Point of care Ultrasound (POCUS)(Physics)

Amit Jaiswal, FRCEMConsultant EM

Regional Ultrasound Lead- East Midland

OBJECTIVE

• Physics

• Settings

• Knobology and terminology

(Language of sonogrpahers)

Ultrasound(USS) Physics

• Ultrasound is a sound wave and is type of mechanical energy

• Frequency used- 2-20 MHz

• Generation of USS requires conversion of Electric energy to Mechanical energy (sound waves).

• Piezoelectric crystals- Lead Zirconate Titanate (PZT).

• Frequency (f) and wavelength (w) – inversely proportion

• High f – high energy, poor penetration and vice-versa

Ultrasound device

• Monitor

• Keyboard

• Transducer (Probes) - Piezoelectric crystals

• Accessories- Storage/printer/CPU

Commonly used Transducers (Probes)

• Linear (Vascular)

• Curvilinear( Abdominal)

• Phased array(Cardiac)

Scanning planes / Probe orientation• Planes - Transverse Longitudinal

• Orientation-

Definitions

• Echogenicity

• Acoustic Impedance

• Resolution

Echogenicity

• Echogenicity- how bright the organ appears (higher the reflection brighter it appears).

• Hyper-echoic, Isoechoic and Hypo-echoic

Hypo

Acoustic Impedance (z)

• It is the RESITANCE to the flow of sound

Resolution

Ability of the imaging system to differentiate b/w two closely placed object or event as distinct.

• Spatial –Resolution in space( x or y plane)

• Contrast- shades of Gray

• Temporal- resolution in time

Knobology

1. Freeze

2. Save

3. Doppler

4. Gain

5. TGC

6. Depth

Gain:

• Compensate attenuation by increases overall amplification of the returning signal.

TGC: Time gain compensation • Selective increase in amplitude of the received signal

Modes of Ultrasound

• A- mode: Amplitude mode ( single dimension- depth and amplitude) Used in Ophthalmology

• B- mode: Brightness mode ( 2 dimensional- ), Most commonly used

• M- mode : motion Mode, combination of A and B mode.

ARTIFACT in USS

A structure in an image which does not directly correlate with actual tissue being scanned (Misinterpretation).

Due to assumption• sound travels at constant speed(1540 m/s)

• sound travels in a straight line

• All sound attenuation gradually

• All waves arise from centre of the beam

Common Artifacts

1. Acoustic shadowing

2. Acoustic enhancement

3. Mirror image

4. Reverberation

5. Edge shadowing

6. Side lobe artifact

7. Duplication

8. Refraction

9. Beam width artifact

10. Slice thickness artifact

ACOUSTIC SHADOW

• Area of low Amplitude echoes behind an area of strongly attenuating tissue.

• Due to either absorption or reflection of the sound waves.

ACOUSTIC ENHANCEMENT

• A localized area of increased echo amplitude behind an area of low attenuation.

• On a scan it appears as an area of increased brightness.

http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/images/large6au.htm

MIRROR IMAGE

• They arise due to reflection of the beam at a large smooth interface.

• An area close to the reflector will be imaged twice,• once by the original beam and

• once by the beam after it has reflected off the reflector.

REVERBERATION

• Due to false echoes from repeated reflections

between 2 interfaces with a high acoustic impedance mismatch.

EDGE SHADOWING

• A combination of Re-fraction and

Re-flection occurring at the edges of rounded structures.

COLOUR DOPPLER

• Doppler Effect- Change in ‘F’ of wave

due to relative motion.

• Used to detect flow ,direction of flow and

velocity.

• It is angle dependent, Max shift at 0 and no shift at 90 degrees

Aliasing-

Imaging error due to under sampling

NOTE

Optimisation of the image-

• Use of – Depth, Gain, TGC

• Use of correct frequency setting – penetration/resolution/general mode

• Use of liver as window for scanning heart.

• If bowel gas is in the way- apply firm and constant pressure to displace it.