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Table 4.2 Probe types.
| Linear array probe | Phased array probe | Large curvilinear probe |
|---|---|---|
| Commonly referred to as vascular probe High frequency (typically 5–10 MHz)Large footprintExcellent image resolution of superficial structures at expense of tissue penetrationUse: vascular, lung (specifically pleura) | Commonly referred to as cardiac probe Small footprint; sound waves originate from single point and fan outwardLow frequency (typically 1–5 MHz)Excellent tissue penetration at expense of image resolutionUse: cardiac, lung, abdomen | Large footprint; sound waves originate from large area and fan outwardLow frequency (typically 2–5 MHz)Excellent tissue penetration at expense of image resolutionUse: abdomen |
Ultrasound equipment
Transducer (probe): sends out US waves that pass through tissue; also senses sound waves reflected back to transducer.
Structures closest to transducer are displayed at top of US screen in ‘near field.’
All probes have an ‘indicator’ (typically a bump or groove) on one side of the transducer that corresponds to an index marker on the US screen. Types of probes: see Figure 4.1 and Table 4.2.
General radiology convention is to position the screen index marker on left side of screen, and ‘point’ the probe indicator to patient’s right side or head. This means images on left side of screen correspond to structures on patient’s right side or toward patient’s head, respectively.
Cardiologists use an opposite convention (discussed in more depth in Procedure section).
It is critical to confirm your probe orientation with gel prior to any US exam or procedure. Relative to you, with the probe placed just above the intended point of contact, tapping under the right side of the probe should result in movement on the right side of the ultrasound screen. If movement occurs on the left side of the screen, rotate the probe 180°.
Basic knobology
Depth: adjusts depth of field of view by increasing or decreasing depth of US beam. Increasing depth will visualize deeper structures and decreasing will enlarge superficial structures.
Gain: adjusts brightness of image by changing amplification of returning echoes.
Time‐gain compensation: adjusts gain at selective depths to account for tissue attenuation; echoes returning from deeper tissues will be weaker.
Freeze: creates ‘still’ or ‘frozen’ 2D images.
Modes:B‐mode (brightness): standard scanning mode using different shades of gray to provide structural information in 2D image.M‐mode (motion): temporal measurement of structures moving toward or away from probe.Color Doppler: distinguishes vascular from non‐vascular structures and shows direction of flow.
Basic terminology
Echogenicity: brightness (amplitude) of image.
Hyperechoic/echogenic: structure appears brighter/whiter by generating more echoes than surrounding tissue.
Hypoechoic: structure appears darker than surrounding tissue by generating few echoes.
Isoechoic: same brightness as surrounding tissue.
Anechoic: area appears black due to complete absence of echoes.
Artifacts of US imaging
Shadowing: partial or total reflection of US waves (gallstones, ribs).
Posterior enhancement: area behind anechoic fluid‐filled structures appears brighter (bladder).
Edge artifact: shadow formed by refraction of US wave at edge of rounded structure.
Mirror artifact: image of structure duplicated as US wave reflects off highly reflective surface (diaphragm).
Reverberation artifact: US wave bounces between two highly reflective surfaces (pleura).
Ring‐down artifact: appearance of needle tip as a hyperechoic structure casting a narrow shadow.
Procedure
Cardiac ultrasonography
Probe selection and orientation
Use phased array ‘cardiac‘ probe.
Conventional cardiology screen/probe orientation is to position the screen index marker on the right side of the screen (reverse of general radiology convention).
Scanning technique
There are four standard views (Figure 4.2).
Parasternal long axis. Position probe just left of sternum at the third or fourth intercostal space. When using conventional cardiology orientation (marker on right side of screen), point probe indicator towards patient’s right shoulder. If you prefer keeping the screen marker fixed on the left side while obtaining views consistent with conventional cardiology imaging, simply point probe in opposite direction towards patient’s left hip. Otherwise, images will be reversed.
Parasternal short axis. Rotate probe 90° from long axis view to obtain circular short axis view of left ventricle. For conventional cardiology orientation, this means pointing probe towards the patient’s left shoulder. For general radiology orientation, point probe towards patient’s right hip. Angling probe through short axis views allows visualization of different segments of the left ventricle, including apex, papillary muscles (mid‐section), mitral valve (base of heart), and aortic valve (‘Mercedes Benz’ sign).
Apical four chamber. Using same orientation as short axis view, slide probe leftward – lateral to nipple line (men) or inframammary crease (women) – to point of maximal impulse. Position probe so ventricular septum is in center of US screen. The left heart will be on right side of screen and vice versa.
Subxiphoid. Position probe just below subxiphoid and angle cephalad toward the patient’s left shoulder using the liver as an acoustic window. If using conventional cardiology orientation, point probe towards patient’s left side. Otherwise, point probe towards patient’s right side. Transition to evaluating the inferior vena cava (IVC) from this view.
Clinical application
Assess for pericardial effusion or tamponade:Effusion appears as anechoic area within pericardial space.Large effusions tend to wrap circumferentially around heart.Clotted blood or exudates appear more