[Ultrasound Physics] OTHERS important for SPI ARDMS

[Contents]

1. Patient Care, Safety, and Communication

2. Physics principles

3. Pulse-Echo Instrumentation

4. Quality Assurance/Quality Control of Equipment

1. Patient Care, Safety, and Communication

* ALARA principle (as low as reasonably achievable) [★]

- time, distance, shielding

- Complete diagnostic scan in a timely fashion at lowest output power that achieves a quality image

- Use the lowest transmit power that allows adequate tissue visualization

 

* Transmit power [★]

- Affect the exposure of the patient to acoustic power

- ↑ transmit power

: penetration ↑

: Acoustic power ↑, voltage applied to transducer elements ↑

: Image brightness ↑

 

* Bioeffect through absorption of sound energy by tissue [★★]

- Heating

: Tissue heating occurs when transmit power increases

 

* Nosocomial infection = hospital-acquired

- To avoid nosocomial & cross infection

: probe cleaning should always precede high-level disinfection

: covering probe with condom alone is insufficient

: disinfection using germicide compatible with the transducer

- When probe is in contact with mucous membranes

- Alcohol wipes are not recommended by manufacturers

: Alcohol degrade the transducer surface over time

 

* Mechanical index (MI)

- Method to evaluate bioeffect of US beam

 - Associated with onset of cavitation

 

* Thermal index (TI) [★]

- Power needed to increase tissue temperature by 1⁰C (Estimated temperature increase in tissue)

- TI > 1: Limit exposure time

- TI = 1: temperature could increase 1⁰C if transducer were held stationary

- most likely: bone (absorber)

- To avoid thermally induced biologic effects

: Avoid local tissue temperature increase exceeding 1⁰C

 

* Advantage of MI, TI

- Information available on screen to help sonographer implement ALARA principle

 

* Hydrophone

- Small needle with crystal at end

- Measure: amplitude (acoustic pressure), period, pulse duration (PD), PRP, PRF, duty factor (DF)

- Cannot measure: impedance

 

* String test object

- Evaluate doppler accuracy

 

* Acoustic streaming

- Acoustically generated transport of fluid within the body of insonated fluid/tissue

- Mechanical interaction of tissue and sound

- Motion of particles in a fluid observed in an intense US beam

2. Physics principles

* Purpose of gel coupling between transducer and skin

- To provide a medium for sound transmission, since US does not propagate through air

 

* Diffraction [★]

- General term for various phenomena in which waves from different parts of a source add or subtract

- ex) Pattern produced by a sound beam after passing through a small aperture

 

* When Image does not show adequate penetration, show noise in far field

- Decrease ultrasound transmit frequency [★★]

- Increase acoustic output

- Move focal zone to deeper position

 

* Doppler pulsed used for diagnostic purposes

- 5-30 cycles long

3. Pulse-Echo Instrumentation

* Tissue harmonics [★★]

1) Harmonic of transmitted frequency is used to create image

2) Harmonic frequency of transmitted pulse is generated within the body

- Selective reception of frequencies that are higher than that of transmitted frequency

generated within the body by nonlinear propagation

- Produces thinner beam

3) Advantage of Tissue harmonic imaging [★★]

- Grating lobe artifacts are reduced

               : improved contrast resolution

                              - contrast resolution is always improved by increasing frequency

                              - harmonics always result in use of higher frequency

                              - harmonics reduce clutter and side lobe artifacts

- lateral resolution is improved

- increase visualization of reflections from blood flow on real-time US image

 

* Signal-to-noise ratio

= relative amplitude of the signal compared to the amplitude of the noise

 

* Volume data set

1) Advantage

- Can be manipulated to show an infinite number of imaging planes with many different images

2) May be obtained by

- a freehand sweep of the probe over the anatomy

- an automated sweep of the transducer within the probe

3) Automated 3-D sweeps can be obtained with specially designed mechanical/electric transducers

- Advantage: Measurements can be accurately obtained

               (the distance of sweep is known)

 

* Cine loop

- Allows user to freeze and then scroll back through the most recently acquired image frames

 

* 3D imaging

1) Advantage [★★]

- Most helpful to obtain accurate anatomic volume measurements

- Ability to display image planes (coronal plane) impossible to obtain with 2D imaging

- Ability to display orthogonal planes simultaneously

2) volume rate: # of volumes displayed per second

3) Voxel

- smallest element of a 3-dimensional volume (3-dimensional pixel element)

- analogous to pixel in 2D imaging

 

* 4D imaging

- 3D imaging with addition of time

 

*

- Isotropic resolution: spatial resolution is equal in all planes

- Anisotropic resolution: unequal resolution between imaging planes

 

* Magnetic field tracking

1) Successful method of obtaining 3D data set

2) Based on a six degree-of-freedom magnetic field sensor

3) requirements for 3D reconstruction

- electromagnetic interference must be minimized

- transmitter must be in close proximity to receiver

- ferrous metals must not be within electromagnetic field

 

* DICOM (Digital Imaging and Communications in Medicine) [★]

- Standard for handling and transferring images and medical information

1) Query/Retrieve

- DICOM feature to recall a previous DICOM study onto system for viewing

2) Worklist

- DICOM feature to select patient name and automatically populate patient information on system.

3) Sonographic images are compressed before sending to PACS

- To reduce time to transmit the image

4) PACS (Picture Archive and Communication System)

- System commonly used to handle the archiving and electronic distribution of sonographic images

               (using a DICOM format)

 

* Advantage of Modality worklist

- Avoiding the need to type patient information

- Reducing patient information error

- Speeding up patient preparation time

- Easily confirming patient information

 

* Pulse inversion harmonics

1) Effective method to filter out fundamental frequency (transmitted frequency)

- leave only the harmonic frequencies for display

2) Uses two pulses of opposite polarity transmitted into the tissue in rapid succession

- The received echoes from the pulses are added together

- Cancels out the transmitted frequency, leaving the harmonics that were generated within tissue

 

* Cardiac output

- volume of blood pumped by the heart per minute

 

* Vessel wall layers

 

- Intima

: Single layer of cells backed by a thin layer of elastin and collagen fibers

               : atherosclerotic disease begins as a fatty streak within intima

- Media: contains smooth muscle fibers

 

* Rouleaux formation

- Stacking up of RBCs that occurs at low velocities and low shear rates

- Produces larger echo: blood flow may be visible

- velocity ↑ → rouleaux formation breaks up

4. Quality Assurance/Quality Control of Equipment

* SMPTE (Society of Motion Picture and Television Engineers) test pattern

- Standard pattern for evaluation of monitors and cameras

- Purpose: to aid in the setup and quality assurance of displays and cameras

 

* Quality control

1) system penetration

2) image uniformity

3) assurance of electric safety

4) distance measurement accuracy

 

* Dead zone

 

- distance from the transducer to the first identifiable echo

 

* sensitivity

- ability of the system to detect weak echoes

 

* transducers can be cleaned with

- soap and water

- ultraviolet light or radiation

- autoclave

- gas

- not) cidex, acetone, iodine, betadine, bleach

- Glutaraldehyde: commonly recommended ingredient in cleansers for intracavitary probes

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler