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Radiology
Ultrasound Physics for the Clinician
Built from Obstetric Imaging
What’s inside
7 sections · 64 slides
Overview
- Scope of this topic
- Components of an imaging system
- The six components in an ultrasound scanner
The ultrasound pulse
The stimulus: what the probe actually sends into the body
- Nature of the ultrasound pulse
- Compression and rarefaction
- Longitudinal and transverse waves
- Amplitude and frequency of the pulse
- 20 Hz – 20 kHz
- Why the prefix ultra-
- Volume occupied by the ultrasound pulse
- Propagation of the ultrasound pulse from the transducer
- Reading the pulse propagation diagram
- Axial, lateral, and elevational dimensions
- Beam geometry and the focus
The body as an acoustic object
What the pulse meets once it is inside the patient
- Acoustic impedance
- Sound speed in soft tissue
- 1750 m/s
The echo
The response: how tissue answers the pulse, and what dims that answer
- Origin of the echo
- Factors that set echo amplitude
- Reflection from large interfaces
- Coupling gel and the acoustic window
- Scattering from tissue microstructure
- Reflection compared with scattering
- Formation of speckle
- Speckle texture on a clinical scan
- Echogenicity and its descriptive terms
- Darker and brighter regions side by side
- Attenuation
- Causes of attenuation
- Decibels and typical attenuation values
- Time-gain compensation
- Detector sensitivity and echo amplitude
Making the image
Transducer, pulse-echo listening, and building the B-mode picture
- The ultrasound transducer
- Piezoelectric elements and the array
- Ultrasound transducer contact face
- The aperture and pulse-echo listening
- Converting echo arrival time into depth
- Formation of a B-mode image
- Echo signal along a single scan line
- Grey-scale B-mode image built from many scan lines
- Building the B-mode image line by line
- Array type and image geometry
- Linear, curvilinear, and phased arrays
- Electronic delays in a phased array
Image quality and its trade-offs
Spatial, temporal, and contrast resolution — and what each one costs
- Three primary determinants of image quality
- Spatial resolution
- Frequency and spatial resolution
- Resolution versus penetration
- Choosing a transducer for the depth of interest
- Practical consequence of the frequency trade-off
- Temporal resolution
- Contrast resolution
- Cost of noise averaging
Safety and the ALARA principle
What the referring physician needs to know about acoustic energy
- Acoustic energy deposition in tissue
- The ALARA principle
- “
- From pulse to picture
- Key points
- Vocabulary to carry away
- References
- References (continued)
- References (continued)
- Suggested readings
- Obstetric Imaging: Fetal Diagnosis and Care, 2nd Edition