Ultrasonography and image viewing techniques

Includes Week 3 of CAR radiographic faults, echocardiology etc.

What is ultrasound

longitudinal pressure waves that produce compressions and rarefactions of ‘particles’

how can the pressure waves be represented

as sinusoidal waves

what is a)wavelength, b)frequency and c)velocity

a)shortest distance b/w 2 points that are in phase (l)

b)the number of waves that would pass a given point in 1 second (f)

c) l x f

what are common diagnostic frequencies and what’s the max human fq that we can hear?

3,5,7 and 10 MHz up to 18MHz

humans can hear up to 20,000Hz

how are images generated with ultrasound (general)

generated based on returning echos reflected at tissue boundaries

What do we use ultrasound for in veterinary medicine?

• imaging soft tissue structures

• guiding certain types of tissue sampling (biopsy)

• view blood flow in circulation (Doppler flow colour)

what’s one limitation of ultrasound use in veterinary medicine

in larger animals some structures are too deep for the wave to reach and return with enough energy to form an image.

how can we adapt ultrasonography for large animals

In cows to view their ovaries, we use a rectal transducer and observe the ovaries directly from above through the rectal wall

What are the general principles of ultrasound imaging

• an ultrasound transducer/probe and coupling transmission gel is applied to the surface

• transducer emits ultrasound and acts as a receiver for incoming echoes

• echoes create an image, depending on strength, time of return and direction of return

• Image is an aggregate of multiple echoes and is updated many times per second

• Images can be paused/frozen, labelled, measured, saved as stills or short loops of moving image.

How is ultrasound generated

Transducer has ‘piezoelectric’ ceramic crystals which deform and vibrate when electrical signal is applied, emitting ultrasound

To generate an image the transducer generates an electrical signal proportional to the strength of the returning echo

approx 1% emit, 99% receive

How is an image created

Amplitude of the returning echo sets the whiteness on the screen (higher = whiter)

Time taken for echo to return sets the position on the screen

Distance = (time taken for echo to return x assumed speed through tissue) / 2

Multiple lines of sound are collected to create a cross-sectional image

How does the machine convert amplitude into grey-scale?

• loudness or amplitude of returning echoes are displayed on the screen with increasing signal strength depicted as whiter.

• Grey scale is used for diagnostic interpretation.

What happens to ultrasound in tissues (4 things)

When sound strikes a boundary b/w two tissues of different acoustic impedance:

1. Transmitted beam = sound continues unaffected

2. Reflected beam = sound is reflected the way it came through

3. Refracted beam = sound changes path, continues through the tissue in a different direction

4. Scattered beam = happens where there are many small uneven boundaries, reflection and refraction in multiple directions

What affects the ability to generate images of deep tissues

Attenuation

When are we able to visualise within tissues when performing ultrasound

when some sound is reflected in passage through a structure

when is reflection the strongest?

at boundaries b/w different tissues especially ones that have a large difference in acoustic impedance

what is acoustic impedance related to

tissue density and speed of the sound in the tissue

what may scattered beams contribute to in an image?

artefacts or improper positioning of an organ or incorrect iconicity.

what do we need to consider with regards to orientation of the image

• a thin slice of tissue image is created

• orientation of the transducer is key

• rotating the transducer enables movement b/w the different ‘cut face’ views.

What do we need to consider for prepping for an ultrasound?

Patient: starvation, sedation, clipping, cleaning, gel application, comfort and position

Room: dim lighting, quietness, comfort for yourself, patient and handler

why is coupling gel used?

it’s water based with similar acoustic impedance to soft tissues so the ultrasound beam can be transmitted into the body and not reflected at the air on the skin.

What types of transducer are there

• usually have several different crystals mounted to permit electrical impulse and frequency variation

Types:

• linear

• curvilinear

• phased array

Outline linear array transducers

• multiple crystals arranged in a line and sequentially triggered

Advantages:

• good near field resolution, parallel beams, no moving parts

Disadvantages:

• large contact area, limited field of view

Common uses:

• abdominal organs, muscles, joints and tendons

Outline curved array/curvilinear

• multiple crystals arranged in an arc and sequentially triggered

Advantages:

• good near field resolution

• no moving parts

• diverging beam

Disadvantages:

• large contact area (less than linear)

• difficult to use on the cranial abdomen (ribs) (solve by using the ones below)

• HOWEVER: microcurved or microconvex transducers have a smaller footprint

common uses:

• abdominal organs, pregnancy diagnosis.

outline phase array transducers

Sector (single crystal) scanner with electronic steering of crystal emission

Advantages:

• small contact area

• diverging beam

• good resolution

Disadvantages:

• reduced near field resolution

Common uses:

• echocardiography, thoracic structures, abdominal organs, regions with small contact area (brain, eye, joints)

what are stand off pads

• used to increase distance b/w transducer and superficial structures, moving the image down the screen to enable interpretation of near structures

• adjust to uneven contours to give better contact

• can be built in or pre-shaped removable pads.

Outline 4 aspects of the ultrasound machine that affect how it works?

• frequency of the ultrasound wave

• gain applied to the returning echo

• time-gain compensation

• where the ultrasound is focused

Often these have been set up initially but change as you move b/w structures

what do we mean by ‘driving skills’

• learning how to adapt the settings of the ultrasound machine to each situation, not just learning numbers.

Outline transducer frequency

• most operate at an optimal frequency governed by the crystal thickness

• some have dual crystals that can operate at more than 1 frequency

• some fire over a broad range of fq and use filters to block out parts of the range and utilise others.

How does wavelength length and fq affect the image

shorter wavelengths are absorbed and attenuated more easily

low fq have a longer wavelength and penetrate better (but poorer quality)

high fq have short wavelength and penetrate worse (but better quality of shallow structures)

outline frequency and resolution in ultrasonography

• resolution = ability to see fine detail in the image

• determined by wavelength (shorter = more accurate in discriminating b/w adjacent structures)

WAVELENGTH IS INVERSELY RELATED TO FQ

How do we select transducer frequence?

• highest that allows adequate penetration (don’t learn exact, know a range)

• if fq is too high, won’t have an image on the deep section (poor penetration)

Give 3 examples of ultrasound frequencies and what they’re used for

1. 5MHz - abdominal in large dogs, and cardiac in med/small dogs

2. 7.5MHz - abdominal and cardiac in small dogs and cats, pelvic and pregnancy in med/small dogs and cats

3. 10MHz - cervical structures (salivary glands, thyroid), superficial structures (eye, skin, mammary glands) and musculoskeletal.

what is Gain?

The amplification applied to the returning echo - makes the overall image look whiter - overall starts at 60%

why would we use gain?

when imaging deep structures, look darker on screen because sound has to travel further so there’s a quieter returning echo.

lower energy echoes have lower amplitude so are shown as darker grey

gain must be adjusted to compensate for this - give a balanced picture

use TIME GAIN COMPENSATION settings to refine this

what is Time Gain Compensation

allows selective amplification of signals from different depth levels

initially all set to 50%

only do once the image is produced on screen.

How and why do we change the focus on an image?

• narrows the beam at certain points improving resolution at the chosen depth

• adjust focal zone to region of interest by selecting focus and moving the arrow to the required depth.

what does the mark/bump on the side of the transducer mean?

• related to the side of the screen marked ‘M’

• should be on the LEFT

How do we orientate with respect to the animal?

• depends on the location of the organ

• use the different planes - frontal, sagittal, transverse

How do we hold our hands when scanning

• find the location of the bump

• move thumb to the thumb indent, knowing where the bump is.

• match the bump to the screen

Sagittal plane:

• bump is cranial and thumb is left side of animal

Transverse

• bump is on the right side of the animal and thumb moves to cranial

Outline the technique used for viewing images

• train your eyes by viewing normal positions

• use the bump, thumb position

• remember you’re looking at a 2D image of a 3D structure

what 6 main terms are there and one additional thing to look out for

• anechoic

• hypoechoic

• medium echogenicity

• hyperechoic

• homogenous

• heterogenous

note any artefacts

what does anechoic mean

• no echo produced

• all sound passes through the tissue

• non is reflected to the ultrasound transducer

• anechoic tissue appears black on an image

examples include fluid (blood, urine, bile)

what does medium echogenicity mean?

• produce a medium echo

• a medium amount of sound is reflected to the transducer and a medium amount passes through

• dark to light grey appearance

examples include soft tissues e.g. liver, spleen, prostate, testes

what does hypoechoic mean

• produce little echo

• most sound passes through

• small is reflected back to the transducer

• dark grey appearance

examples: high-water content tissues e.g. cartilage, muscle and renal medulla

what does hyperechoic mean

• produce much echo

• little sound passes through

• most is reflected back to the transducer

• often appear white on image

example is dense connective tissue

where else may we get full reflection?

• gas and bone interface (may appear as acoustic shadow deep to structure)

what does heterogeneous and homogeneous mean

Heterogeneous (non-uniformed, mixed) = mixed/irregular pattern (common in finding diseased tissue)

Homogeneous (uniform) = similar and regular pattern throughout (most normal tissues)

define an x-ray

electromagnetic radiation used to create an image

what is a radiograph

an image on a display screen

define radiography

the art or process of making the radiographic image

includes radiation safety, radiographic equipment and radiographic technique

define radiology

the interpretation of the radiograph leading to diagnosis of disease using X-rays/radioactive material.

• includes normal radiographic anatomy and appearance which changes with different diseases.

How many radiographs should we take, why and how

Minimum of 2 views perpendicular to each other

• animal is 3D

• images are 2D

• need the two images to gain an understanding of 3D structure

• called orthogonal views

what are X-rays

• a form of short wavelength, high energy electromagnetic radiation

• sinusoidal waveform - transverse waves

• J= amplitude (height of each wave)

•  λ is wavelength

• frequency = number of wavelengths per second

on the electromagnetic spectrum how does wavelength and energy change

Higher up the spectrum e.g. with gamma, xrays and cosmic waves, higher energy, shorter wavelength

Lower down the spectrum e.g. electric power or Tv shortwave radio, lower energy and longer wavelength

Give 9 properties of X-rays (and gamma rays)

1. no charge, no mass

2. invisible

3. cannot be felt

4. travel at speed of light

5. travel in a straight line

6. penetrate all matter (to some degree)

7. can cause some substances to fluoresce

8. expose photographic emulsion

9. ionise atoms

give 3 important features of X-rays to consider for medical use

1. high energy and short wavelength

2. penetrate materials and cause changes atomically

3. can lead to ionisation which can have potentially damaging effects on living tissue = IONISING RADIATION

outline the risk of ionising radiation on living tissue

• harmful to living tissue

single large doses = SOMATIC EFFECTS which includes tissue damage seen or felt e.g. radiation burns or sickness

multiple, very small doses = GENETIC EFFECTS which includes tissue damage to cellular DNA, can cause mutations or cancer

• patients are unlikely to have significant risk, however accumulation of small amounts can be a risk.

Outline radiation safety for personnel

ALARP PRINCIPLE:

As Low As Reasonably Practical

Radiation Regulations 2017, enforced by Health and Safety Executive

What are 14 safety considerations for radiation

• Never expose to primary beam

• U16 and pregnant/breast feeding = not in the room

• min. no. people needed

• always wear protective clothing

• always use vertical beam

• only manually restrain in exceptional circumstances

• monitor radiation for staff

• minimal time for exposure as possible

• use lead screens

• control from outside the area if possible

• lead clothing

• wear gloves

• cassette holder

• x-ray machine holder?

what are dosimeters

a health requirement to track individual ionising radiation exposure IN ADDITION to other sensible practices

• site specific

• stored outside radiography room

• worn on the front of the body, around the torse and clipped to clothing

• under leaded PPE too

• sent for readings every 1-3 months (for development and interpretation)

how are x rays produced?

1. Cathode in an X-ray tube generates a stream of electrons from a coiled tungsten wire filament when current passes through

2. cathode is a cup shape to focus stream

3. number of electrons generated is determined by mA (miliamperes) and exposure time

4. electrons hit and interact with atoms w/in target area of tungsten anode which releases X-rays

5. potential difference is applied across tube, accelerating electrons towards +vely charged anode

6. electron energy is determined by kV (kilovolts, sometimes kVp = kilovolt peak)

why is tungsten metal used in X-ray production

• high melting point

• high atomic number

• heat is produced during the process

How is an X-ray tube constructed

• produced in tube head of machine due to interactions b/w anode and cathode

• whole arrangement is kept in evacuated glass envelope surrounded by oil to absorb heat produced

• placed in a metal casing

• absorbs x-rays with a window to allow them to emerge in only one direction

• beam passes through a collimator that has lead plates to adjust size of the x-ray

• light shines through the gap to indicate extent of x-ray beam

• collimator is referred to as light beam diaphragm

what are the 2 properties of X-ray beam

1. quality = penetrating power of beam (comes from energy)

2. Intensity = amount of radiation in the beam (comes from number of xrays)

first x-ray beam = primary beam

what is the effect of increasing kilovoltage

• increase kV

• increased electron acceleration

• increased energy of electrons

• grater number of x-rays produced

• increased energy = increased penetrating power

what is the effect of mA

• governs the current applied to the filament and is applied for a specific time = sec

• increased mA

• increased tube current

• increased number of electrons

• GREATER NUMBER OF X-RAYS ARE PRODUCED

• energy remains unchanged. (same penetrating power)

what is the effect of exposure time?

if time over which current is implied INCREASES, more x-rays produced, energy remains unchanged

mA x sec = mAs

how can we increase mAs?

• increase time

• increase mA

what is source to image-receptor distance (SID)

• closer the x-ray tube to the image receptor, the more ‘concentrated’ the x-ray beam (vice versa)

• exposure varies according to inverse square law

e.g.

double source to image receptor distance - ¼ amount of radiation lands on given area of image receptor.

How did we use to form radiographic images vs now?

Historically: recorded on photographic film, chemically processed

Now: digital image - recorded on digital detector, electronically processed and displayed on a computer screen.

examples: computer radiography and direct (digital) radiography

uses substantially less energy

what is computed radiology (CR)?

• phosphor plate held in cassette

• X-rays strike plate, electrons are energised and form an ‘invisible’, latent image.

• cassette placed into reader, plate removed

• scanned by a laser, trapped energy is released as visible light

• photomultiplier detects light, converts it to an electrical signal

• processed by a computer

• image is produced and can be manipulated post processing

what is Direct Digital Radiography (DR)

• x-rays stimulate sensor panel, sends electrical signals direct to computer

• image manipulated and stored as before

• sensor panel separate/mounted in table

how are images stored and distributed?

stored to enable:

• easy searching of archived images

• back-up of digital files

• PACS (picture archiving and communication system) often used

• consider back ups (cloud/physical)

• be able to share them with other professionals e.g. for referrals.

what are 3 ways the x-ray interacts with the patient

1. Absorbed completely

2. pass through unaffected

3. interact with atoms and scatter

How is a radiographic image produced

• animal is between x-ray tube and image receptor

• low energy rays absorbed by patient

• differential absorption by tissues/structures produces a radiation pattern from the patient, used to create the image

is it possible to have a thick, low-density structure showing as whiter than a thin high-density structure

YES

e.g. thick area of fat vs thin area of bone

what is meant by radiographic density

• ability of tissues to absorb radiation

outline the different radiographic tissue types

5 tissues with relative radiographic densities

• air = lowest value, shows as radiolucency

• fat is next

• water, soft tissue/fluid

• bone shows as radiopaque

• metal is bright white

note:

• all fluid shows same opacity so can’t distinguish type of fluid, same as soft tissue

why do different tissues have different radiographic densities?

• atoms get closer/further apart

• interact differently with x-rays

outline what scatter is

• low energy x-rays emitted in random directions as it bounces off atoms

• increases as volume of irradiated tissue increases

why is scatter a problem?

• can fall onto image receptor

• causes blackening/fogging that’s unrelated to tissue pattern

• reduces image quality

• increases radiation dose to personnel outside primary beam = health risk

how can we limit impact of scatter?

1. use a GRID for tissue thicker than 10cm

2. collimate beam to minimum size necessary

3. reduce kV

4. ensure personnel are outside room at exposure

how does a grid work?

• series of lead foil strips in a grid pattern

• separated by radiolucent spacers

• placed between patient and image receptor when tissue depth >10cm

• multidirectional scattered radiation is absorbed by lead strips

• only radiation in parallel with transmitted beam produces the image

• image isn’t affected

what are some practical considerations for using a grid?

• exposure needs to be slightly higher (some of the radiation is absorbed by grid)

• care is needed to time when exposure is taken with respiratory pauses (blur more likely when animal breathes, increased mAs)

• faint grid lines are visible if stationary grid is used

how can we prevent an image having faint grid lines?

• bucky tray is used

• placed below the table

• grid will oscillate slightly at exposure

• therefore grid lines blur slightly and aren’t visible on final image

• doesn’t give moment blur to patient image as image receptor stays still.

how do we take a good radiograph?

1. depicts accurate portrayal of structures

2. enables easy perception, sharp shadows, wide shades of grey

3. no misleading artefacts

4. no unnecessary risks to patient or personnel

5. was taken using a standardised technique for consistent results

take attention to detail and care in radiographic technique

what is a pneumonic for radiograph technique?

PINK CAMELS COLLECT EXTRA LARGE APPLES

Positioning

Centring

Collimation

Exposure

Labelling

Artefacts

• choose correct equipment before this

• process and store the image

when may blurring occur and how can we overcome this?

if the animal moves when exposure is taken:

• involuntary movement e.g. breathing

• voluntary movement e.g. limbs, trying to get off table

overcome?

• sedation with positioning aids e.g. sandbags (NEVER USE TIES if awake/lightly sedated)

• GA - then use ties as well as sandbags

• time the exposure to minimise impact of breathing

what do we need to consider about magnification during positioning for a radiograph

• x-ray beam diverges with distance from tube

• position part of interest as close to receptor as possible - clear and accurate image

to minimise magnification:

• minimal OIRD (object to image receptor distance)

• maximal FOD (focus to object distance)

What do we need to consider about geometric distortion for positioning during a radiograph

1. position accurately with part of interest parallel to image-receptor

2. centring point in the middle of cassette and area of interest

what will happen if an object is not parallel to beam/in the centre of the beam for a radiograph?

1. distortion

2. structures may appear elongated

3. may appear foreshortened

4. shapes won’t be accurate

5. superimposition

what do we need to consider for rotation for a radiograph

• avoid axial rotation of part of interest

• ensures accurate portrayal of part

• makes positioning for limb views hard

can use foam blocks for example.

how can we assess axial rotation in lateral thoracic views?

• caudodorsal rib heads should be superimposed

how do we correctly centre?

• centre primary beam over area of interest

• especially important in large animal limbs, all joints and spines

relative position may shift if not central

outline collimation

• scattered radiation contributes to general image opacity and increases radiation hazard

collimation:

• reduces production of scattered radiation

• beam is collimated to minimize size necessary to include only area of interest

why is collimation important in radiography?

• important for appropriate image processing

• collimate closely but include area of interest

give examples of how much of the surrounding tissue should be included in radiographs

Joints: include 1/3 of adjacent associated bone

long bones: whole bone, adjacent joints

abdomen OR thorax, not both

when evaluating images: describe how closely it’s been collimated to the area of interest

what are some important concepts for exposure factors in radiography

• need a wide range of well differentiated shades of grey

  need to balance kV and mAs:

• enough penetration with sufficient x-rays for a good image to form

• optimum difference b/w different tissues (contrast)

• ensure different structures can be viewed.

• use exposure factors for consistent results

how does kV affect image

• increasing = greater number of x-rays produced and increased energy, therefore increased penetrating power

If

a) increase kV = increase overall amount of radiation emerging from patient (may lead to over exposure), reduces contrast

b) decreasing kV = decreases exposure and increases difference. Energy is reduced therefore absorption is increased = increased contrast