Frederick W. Kremkau

Physical principles of ultrasound

U LTRASOUND has characteristics that enable it to yield information concerning internal body structures. Let us first describe what ultrasound is and then consider those characteristics that make it useful in sonography. Then we will discuss the means by which ultrasound is generated and received. A list of definitions is presented in the Glossary for ready reference. Greater detail may be found in the References.le3

Ultrasonic enhancement of nitrogen mustard cytotoxicity in mouse leukemia.

Mouse leukemia L1210 cells were exposed to continuous wave 2 MHz, 10 W/cm2 ultrasound for 10 minutes while suspended in nitrogen mustard solution in vitro. Mice subsequently inoculated with these cells had longer survival times than control animals that received cells exposed to the drug but not ultrasound. Without the drug, ultrasound did not alter survival time. Tracer studies revealed increased cellular accumulation of drug under the influence of ultrasound.

Peak velocity overestimation and linear-array spectral Doppler.

Ultrasound Instruments are used to evaluate blood flow velocities in the human body. Most clinical instruments perform velocity calculations based on the Doppler principle and measure the frequency shift of a reflected ultrasound beam. Doppler‐only instruments use single‐frequency, s1ngle‐crystal transducers. Linearand annular‐array multiple‐crystal transducers are used for duplex scanning (simultaneous B‐mode image and Doppler). Clinical interpretation relies primarily on determination of peak velocities or frequency shifts as identified by the Doppler spectrum. Understanding of the validity of these measurements is important for Instruments in clnical use. The present study exammed the accuracy with which several ultrasound instruments could estimate velocities based on the Identification of the peak of the Doppler spectrum, across a range of different angles of insonation, on a Doppler string phantom. The stnng was running in a water tank at constant speeds of 50, 100, and 150 em/sec and also in a sine wave pattern at 100ߚ or 150ߚcm/sec amplitude. Angles of msonatlon were 30, 45, 60, and 70 degrees. The single‐frequency, single‐crystal transducers (PC Dop 842, 2‐M Hz pulsed‐wave, 4‐MHz continuous‐wave) provided acceptably accurate velocity estimates at all tested velocities independent of the angle of insonation. All duplex Doppler instruments with linear‐array transducers (Philips P700, 5.0‐MHz; Hewlett‐Packard Sonos 1000, 7.5‐MHz; ATL Ultramark 9 HDI, 7 5‐MHz) exhibited a consistent overestimation of the true flow velocity due to mcreasing intrinsic spectral broadening with increasing angle of insonation. At an angle of insonation of 60 degrees the peak of the Doppler spectrum overestimated the true flow velocity by 25% and at 70 degrees, by 33%. The most likely explanation of this phenomenon is the wide aperture of these probes, with potential angle diversity of the reflected ultrasound beams along the surface area (footprint) of the probe. The use of an Intensity‐weighted peak velocity (centroid), with calculation of the velocity which includes 95% of the spectral points, rather than the absolute peak of the spectrum, proved to be more accurate and was angle Independent in these Instruments.

Color velocity imaging: introduction to a new ultrasound technology.

Noninvasive ultrasound is the preferred methodology for the initial evaluation of carotid atherosclerosis. Since the early use of continuous‐wave Doppler to assess carotid artery flow velocity blindly, neurosonology has progressed through crude B‐mode imaging, spectral analysis of the Doppler signal, and gray‐scale duplex Doppler/B‐mode imaging, to color‐flow Doppler duplex imaging. The latter allows color coding of Doppler data based on the velocity of blood flow. The combination of color‐flow Doppler with gray‐scale B‐mode imaging allows simultaneous visual display of anatomical and hemodynamic information. Physical limitations of color‐flow duplex Doppler imaging may affect the clinical utility of these techniques. Problems with pulse repetition frequency, aliasing, resolution capability of the color data, and interpolation of data make some applications difficult. Color velocity imaging uses the data contained in the gray‐scale B‐mode image scan lines to determine velocity of blood flow, and it offers potential advantages over conventional color‐flow duplex Doppler for the assessment of carotid atherosclerosis and hemodynamics. Initial comparison of spectral Doppler and color velocity imaging data suggests that the latter is an accurate method to assess blood flow velocity. Understanding of the validity, utility, and prognostic advantages offered by color velocity imaging awaits careful prospective clinical studies.

Ultrasonic enhancement of cancer radiotherapy.

Ultrasound treatment (1.5 W/cm2, 1.9 MHz, C.W.) for 15 minutes either before or after x irradiation reduced the TCD50 of sarcoma-180 by approximately 40% while similar ultrasound treatment for up to 30 minutes did not reduce the TCD50 of the C3HBA mammary adenocarcinoma. Water bath heating (44.5 degrees C for 15 minutes) after x irradiation reduced the TCD50 of both tumors. Ultrasound alone for up to 30 minutes had no effect on growth or cure rate of either tumor.

AIUM Standard for the Performance of an Ultrasound Examination of the Abdomen or Retroperitoneum

The American Institute of Ultrasound in Medicine (AIUM) is an educational, scientific, and professional society concerned with the advancement of the art and science of ultrasound in medicine and research. To promote this mission, the AIUM is pleased to publish, in conjunction with the American College of Radiology (ACR), the updated Standard for Performance of an Ultrasound Examination of the Abdomen or Retroperitoneum. We are indebted to the many volunteers who contributed their time, knowledge, and energy to bringing this document to completion.

Mirror-image artifact can affect transcranial Doppler interpretation.

Transcranial Doppler ultrasonography (TCD) allows evaluation of blood‐flow velocity in intracranial arteries detection and monitoring of vasospasm in patients with subarachnoid hemorrhage. Spectral Doppler artifacts can affect TCD data. A 1‐month series of TCD findings showed marked fluctuation in blood‐flow velocity values in both the middle and anterior cerebral arteries of a patient with subarachnoid hemorrhage. A mirror‐image artifact of the Doppler fast Fourier transform velocity spectrum resulted in erroneous interpretation of higher flow velocity in certain vessels. This artifact may cause misinterpretation of TCD flow‐velocity data and lead to improper diagnosis of the condition and treatment of patients.

Diminishing Exposure to Doppler Ultrasound The Sonographer's Role

Diagnostic ultrasound has an unblemished record for safety. The introduction of pulsed Doppler may involve higher intensities exceeding 1 W/cm2 (SPTA). These intensities must cause concern for fetal exposure, and the FDA has introduced strict guidelines to limit such exposure. Presented here are practical ways in which sonography can minimize exposure. These include the use of Doppler only when clinically indicated or part of an approved research protocol, knowledge of the emitted intensity, and how to attenuate this to prudent levels. It is desirable to use sensitive equipment and increase the TGC before increasing power. Such prudent use will allow ultrasound to be used in these new applications without hazard.

Principles of Spectral Doppler

Spectral-Doppler ultrasound provides a quantitative visual presentation of blood-flow information. The spectral display is a real-time presentation of Doppler shift vs. time on the vertical and horizontal display axes, respectively. The vertical axis is normally calibrated in flow-speed units by solving the Doppler equation with Doppler angle incorporation (by the instrument operator). The spectral display provides flow information (presence, direction, speed, character) at the site of observation (sample volume location). Information regarding proximal and distal flow conditions can also be derived from the spectral display. Pulsed wave Doppler systems provide the ability to select the depth from which Doppler information is received. Spectral analysis provides visual information on the distribution (spectrum) of Doppler-shift frequencies resulting from the distribution of scatterer velocities (speeds and directions) encountered within the sample volume. The spectrum is derived electronically using the fast Fourier transform and is presented on the display as Doppler shift versus time, with brightness or color indicating the strength of the Doppler shifts. Flow conditions at the site of measurement are indicated by the (vertical) width of the spectrum, with spectral broadening indicative of disturbed and turbulent flow. Flow conditions downstream, especially distal flow impedance, are indicated by the relationship between peak systolic and end-diastolic flow speeds. Proximal stenosis is indicated by the tardusparvus waveform. Aliasing displaces systolic Doppler shifts to the wrong side of the baseline when the Nyquist limit is exceeded. Aliasing is corrected by baseline shifting and, in extreme cases, scale change, which increases the sampling rate (PRF).

The New Paradigm for Understanding, Teaching, and Testing Sonographic Principles

Two alternative fundamental principles of operation are present in the array of sonographic equipment commercially available currently. Principle 1 has been the operating principle for over 50 years. Recently, Principle 2 has appeared. In Principle 1, there is a one-to-one correspondence between the echo stream from an emitted ultrasound pulse and its displayed scan line, ie, physical beam-forming is directly coupled with displayed scan lines. In Principle 2, fewer pulses are required and focusing is not necessary, and yet the entire image is in focus (ie, excellent detail resolution) and with higher frame rates (improved temporal resolution). This “virtual beam-forming” is accomplished through massive, parallel, high-speed computational postprocessing. The resulting images are similar to what extremely thin, laser-like physical ultrasound beams would produce. However, such beams cannot be physically produced in the frequency range required for imaging depths appropriate for human anatomic imaging. Virtual beam-forming significantly improves nearly every aspect of sonographic, anatomic imaging and Doppler motion detection and presentation.