of soft tissue.

How it works[edit]

Acoustic radiation force is a phenomenon associated with the propagation of acoustic waves in attenuating media. Attenuation includes both scattering and absorption of the acoustic wave. Attenuation is a frequency dependent phenomenon, and in soft tissues it is dominated by absorption. With increasing acoustic frequencies, the tissue does not respond fast enough to the transitions between positive and negative pressures, thus its motion becomes out of phase with the acoustic wave, and energy is deposited into the tissue. This energy results in a momentum transfer in the direction of wave propagation and tissue heating. The momentum transfer generates a force that causes displacement of the tissue, and the time scale of this response is much slower than that of the ultrasonic wave propagation. This displacement (typically a few micrometers), which is typically detected by computing the correlation of ultrasonic RF signal, can be used to derive additional information about the tissue beyond what is normally provided in an ultrasonic image. The magnitude, location, spatial extent, and duration of acoustic radiation force can be controlled to interrogate the mechanical properties of the tissue.

Acoustic Radiation Force Impulse vs. FibroScan[edit]

ARFI technology is very useful for distinction between malignant/benign tumors. However, FibroScan is more valid and accurate in detecting and quantifying liver fibrosis for various chronic liver diseases.[1] These are the details of their differences:

Method[edit]

  • FibroScan: Uses VCTE (Vibration Controlled Transient Elastography) with controlled parameters.
  • ARFI: A high-frequency ultrasound is used in ARFI technology. This is primarily used for qualitative focal assessment of deep tissues.

Parameters[edit]

  • FibroScan: Shear wave frequency at 50 Hz sent in the liver. FibroScanhas a scoring card to compare liver stiffness measurements
  • ARFI: measures shear wave velocity and results, expressed in m/s=Vs. ARFI results cannot be used to compare with validated liver stiffness measurements and cut-off values expressed in kPa by FibroScan®.

Method development[edit]

  • FibroScan: The VCTE algorithm for stiffness measurement has never been changed since 2003. Result: allows accurate longitudinal follow-up of patients.
  • ARFI: the algorithm is still under development with numerous updates. Result: follow-up of patients might be difficult.

Validation[edit]

  • FibroScan: has been validated by numerous independent and peer reviewed studies (more than 700 original publications). FibroScan has been listed in international Hepatology Guidelines in many countries such as France, Spain, Canada, EASL, China.
  • ARFI: ARFI technology has not yet been validated, with only 45 papers available. ARFI has no recommendation for use in any guidelines.

Reproducibility[edit]

  • FibroScan: The frequency, gain and depth are fixed. The operator has no control over these parameters. Result: the examination is device and operator-independent. Results are reproducible.
  • ARFI: The operator chooses the place and depth, then the image is frozen for calculation. Frequencies of shear waves are different according to the region of the measurement. Result: It is operator-dependent. The examination is not reproducible because the results may be biased, therefore less reliable.

Region of interest[edit]

  • FibroScan: About 3cm3. Result: larger measurement
  • ARFI: About 0.5cm x 1cm. Result: smaller measurement

Shear wave frequency: a critical point to be taken into account since it is closely related to stiffness results[edit]

  • FibroScan: VCTE uses a controlled and fixed low shear wave frequency at 50 Hz. Result: Examinations are reliable and reproducible, no matter which device or probe is being used.
  • ARFI: this device, as well as devices by other manufacturers, do not control the frequency of shear waves. Large bandwidth of frequencies is used. Result: Reproducibility of examinations is affected; therefore results may not be reliable. The higher the shear wave frequency, the higher the stiffness results.

Steatosis evaluation[edit]

  • FibroScan: With installation of CAP Module on FibroScan®, it is now possible to detect and quantify ultrasound attenuation (correlated to fatty liver/steatosis) at the same time as liver stiffness, using the same machine and same probe (M probe only)
  • ARFI: Steatosis evaluation is not available with ARFI.

Generation of Shear Waves[edit]

  • FibroScan: VCTE technology artificially creates shear wave with a small mechanical impulse on the skin of the patient by applying a dedicated probe positioned in front of the right lobe of the liver. The measurement depths are fixed. They depend on the probe used: Depth of 25-65 mm for medium probe, depth of 35-75 mm for XL probe.
  • ARFI: Shear waves are generated into the liver using radiation pressure by a high intensity ultrasound beam from a probe. With highly focused acoustic beams being sent into the liver to generate shear waves, there is a possibility of an increase in temperature. Result: increase in temperature can be dangerous to the patient and slow down the rhythm of the exam.[2]

Influence of BMI[edit]

  • FibroScan: Specific patient body types have designated probes. FibroScan® XL probe should be used as a first line on obese patients.
  • ARFI: ARFI performances are affected by the BMI of patients. Result: There is a significant decrease in ARFI performance with obese patients.

Dedication to liver[edit]

  • FibroScan: a bedside tool fully dedicated to liver diagnosis.
  • ARFI: ARFI and all other available devices are not primary dedicated to liver assessment. They are basically ultrasound devices with an add-on tool for liver fibrosis detection.

Clinical applications[edit]

Clinical applications of ARFI imaging include:

Liver fibrosis quantification[edit]

The speed at which shear waves propagate in tissue can be used to quantify the shear modulus of the tissue. Acoustic radiation force-based imaging modalities are being studied to non-invasively characterize the liver without the need for liver biopsy.[3]

Liver Fibrosis Quantification efforts at Duke University

Acoustic Radiation Force Impulse Imaging (ARFI): a New Technique to Assess Liver Elasticity

Yoneda et al. also recently compared ARFI shear wave imaging as implemented on the Siemens Acuson S2000 with transient elastography using the FibroScan system (EchoSens, Paris, France) in the context of evaluating patients with Non-alcoholic Fatty Liver Disease (NAFLD).[4]

Breast mass imaging[edit]

Focused acoustic waves that propagate through tissue are absorbed and generate radiation force. Acoustic radiation force results from a transfer of momentum from an acoustic wave to tissue in the direction of wave propagation, arising from absorption and scattering of the wave. The tissue displacement response to radiation force excitation occurs on a slower time-scale than ultrasonic wave propagation, thus, conventional ultrasonic methods can be used to monitor the tissue response to radiation force. For breast imaging, experiments have been conducted to measure the potential for ARFI images to provide adjunctive information to matched ultrasonic images in order to enhance clinical confidence in diagnosis of breast masses. Patients scheduled for core biopsy of breast masses are recruited and matched B-mode and ARFI images of masses were obtained. The images were then correlated with biopsy result.

Colorectal tumor imaging/staging[edit]

Colorectal cancer is the second leading cause of cancer death in the United States (after lung cancer), and the third most common cancer overall in men (after prostate and lung cancer) and women (after breast and lung cancer). Once identified, the treatment approach for rectal cancer is dictated by the stage of the tumor (T-stage) and local lymph nodes (N-stage). There are currently no imaging methods that provide reliable N-staging accuracy of colorectal cancers. While endorectal ultrasound (EUS) is the standard for staging the degree of wall invasion of rectal cancers (T-stage), its accuracy is poor in the critical determination between uT2 and uT3, with 10-35% of uT2 tumors being overstaged. The consequences for the patient are dramatic. A stage uT3 rectal carcinoma is treated by radical surgical resection, and is typically treated with neoadjuvant chemotherapy and radiation. Neoadjuvant treatment damages adjacent healthy tissues, and can impair the healing process following surgery, leading to increased complications of infection, bleeding, decreased colonic mobility, and incontinence. Whereas stage uT2, N- and lower tumors generally receive a transanal local excision and are not treated neoadjuvantly, eliminating these additional risks to the patient.

The objective of ARFI Imaging development is to develop and evaluate the ability of ARFI imaging techniques to image layered tissue structures as found in the gastrointestinal tract, and to guide treatment decisions through improved preoperative tumor and lymph node staging.

Prostate imaging[edit]

Prostate cancer is the most common cancer and the second leading cause of cancer death in American men. According to the American Cancer Society (ACS), an estimated 218,890 new cases of prostate cancer will be found and 27,050 men will die of this disease in 2007. Early diagnosis is essential for better treatment and increasing survival rate. Although the current screening techniques, antigen (PSA) blood testing and digital rectal examination (DRE) are considered sensitive enough for cancer screening, follow-up biopsies have significant shortcomings. Without a good imaging technique to target the needle biopsy in the prostate gland, only about 25% of tests are positive for cancer in more than 1 million prostate biopsies performed each year; the false negative rates range from 25-45% based on the first time biopsy.

Although imaging techniques are essential for cancer diagnosis, imaging the structures and lesions within prostates has been a challenging task. The goal of ARFI imaging is to optimize and evaluate ARFI techniques for prostate imaging, which will potentially provide a more accurate, low cost and patient friendly imaging guidance for targeting prostate biopsies.

In vivo imaging of malignant tumors[5][edit]

External links[edit]

References[edit]

  1. ^ http://link.springer.com/article/10.1007%2Fs10396-012-0357-8
  2. ^ http://link.springer.com/article/10.1007%2Fs10396-012-0357-8
  3. ^ "Quantifying hepatic shear modulus in vivo using acoustic radiation force". Ultrasound Med Biol 34 (4): 546–58. April 2008. doi:10.1016/j.ultrasmedbio.2007.10.009. PMC 2362504. PMID 18222031. 
  4. ^ http://radiology.rsna.org/content/256/2/640.abstract
  5. ^ Fahey, B.J. et al. "In vivo visualization of abdominal malignancies with acoustic radiation force elastography", Phys Med Biol, 53(1), 279-93, 2008.