As Low As Reasonably Achievable

Prudent use implies that steps should be taken to minimize the risks of bioeffects to patients.

In 30 years of clinical use, there are no recorded instances of bioeffects, however, the possibility exists they may be identified in the future.

To maintain the ALARA principle, use of high receiver gain and low output power is recommended to minimize exposure to the patient.

Pulsewave (doppler) of the fetal heart, is considered a non-scanned modality, while B-mode is a scanned modality. For a scanned modality, there is time between transmits of sound for heat to dissipate over a larger field. A non-scanned modality (pulsewave) transmits repeatedly in the same direction allowing for heat to build up, increasing the risk for thermal bioeffects. Therefore, dopplers are not to be utilized routinely, but rather at the technologist’s discretion or of that which has been established by the medical director of each center, keeping in mind that the benefit must outweight the risk in order to justify the use of this feature.

The ultrasound machine assumes that sound is traveling through soft tissue at a speed of 1540 m/sec. and produces an image based on that speed.

Sound travels at different speeds through different media. Sound travels slowest through air and fastest through bone. Since the machine assumes the sound is only traveling through soft tissue, the speed propagation through media other than soft tissue could create an erroneous image on the ultrasound screen called an “artifact”.

 Artifact- Structures or echoes that appear on the image but do not have a corresponding anatomical structure.

Artifacts should be recognized and:

ignored
eliminated
assist in diagnosis

Common Artifacts

Shadow: Sound travels fastest through bone causing structures posterior to it, with differing acoustic impedances, to be shadowed out. In a limited obstetrical study we see shadowing posterior to the ossified bones of a 2nd trimester fetus.

Edge Shadow: When obtaining a second trimester measurement of the fetus’ Biparietal Diameter (BPD) we see shadowing coming from the edges of the baby’s cranial bones.

Enhancement: We mainly see this artifact through fluid-filled structures such as a full bladder or a corpus luteum cyst in an ovary.

Reverberation: Occurs when sound bounces between structures and creates equally spaced lines on the ultrasound image.