Question

2) For the following examination types, what mode(s) and probe (type and frequency range) should be used? Explain why:

a. Thyroid

b. Gall bladder

c. Echocardiogram

Answer 1

Thyroid - High-resolution ultrasonography (USG) is the most sensitive imaging modality available for examination of the thyroid gland and associated abnormalities. The major limitation of ultrasound in thyroid imaging is that it cannot determine thyroid function, i.e., whether the thyroid gland is underactive, overactive or normal in function; for which a blood test or radioactive isotope uptake test is generally required.

.A probe that contains a piezoelectric crystal called a transducer is applied to the neck but since air does not transmit ultrasound, it must be coupled to the skin with a liquid medium or a gel. This instrument rapidly alternates as the generator of the ultrasound and the receiver of the signal that has been reflected by internal tissues. The signal is organized electronically into numerous shades of gray and is processed electronically to produce an image instantaneously.  Two-dimensional images have been standard and 3-dimentional images are an improvement in certain circumstances.

The depth penetration and resolving power of ultrasound depends greatly on frequency. Depth penetration is inversely related and spatial resolution is directly related to the frequency of the ultrasound. For thyroid, a frequency of 7.5 to 10 - 15 megahertz is generally optimal for all but the largest goiters. Using these frequencies, nodules as small as two to three millimeters can be identified.

Echocardiogram - Ultrasound has a frequency above the range audible by humans (ie, >20,000 Hz). For adult cardiac imaging, ultrasound waves in the range of 4–7 MHz are used (intravascular ultrasound uses frequencies as high as 30 MHz). These are created within the ultrasound probe by striking piezo-electric crystals with an electric pulse, which stimulates the crystals to release sound waves. The central principle of ultrasound imaging is that, while most waves are absorbed by the body, those at interfaces between different tissue densities are reflected. In addition to emitting the ultrasound waves, the transducer detects the returning waves, processes the information, and displays it as characteristic images. Higher frequency ultrasound waves increase resolution, but decrease tissue penetration.

The principal modes of ultrasound used in echocardiography are

1.2-D or 2 dimensional mode - This is the default mode that comes on when any ultrasound / echo machine is turned on. It is a 2 dimensional cross sectional view of the underlying structures and is made up of numerous B-mode (brightness mode) scan lines. The field of view is the portion of the organs or tissues that are intersected by the scanning plane. Depending on the probe used, the shape of this field could be a sector – commonly seen with Echo and abdominal ultrasound probes or rectangular or trapezoid – seen with superficial or vascular probes.The main uses for 2-D mode are to measure cardiac chamber dimensions, assess valvular structure and function, estimate global and segmental ventricular systolic function and improve accuracy of interpretation of Doppler modalities.
2.M-mode or motion mode - This represents movement of structures over time. Initially a 2-D image is acquired and a single scan line is placed along the area of interest. The M-mode has good temporal resolution, so it is useful in detecting and recording rapid movements. We can also correlate and time events with ECG or respiratory pressure waveforms traced alongside the M-mode tracings. The M-mode is commonly used for measuring chamber dimensions and calculating fractional shortening and ejection fraction.
3.Colour flow doppler imaging - In this mode, the velocity and direction of blood flows are depicted in a color map superimposed on the 2-D image.It uses pulse wave Doppler signals to derive this image. This is usually done with lower frequency ultrasound waves and hence the resolution of the 2-D image deteriorates in this mode. As it takes many pulses in each scan line to derive the color image, the frame rate is reduced compared to 2-D mode. Reducing the depth and size of the color box and reducing the scanning sector width can compensate for this.

Although it can be changed, by convention, blood flowing away from the probe is depicted in blue and that flowing toward the probe in red. (BART: blue away, red toward). Blood flowing perpendicular to the scanning plane will appear black. Areas of turbulent flow may be depicted in green or white.

4.Pulse wave doppler - This is a pulsed Doppler technique in which the Doppler signal arising from a specific position in the scanned tissue is analyzed to depict velocity and direction of flow.The transducer crystal transmits the ultrasound and receives it after a preset delay. This allows it to precisely localize the site of origin of a velocity signal. when the Doppler shift being measured exceeds one-half of the pulse repetition frequency (PRF). At usual settings, this is seen to happen above a velocity of 2m/sec.

5.Continuous wave doppler - In this mode, a part of the transducer is continuously transmitting and a part of the transducer is continuously receiving the Doppler signal along a single line that is placed on the 2-D image. This method gives very good resolution of high velocities, but it does not give any information about the location of the signal, which may originate anywhere along the preset line of the ultrasound beam. As it measures velocities along the entire line, there will be a range of RBC velocities and the Doppler waveform is normally filled-in in contrast to the PWD.

6.Tissue doppler - This mode is similar to the Pulsed Wave Doppler except that it is used to measure velocities of tissue movement, which are much lower than blood velocities. The cursor or sample volume is placed on the 2-D image over the tissue of interest and the Doppler waveforms are acquired. The machine filters out the high velocities and displays a waveform that is very similar in appearance to the PWD waveform.

Frequency - . The sound waves used to obtain cardiac images in echocardiography are ultrasound waves that range in frequency from 2-12 MHz. As a result, adults are usually imaged using a 2-4 MHz transducer, while pediatric patients are imaged using a 7-12 MHz transducer.