Ultrasound In Radiology

Ultrasound machine
Ultrasound Machine.

Sound is propagated through a medium (e.g. air) as a mechanical vibration of the particles of that medium and in simple terms may be categorised by its loudness and pitch or frequency. “Ultra” means beyond, ultrasound is sound with a frequency beyond that of human perception (i.e. >20 kHz), and has the same physical properties as “audio” sound. Most clinical diagnostic applications of ultrasound employ frequencies in the range 2 - 10 MHz.
Ultrasonic energy travels through a medium in the form of a wave. Although a number of different wave modes are possible, in almost all diagnostic applications, ultrasound propagates in the form of a longitudinal wave, where the particles of the medium oscillate in the direction of propagation of the sound. Energy is transferred through the medium in a direction parallel to that of the oscillations of the particles. The particles themselves do not move through the medium. They simply vibrate to and fro about their mean position.

It is often useful to think of the source of ultrasound, the transducer, as a vibrating piston. As it moves it displaces the adjacent particles of the adjoining medium. These in turn displace more particles throughout the medium. Since the particles are not rigidly fixed to each other, they do not all move together. There is a delay between the movement of adjacent particles (analogous to a series of balls connected by springs). 

At a particular time there will be some regions where the particles are closer together and the pressure and density of the medium is increased (regions of compression) and areas where the particles are further apart and the pressure and density of the medium is decreased (regions of rarefaction). These regions of compression or rarefaction move through the medium as a wave.

Practical measures for the reduction of patient radiation dose

Radiation Sign. Please Take care!
Radiation Sign. Please Take care.

   (A) Some dose-saving equipment:
1.  Fast screen-film combinations (e.g. rare earth)
2. Low attenuation (e.g. carbon fiber) materials for cassette fronts, antiscatter grid interspacing.
3. Constant potential generators with appropriate kilovoltage.
4. Appropriate beam filtration (minimum 2.5 mm Al for general radiography).
5.  Specialized equipment for mammography and pediatrics
6. Pulsed and frame-hold (image storage) fluoroscopy equipment.
7.  Digital radiography equipment.
8. Dose-area product meter to monitor patient exposure.

   (B) Some dose-saving techniques:
1. Use smallest possible field size and good collimation.
2. Collimate to exclude radiosensitive organs (gonads, breasts and eyes).
3. When gonads lie outside the primary beam, make distance between the edge of the field and the gonads as large as possible.
4. Shield breasts, eyes, and gonads unless the area of interest would be masked. Dose to ovary can be halved and that to testes reduced by a factor of 20.
5. Use largest practicable focus to skin distance: never less than 30 cm, especially in mobile radiography.
6. Position the patient carefully. Reduce the dose to the female breast and, in skull radiography, to the eye by postero-anterior projection. Minimize the gap between patient and film-screen.
7. Use compression of patient where possible.
8. Use non-grid techniques when examining children and small adults.
9. Keep film reject rate due to all causes down to 5%. Check the factors before exposure. Quality assurance, particularly of automatic processors, is important.
10. In fluoroscopy use the minimal field size and minimal screening time essential for good diagnosis.
11. Use zoom or small field techniques, which require a higher dose rate, with discretion.

(C) High-risk examinations:
1. Keep pediatric radiation doses to an absolute minimum consistent with adequate diagnosis as children up to the age of 10 years are believed to be 3-4 times more radiosensitive than adults.
2. In pelvimetry: use MRI or CT scanography where possible; otherwise use fast rare earth screens and carbon fiber components.
3. Mammography is not generally performed on women younger than 50 years unless there is a family history of breast cancer or the patient has related symptoms.
4. In CT scanning, take the minimum number of slices, position the patient to avoid the eyes and other critical organs; reduce milliamperage if appropriate, e.g. for the chest.
5.  Patients who are or might be pregnant.
6. Interventional radiology needs  care  to avoid  skin  reactions;  use  pulsed and frame-hold systems: minimize screening times.