Ultrasound Flashcards
1
Q
Sound waves
A
Mechanical radiation, propagating by vibration of molecules of a medium
- manifests as longitudinal pressure (compression) wave and transverse (shear) wave
- sound production requires a vibrating source
2
Q
Compression and rarefaction
A
- back and forth displacement of the source squuezes and pulls on particles in the medium
3
Q
Basic acoustics
A
- the wavelength is the distance between two points of compression
- the frequency is how many waves in a particular time
- C = dependent on the medium there will be a different speed of sound
4
Q
What determines speed?
A
- material properties - density (p) and Bulk modulus (stiffness (B))
- the higher the density the slower the speed
- the more squishy the tissue is, the slower the speed
- Fat, blood, muscle and liver are around 1500m/s, whereas bone is much faster at 4080m/s and air is much slower at 343m/s
- average is 1540m/s
5
Q
Interactions of US with tissue
A
- Absorption - US energy is converted to heat
- Reflection - some energy reflected from a boundary
- interference - artefacts in the image
- refraction - some or all energy is diverted from its original path
- diffraction - natural divergence of beam from sound source (defocusing)
- Scatter - some energy is dispersed in all directions
6
Q
Absorption
A
- Ultrasound energy is converted to heat
- amplitude is reduced
- depends on tissue properties, frequency and the distance it has to travel
- if you are scanning the abdomen, you need a lower frequency than if you are scanning the neck as you need to get the extra distance
7
Q
Reflection and Transmission
A
- reflections occur at tissue boundaries where there is a change in acoustic impedance (Z)
- The density and speed of sound in the tissue determines the impedance
- the bigger the difference in impedance, the bigger the reflection
8
Q
Types of reflectors
A
- Specular reflectors - returns echoes to the transducer only when the sound beam is perpendicular to the interface
>straight, flat interfaces
>if you hit the reflector at an angle, the angle of reflection is going off the plane and so wont be picked up by the scanner - Scattering
>much smaller interfaces within solid organs - scatter echoes in all directions
9
Q
Interference
A
- Constructive = two waves meet and their crests line up - increase amplitude
- Destructive = two waves meet but one peak meets a trough - results in lower amplitude
- causes speckle in the image
10
Q
Attenuation and Decibels
A
- attenuation is the loss of power or amplitude of the ultrasound signal as it passes through tissue
- Attenuation = Absorption + Scatter + Reflection
- Attenuation loss measured in decibels (a ratio)
11
Q
Frequency vs Attenuation
A
- attenuation is nearly proprtional to frequency - the higher the freq., the more loss of sound
- in soft tissue, attenuation ~1dB/cm/MHz
12
Q
US Machine
A
- transmitter - energises transducer
- transducer - piezoelectric ceramics converts electrical energy to mechanical vibrations
- receiver, ADC and processer - to detect and amplify backscattered energy
- image display - presents US image
13
Q
Transducers
A
- transducer contains multiple elements (120-250) each with its own electrodes
- linear - sound waves parallel to each other giving rectangular image. (+ = good near field resolution. - = artefacts when applied to curved part of body as gives air gaps between skin and transducer)
- curved - a compromise of linear and sector. Density of scan lines decreases with increaseing distance from transducer
- phased - sector image so can look inbetween ribs
- annular (old)
14
Q
ultrasound generation
A
- piezoelectric element inside
- pass a current across it and it will vibrate (expand). if you hit it, it will generate an electric current
- generally use ceramic material, but new materials have been researched recently
- damping material behind element allows mot current to go forwards
- matching layer means you minimise reflection
15
Q
Continuous and Pulsed wave
A
- Continuous - just turn it on, giving continuous doppler tone - can measure blood flow in vessels
> need to have a different receiver however as first is ringing at all times, cannot receive as well - Tone burst - used to break up haematomas (much higher power than imaging)
- Short pulse - ring it, then stop it by reversing polarity - gives high spatial resolution (main use in imaging)
16
Q
Pulse-echo principles (A-scan)
A
- amplitude scanning
- most has gone into the body, and a little comes back
- then goes further into the body, hits another interface and another bit is reflected
- bu this point, some from the first burst has come back to the detector, generating a tiny electric current in the detector (peak on graph)
- bit later, second echo comes back, a little weaker as it has had to travel further - lost energy from attenuation/absorption (smaller peak on graph)
17
Q
B-mode image
A
- brightness mode
- rather than a graphical display with points on a graph, you get an image display with different dots of different brightnesses
- large number of pulse-echo lines
- each echo is displayed along a line as a dot
- the brightness of each dot is deteremined by the strength of the echo
- the distance down the display relates to its depth below the transducer
- image builds up from left to right, sweeping across, then starts again
- now with digital imaging we can image from different points simultaneously and so dont get sweeping as frame rate is faster
18
Q
Transmit focussing
A
- Want the beam to be narrow so that we know where the echo has come from (not spread out in all directions)
- by delaying the transmission, you can get it to focus at different depths
- all the wave fronts will come together at a single point
19
Q
Amplification andTime gain compensation
A
- Echo signal generated at transducer elements initially undergoes linear amplification (overall gain)
- echoes returning from deeper structure are weaker and so need to be amplified to produce uniform tissue echo appearance
- manually controlled time gain compensation has a profound effect on image quality for interpretation - built into machien but can alter manually
- turn up overall gain - everything is turned up
- can turn up gain in just deeper part, or nearer part
20
Q
Image storage
A
- each echo is assigned to the image memory
- divides the image into a 2d array of pixels - each has “an address”
- if image writing is stopped, the image in the memory is repeatedly read out to the display
21
Q
The doppler effects
A
- the doppler shift frequency (fd) is the difference between the transmitted frequency (ft) and the received frequency (fr)
- if neither source nor observer are moving, frequency emitted will be equal to frequency absorbed
- if the source is moving towards the observer, the wave front gets pushed together = higher received frequency (fr > ft)
- if source is moving away from you, wave fronts get further apart, giving a lower received frequency (fr < ft)
- use it to look at RBC movements - can work out direction and speed of movement
22
Q
Doppler equation
A
- calculates the freq. shift when you hit a moving object (RBC)
- the angle matters, the best would be if the RBC was coming straight towards you
- the closer it gets to 90, the less accurate it gets when trying to calculate it
- if it is 90 degress, the cosin will be 0 - wont be able to see it
23
Q
Colour doppler
A
- red = towards, blue = away
- information on speed and direction of flow (probe face always at top)
- image noise results in random falshes of colour
- angle dependent (no flow shown at 90)
- each pixel has a colour - the brighter the hue, the faster the flow
24
Q
Power doppler
A
- nothing about speed - just strength of doppler signal
- Only detects blood flow but more sensitive to low vol flow
- angle independent as doesnt use doppler equation
- better boundary detection - nice to look at architecture of vessels
- very sensitive to soft tissue motion
25
Performing the scan
- referral
- machine
- appopriate surroundigns for where youre iamging
- probes - freq/array
- gel (reduce impedance mismatch)
- image record (thermal paper, film, CD etc)
26
Standard scanning planes
- Static images are usually recorded as Transverse section (TS) - right side of pt is on the left of the image
- Longitudingal sections (LS) - cranial aspect of patient is on the left of the image
27
Advantages and Disadvantages of US
A - real-time guidance, very high FPS, can visualise blood flow, very cheap, non-ionising, portable ones, dont need contrast injections
- D - sound penetration (bone and gas), operator dependence, patient size, artefacts
28
Acoustic shadowing
- ragion has much higher attenuation coefficient than adjacent tissues
- tissues deep to it appear black
- a lot gets absorbed in calcium to there will be shadow below it - useful for detectign gallstones
29
Acoustic enhancement
- region has a lower attenuation coefficient than adjacent tissues
- machine overcompensates - provides a clue to tissue composition
- cysts wont absorb as much and so more gets to the tissue behind it - gives bright stripe from overcompensation
30
Uses in medicine
- Imaging - abdominal, obstetrics, gynaecology, mammography, MSK, trans-rectal, cardiology, neonatal head, intra-operative, trans-oesophageal, vascular, IV ultrasound, point-of-care (needle guide)
- Doppler - vascular, cardiology in particular
- Therapeutic - physiotherapy, lithotherapy, High intensity focused ultrasound, surgery (scalpel)
31
Abdominal imaging
- good structural and textural info on liver, biliary system, spleen and kidneys
- more limited info on pancreas, adrenals, bowel and retroperitoneum
- good eval. of organ vascularity, able to detect vascular stenoses and aneurysms
- initial assessment in trauma
32
Pelvis (Gynae and prostate)
- good visualisation of uterus and adnexal structures
- needs full bladder for transabdominal approach, but transvaginal avoids this
- essential tool in management of infertility
- transrectal approach to prostate for diagnosis and biopsy and therapy
33
US of small parts
- expanding field due to improved probe design with better near field focussing and higher frequencies
- neck glands, subcutaneous tissues, MSK, scrot, breast, eye, skin
34
Obstetrics
- use of US dates back to A-scan
- used throughout pregnancy to diagnose, assess development, diagnose anomalies or complications (e.g. measuring nuchal fold to detect Down's)
- measure amniotic fluid vol
- assess placental function and position
- guide CVS, amniocentesis and foetal interventional procedures
35
Vascular
- peripheral and central vessels, arterial and venous
- arteries - artheromas, stenoses, occlusions, aneurysms, post surgical followups
- veins - DVT and central thrombosis, tumour invasion, incompetence
- venous/arterial access
36
Paediatrics
- often first line imaging investigation as no ionising radiation
- dont have to lie still
- very hard to do MRI as have to put them to sleep
- replaced plain films for diagnosing pyloric stenosis, intussusception, CDH
37
Chest
- limited because of air in lung
- confirms pleural effusions and guides drainage
- confirming consolidation
- pleural masses and peripheral lung masses
- diagnosing pneumothorax
- diaphragm movement
38
Brain
- almost exclusively confined to neonates as fontanelle acts as acoustic window
- doppler imaging possible in adults (at low freq)
- intraoperative US to locate and assess vascularity of tumour/masses
39
Cardiology
- trans-thoracic and trans-oesophageal
- assess myocarial function, shunts, ventricles, valve function, pericardial disease and proximal great vessels
- recent advances may allow myocardial perfusion assessment
40
Endoscopic/IV
- detailed assessments of biliary tree, pancreas, upper GI tumours
- regional lymph node staging
- stenting/drainage procedures
- evaluation of intimal plaque, wall movement and stent placement
41
intaoperative/Laparoscopic
- allows high resolution scanning of abdo organs, for detection of tumours (pancreas), liver metastases and CBD stones
- guidance for ablation procedures
42
Interventional US
- real time and portable offer advantages over other image guidance (irradiation of operator from CT and MR)
- biopsies, FNA, drainage procedures, vascular access
- guidance for ablation procedures
- sterile probe covers required
43
Microbubbles
- Tiny gass filled balls that are able to cross pulmonary circulation, and capillary beds
- vascular markers
- stay in body 5-10 mins
- very safe
- resonant frequency 2-15MHz - when you hit them, they act as a reflector and vibrate, generating a higher freq
44
Free-hand elasticity imaging
- US transducer is used to compress tissue
- compression is simultaneuosuly imaged
- images are further process to generated stiffness images
- uses shear wave - calculates physical properties of tissue
- can calculate which tissues are more easily compressed than others
45
Sono-ealstography
- breast - improve differentiation between benign and malignant disease - assess treatment
- thyroid - additional features of malignancy - target biopsy of complex lesion
- prostate - improve visualisation of cancer - target biopsy
- pancrease - differentiate between benign and malignant tumours
- Liver - obsever progressive degree of liber fibrosis
46
3D/4D ultrasound
- A = volume measurement, viewing orthogonal plane, assessment of surface and complex shapes, separation of B-mode data from colour
- D = acquisition, lots of artefacts
- 3d images produced either by segmenting out an object of image from sequential 2D images (time consuming and susceptible to errors), or 2D images built into a voxel based volume with gaps filled by interpolation (large data sets)
47
Multi-planar reformatting
- produces images in the 3 orthogonal planes
- can be rotated or scrolled through
- Power doppler information can be combined and on 4D systems this can be viewed in real time
