Equipment and Knobology

Equipment and Knobology

Frequency

Scanning is made possible by the piezoelectric effect, namely that if you apply voltage to a crystal, it will release sound. The higher the frequency of these emissions, the greater the resolution and the worse the penetration.

Power

Adjusts the amount of energy leaving the probe. Conceivably higher powers can have biologic consequences. Theory of ALARA, as low as reasonably achievable, should be followed

Gain

Amplifies returning signals, does not effect probe emission like power. Amplifies all segments of returning signals unlike the selective TGC.

Time-Gain Compensation (TGC)

As sound waves travel further from the probe, they are attenuated. All machines have built in compensation for this, but sometimes manual adjustments are necessary, so all machines have some way to adjust gain. This might be precise like a set of 8 slider knobs or simply one dial to adjust near and one dial to adjust far gain

Depth

Magnifies and reduces workload for near images

Focus and Zoom

Focus adjustment allows maximum information to be obtained from the area of interest by altering crystal emission timing. Zoom takes a segment of an image and blows it up.

Tissue Harmonics

Takes a different frequency of returning waves than the one sent out by the probe reducing artifact.

B Mode

The normal 2d image to which we are accustomed

M Mode

An ice pick through one vertical axis of the image which then shows motion through that area plotted against time. Useful for cardiac measurements and fetal heart rate.

Transducers

Convex

Sector scanning, good general, all purpose probe

Linear

Flat head, good for higher frequency, high resolution superficial scanning. E.g. DVTs, central line placement, ocular, testicular scans

Phased Array

flat head, but crystals fire at variable time giving a sector image. Useful b/c small footprint allows scanning between ribs with a wide area of scan,

Endovaginal

High frequency probes with sector scanning surface just off the horizontal axis. Designed for the anteverted uterus. If the uterus is retroverted, turn 180° and reverse the image on the screen.

2-D Imaging

Echogenicity

The amplitude of the returning echoes. Hyperechoic structures are white on the screen. Hypoechoic structures are black. Fluid is hypoechoic/anechoic.

Probe Orientation

For all but echocardiography, the probe orientation is thus:

Sagittal (Longitudinal): Indicator towards the head on the ventral plane of the body

Transverse: Indicator towards the patient’s right on the ventral plane of the body

Coronal: Indicator towards the head on the lateral plane of the body

Artifacts

Shadowing

Seen when the sound waves encounter a highly reflective surface (analogy of shining flashlight in a mirror.)

Acoustic Enhancement

The opposite of shadowing. When the wave encounters an area which sound passes through especially easily, i.e. Cystic structure, empty distended gallbladder, urinary bladder, the area distal to this structure will be more hyperechoic than expected.

Refraction

Edge artifact. Shadowing secondary to the interface between structures with very different echogenicity when the sound beam is slightly tangential. Analogous to placing a pencil in a glass of water and having it seem broken.

Gas

Bowel gas is the enemy of abdominal ultrasonographers. Attempt probe pressure or changes in patient positioning

Reverberation

Concurrent arcs when scanning structures close to the surface from reflection of the skin wall. Like grooves on a record

Mirroring

Duplication of structures b/c machine is fooled by multiple reflections. Seen often at the interface of liver and diaphragm with the liver being repeated distal to diaphragm.

Motion Artifacts

Blurring from probe movement, often when going to press the freeze button. Use cine to correct.

Holding the Probe

Pencil grip with 4th and 5th fingers touching the skin

If doing Sub-Xiphoid view adjust grip so that 2nd and 3rd fingers are over the probe in overhand grip

| | |