1.Explain the source of contrast in ultrasound images. Specifically, explain wha

Question

1.Explain the source of contrast in ultrasound images. Specifically, explain what physical property the resulting image intensty is related to. (5 points)

2.What is the purpose of ultrasound gel? Specifically, why would ultrasound imaging all but fail in the absence of ultrasound gel? (3 points)

3.Briefly explain what A-mode, B-mode, and M-mode ultrasound scans are. (7 points)

4.For a 15MHz ultrasound probe, how thick of a PZT layer is needed? And what is the approximate acoustical impedance Z of the matching layer in a single-layer system? (5 points)

5.Describe what limits the resolution in the axial and the lateral direction. Give examples for typical resolution limits for a 5MHz ultrasound system. (5 points)

6.What limits the depth of a scan? How deep can we scan with a 5MHz probe under ideal conditions when we require a signal-to-noise ratio of 110dB? How is the depth of a scan related to the pulse repetition rate? (5 points)

7.Why is there a maximum velocity (e.g., blood flow) that can be measured with pulsed Doppler ultrasound? What is the maximum flow rate that can be measured with a 5MHz scanner under an observation angle of 45 degrees and with a pulse repetition rate of 8kHz? (5 points)

Explanation / Answer

1). Contrast-enhanced ultrasound (CEUS) is the application of ultrasound contrast medium to traditional medical sonography. Ultrasound contrast agents rely on the different ways in which sound waves are reflected from interfaces between substances. This may be the surface of a small air bubble or a more complex structure. Commercially available contrast media are gas-filled microbubbles that are administered intravenously to the systemic circulation. Microbubbles have a high degree of echogenicity (the ability of an object to reflect ultrasound waves). There are a variety of microbubble contrast agents. Microbubbles differ in their shell makeup, gas core makeup, and whether or not they are targeted.

Intensity is the rate at which ultrasound energy is applied to a specific tissue location within the patient's body. It is the quantity that must be considered with respect to producing biological effects and safety. The intensity of most diagnostic ultrasound beams at the transducer surface is on the order of a few milliwatts per square centimeter.

   Intensity is related to the pressure amplitude of the individual pulses and the pulse rate. Since the pulse rate is fixed in most systems, the intensity is determined by the pulse amplitude.

The relative intensity of two pulses (I1 and I2) can be expressed in the units of decibels by:

Relative Intensity = 10 log I2/I1.

2).

Ultrasound waves don’t travel very well through air. So any little bit of air that would be between the probe and the skin would be a problem. And dry skin poses a lot of tiny pockets of air.

Ultrasound gel is a type of conductive medium that enables a tight bond between the skin and the probe or transducer, letting the waves transmit directly to the tissues beneath and to the parts that need to be imaged. It is formulated to act as a coupling agent and reduce static. In fact, the more gel, the better.

3). A-Mode, or Amplitude Modulation, is the display of amplitude spikes of different heights. It is used for opthamalolgy studes to detect finding in the optic nerve. A-Mode consists of a x and y axis, where x represents the depth and y represents the Amplitude.
B-Mode, or Brightness Modulation, is the display of 2D map of B-Mode data, and is the most common form of ultrasound imaging. Unlike A-Mode, B-Mode is based on brightness with the absence of vertical spikes. Therefore, the brightness depends upon the amplitude or intensity of the echo. There is no y axis on B-Mode, instead, there is a z axis, which represents the echo intensity or amplitude, and a x axis, which represents depth. B-Mode will display an image of large and small dots, which represent strong and weak echoes, respectively. Below is an example of B-Mode imaging of an echogenic mass in a particular organ.

M-Mode, or Motion Mode (also called Time Motion or TM-Mode), is the display of a one-dimensional image that is used for analyzing moving body parts commonly in cardiac and fetal cardiac imaging. This can be accomplished by recording the amplitude and rate of motion in real time by repeatedly measuring the distance of the object from the single transducer at a given moment. The single sound beam is transmitted and the reflected echoes are displayed as dots of varying intensities thus creating lines across the screen. Below is an TM-Mode one dimensional imaging in a pt with moderate mitral stenosis (calcific) that shows evidence of multiple echoes of the anterior mitral leaflet.

4). Its around 50 micrometre.

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