Vascular Science Flashcards
Types of conditions a vascular scientist encounters?
- Claudication/ limb ischaemia (Peripheral vascular disease)
- TIA/ stroke (cerebrovascular disease)
- Aneurysm
- Deep vein thrombosis
- Varicose veins
What are the 3 vascular beds in the body?
- Peripheral : PVD is a disorder of the blood vessels outside of the heart and brain
- Cerebral: Cerebrovascular is a disorder of the blood vessels leading to and in the brain
Coronary: Coronary disease is a disorder of the blood vessels feeding the heart
What can vascular disease do to blood vessels?
Can cause blood vessels to:
- widen (aneurysm)
- narrow (atherosclerosis/thrombosis)
- spasm (arteritis)
This can occur in arteries or veins
What are the 5 types of blood vessels? What are their functions?
Arteries: Transport high pressure blood from the heart to smaller arteries and arterioles
Arterioles: Connect arteries and capillaries
Capillaries: Allow gas exchange, nutrient transfer and waste removal between blood and tissue fluid
Venules: Connect capillaries and veins
Veins: Act as a reservoir of blood and transport low-pressure blood from venues to heart
Properties of arteries, capillaries and veins?
Arteries: blood away from heart, thicker walls, withstand high pressure
Capillaries: Walls 1-cell thick, exchange gases between blood and tissue cells
Veins: Blood back to heart, thinner walls, low pressure, large lumen, valves prevent back flow of blood, skeletal muscles enhance venous return
What are the layers of arteries and venous vessels?
- Tunica intima
- Endothelium
- Subendothelial layer
- Internal elastic lamina (arteries)
- Tunica media (smooth muscle and elastic fibres)
- External elastic lamina (arteries) - Tunica externa (collagen fibres)
Constituent’s of capillaries?
simple squamous epithelial (endothelial) cells forming a basement membrane (tunica intima)
Risk factors of vascular disease, CVD and coronary artery disease?
Raised cholesterol
Diabetes
Smoking
High blood pressure
Genetics
Gender
Age
Obesity
Vascular disease assessments
- Clinical signs & symptoms
- Physiological measurements (ABPI, toe pressures)
- Ultrasound/Doppler
Diagnostic tests performed for vascular disease?
Duplex (Doppler ultrasound)
ABPI
Is ultrasound safe?
British Medical Ultrasound Society (BMUS)
Ultrasound should only be used:
For medical diagnosis
By trained professional
For the shortest time achievable
With the lowest power output achievable
Generally considered safe as it doesn’t use ionising radiation to create images, only high frequency sound waves. It has been used in medical imagery for decades, and is a painless procedure. Ultrasound machines display mechanical and thermal indexes as a way to reduce ultrasound-induced bioeffects. Thermal index: ultrasound induced bioeffect in which sound waves absorbed by the skin could heat tissue. Mechanical: ultrasound beam can cause non-thermal effects such as cavitation (when a liquid is exposed to high frequency ultrasound, gas pockets form then collapse violently - can cause tissue damage.
What is ultrasound?
Used for diagnostic observation for over 50 years
Continued research
US government relaxed regulations in 1992 to allow power output x8 times prev level
This allowed better image quality but long term effects unknown
What are the risks associated with ultrasound?
1.Thermal effects
(some u/s machines are capable of causing a 6 degree increase) potential risk to foetus/eye
2.Mechanical effects (cavitation)
Pulsation of air bubbles can cause increase movement of cell, cell
damage, increase in platelets
How else can ultrasound be used?
Lithotripsy to break up kidney stones
How do we reduce risks of ultrasound?
‘ALARA’ principle:
As
Low
As
Reasonably
Achievable
Reduce the power output
Only use B mode where possible keeping colour and spectral to a minimum
Reduce exposure time
Keep the probe moving
Change probe frequency
Move focal zone
When should care especially be taken?
Imaging tissue with low blood supply -The eye
Imaging air filled areas – lungs/bowel
Using focused beam
Using spectral/colour Doppler
Areas near bone – Foetus
Using contrast agents (microbubbles)
How does the ultrasound machine ensure safety?
U/S equipment now has to display 2 figures which warn the operator of these risks
These are; Thermal Index (TI) and Mechanical Index (MI)
TI – Gives the indication of the relative potential for a tissue temperature rise
MI – gives the indication of the relative potential for ultrasound to induce an adverse bio effect by anon thermal mechanism such as cavitation
For Vascular Studies, what should TI and MI be?
TI should be kept below 1.0 (BMUS)
MI should be kept below 0.7 (BMUS)
Limitations of Doppler ultrasound?
- Operator dependant skill
- Calculation error (5% angle error can result in measurement error)
- Calcification
- Bowel gas
Are there any side effects to ABPI?
No known adverse effects of continuous wave Doppler
However;
Should not perform an ABPI on a patient with a fistula, had a mastectomy or a distal bypass graft
Be aware of any existing heart or breathing problems in patients who are to perform an exercise test
Limitations fo ABPI?
May give inaccurate readings if vessel calcified, large dressings or oedematous leg
How do you ensure equipment error/safety?
Ensure equipment serviced
Ensure regular equipment QA
What is ultrasound?
Sound is a form of mechanical energy
Sound is a vibration that is transmitted through an elastic solid, liquid or gas
Sound is measured in frequency - Hertz (Hz – cycles per second)
Audible frequency is in range 20 – 20,000Hz
Ultrasound is all sound above audible range so >20KHz but typically 2 – 12MHz for medical applications.
Infrasound is <20Hz.
Ultrasound is a noninvasive imaging test that shows structures inside your body using high-intensity sound waves. An ultrasound picture is called a sonogram.
A device called a transducer, or probe, emits high-frequency sound waves into the body.
These sound waves travel through the body and are reflected back to the transducer when they encounter different tissues and organs.
The transducer contains piezoelectric crystals that convert electrical energy into sound waves and vice versa.
The transducer detects the returning sound waves (echoes) and converts them into electrical signals.
A computer analyzes the electrical signals, taking into account the speed of sound and the time it takes for the echoes to return, to create a two-dimensional image of the internal structures.
Different tissues and organs reflect sound waves differently, allowing the ultrasound machine to distinguish between them.
Real-time Imaging:
Ultrasound imaging can provide real-time images, allowing doctors to see the movement of organs and blood flow.
A special type of ultrasound, called Doppler ultrasound, can show the movement of blood flow and other moving structures.
Ultrasound imaging is considered safe because it does not use ionizing radiation
How is U/S generated?
Piezoelectric effect
Deforming some materials will produce a voltage, and conversely applying a voltage will cause deformity.
Applying an alternating voltage to the transducer causes the piezoelectric material to deform/oscillate which produces ultrasound.
Frequency of oscillation is proportional to frequency of ultrasound produced.
Typical material is lead zirconate titanate – made piezoelectric by placing in a strong electric field at high temp during manufacture.
Combining piezoelectric ceramic with non-piezoelectric polymers improves performance of the performance of the transducer.
Types of ultrasound
Continuous wave and pulsed wave
Features of continuous wave U/S
- Used for audible or spectral analysis of blood flow
- Continuously sending and receiving sound waves
- Uses Doppler principle
- Can’t produce an image
- Hand-held Doppler
Features of pulsed wave U/S
Pulsed-wave Doppler ultrasound utilizes the Doppler effect, where the frequency of sound waves changes as they bounce off moving objects (like red blood cells).
It sends out short bursts of ultrasound and then listens for the returning echoes.
By analyzing the frequency shifts of the returning echoes, the ultrasound machine can determine the velocity and direction of blood flow.
It allows for the measurement of blood flow velocity at a specific point in the vessel (range resolution).
The user defines a small area (the sample “volume” or “gate”) within the B-mode image, and only the Doppler shifts from that area are recorded
-
- - Used for imaging, colour and spectral analysis
- Uses Doppler effect and pulse-echo principle and detects time between pulses, strength of pulse
- Location of source of reflected pulse is displayed in position on image and brightness reflects strength of signal
What is the Doppler effect?
Doppler shift is the difference between the emitted frequency and the echo frequency returning from moving target
Think of change in sound as an ambulance travels past you.
How is an ultrasound image produced?
- Pulsed wave ultrasound
- Beam of ultrasound directed into the skin
- As sound hits a target the sound wave is reflected back to the transducer
- Time for reflection detected
- Strength of returning signal displayed as brightness of pixel
What are some types of image resolution?
TEMPORAL RESOLUTION
How quickly the image updates – important for real-time imaging.
AXIAL RESOLUTION
How well the machine can distinguish two objects along the axis of the beam.
Determined by length of pulse: need short pulses for good axial resolution.
LATERAL RESOLUTION
How well the machine can distinguish two objects perpendicular to the beam.
Determined by beam width/focussing.
Transducer structure
Protective surface layer
Matching layer – improves sound transmission
Piezoelectric element – rows of rectangular shaped crystals
Backing or damping layer reduces pulse duration – improves axial resolution.
Lens – to focus beam
How controls affect image
- Gain – increases intensity of the returning echo
- Time gain compensation -increases intensity of the echo at a particular depth compensating for attenuation.
- Depth of image or penetration of ultrasound – increases as frequency decreases, is limited by attenuation (weakening of signal).
- Focus – best quality image at this point.
Controls on ultrasound
- Focus – beam can be focused by using several elements with a range of delays between excitation.
- Wave fronts will interfere producing a concave beam
- Narrow beam improves image at that point
What is attenuation?
- Weakening of the sound wave as it travels
- Mostly caused by energy conversion from sound to heat in the tissues (absorption).
Some weakening caused by scattering and refection of sound waves.
What affects image quality?
- Attenuation of signal
- Depth vs resolution
- Low frequency/low resolution/deep
- High frequency/high resolution/shallow
- Calcification ( acoustic shadowing)
- Oedema
- Controls!
Properties of colour flow?
- Key tool for assessing blood flow
- Doppler Effect
- Probe detects flow toward or away from probe
- Colour displayed according to colour map (red/blue)
- If Doppler shift increases calculated speed increases
- PRF – pulse repetition frequency controls colour display
- Different settings for different applications
Properties of power doppler
- Displays amplitude of echoes rather than speed/direction.
- Is more sensitive to slow, deep or narrow flow.
- Useful for distinguishing occlusion/trickle flow.
Is independent of angle
Properties of spectral doppler
- Doppler effect.
- Used to work out velocity – essential for grading stenoses.
- Doppler equation:
FD= Ft x (2 x v x (cos Ɵ)
c
Where FD = Doppler shift frequency
Ft = transducer frequency
V – velocity
c – speed of sound (in tissue 1540cm/s)
Ɵ – Doppler angle
Spectral display
‘x’ axis displays time
‘y’ axis displays velocity
Spectrum displays acceleration/direction of flow across cardiac cycle.
Angle dependent – try to maintain angle at 60°
Assess velocity, turbulence, signal strength, damping (time).
Waveforms
For peripheral arteries (carotid arteries have different waveforms):
- TRIPHASIC
Healthy, no significant disease
- BIPHASIC
Mild disease, aging
- MONOPHASIC
Significant disease (usually)
Quality assurance ultrasound
Equipment:
Regular Instrumentation service
QA testing using phantoms
Can check callipers, spectral Doppler etc.
Crystal drop out
Operator:
Regular audit
Against other staff
Against another modality eg. MRA or angiography
CQC guidelines
Different Vascular tests I’ve observed, and can I describe them a little?
- Carotid scans
- Deep Vein Thrombosis (DVT) lower limb extremities scan after + d-dimer results (>500ng/ml) and/or >2 WELLS risk score
- Abdominal aortic aneurysm screening
- Arteriovenous dialysis fistula scan
- Varicose vein evaluation
- Peripherally Inserted Central Catheter (PICC line) scan for thrombosis
- Artery assessment – narrowing or blockages that could suggest stroke risk
- Post-operative stent/graft monitoring
- General vascular investigations
Vascular Duplex ultrasound explained
Duplex ultrasound employs both Brightness mode (B-mode) and Doppler (colour flow and spectral) to visualise vessel anatomy and blood flow. B-mode is the most standard image produced by ultrasound, with greyscale images of tissue and structural anatomy. Colour flow allows for the assessment of lumen patency, possible stenosis or any occlusion that may be missed with B-mode. Spectral Doppler displays blood flow velocity and can be used to assess any potential stenoses or occlusions.
What would normal, narrow and stenosed flow look like on spectral Doppler?
Normal - peaked
Narrowed - spectral broadening
Stenosed - High-velocity marked spectral broadening
What is triphasic
x
