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What is ultrasound?
Ultrasound is a common imaging modality that allows visualisation in real-time. As such it is becoming increasingly popular on the wards for diagnosis and management purposes. You should be familiar with its operation and know in which situations it may help your clinical decision making.
What is ultrasound used for?
Ultrasound can be used for:
Assessment of jugular venous pressure (JVP)
Focused assessment for screening in trauma (FAST)
Evaluation of abdominal organs
How does ultrasound work?¹
1. High-frequency sound waves are transmitted from a transducer.
2. These sound waves are then reflected by different tissue types in different ways.
3. The reflected sound waves are then picked up by the ultrasound transducer.
4. The sound waves are then transformed into an image by special software.
How do tissue types differ in their reflection of sound waves?
Bones, fat and stones
Bones, fat and stones produce a hyperechoicsignal.
A hyperechoicsignal is bright as most ultrasound waves are reflected.
Cartilage and muscle
Cartilage and muscle produce a hypoechoicsignal.
A hypoechoicsignal appears dark as most waves pass through the tissue.
Fluid and fluid-filled structures
Fluid and fluid-filled structures produce an anechoicsignal.
An anechoicsignal appears black as there is no reflection of ultrasound waves.
A shadow may be noted on an ultrasound when a hypoechoicarea is located behind a hyperechoicstructure.
The firststeps of performing an ultrasound involve:
Turning on the machine (easy, but often overlooked; often a button in the upper left or right corner of the keypad).
Entering the patient’s information (e.g. name, date of birth, hospital number).
Selecting an appropriate ultrasound probe for the area being examined.
How do I know which probe I should use?
Typically there are 3 different types of ultrasound probe: linear, curvilinear and phased.
High frequency (7-15MHz):
High resolution but superficial (1-6cm) depth
Good for vascular access, nerve blocks, assessment of testes and superficial lung tissue
Low frequency (2-5MHz)
Low resolution, but greater depth (10-20cm)
Useful for abdominal, pelvic, obstetric and deep lung tissue
The lowest frequency (1-3MHz)
Useful for echocardiography
How do I hold the probe?
The image below demonstrates how to appropriately hold an ultrasound probe.
Typically, there is a dot or a cross on the probe, this correlates with a dot on the left side of the screen.
This marker should be toward the patient’s right in transverse and head in longitudinal.
If you are unsure, it is best to place your finger on one side of the probe and look for movement on the screen (the side that shows movement by the dot is the side that should face the patient’s right).
Once you’ve chosen an appropriate probe and are holding it right, the next steps of performing an ultrasound involve:
Applying gel to the probe and patient.
Placing the probe onto the patient and observing the images on the screen.
Adjusting the settings to achieve an optimal view.
Common settings for achieving an optimal view
Adjusting the gain of an ultrasound changes the brightness of the image.
Gain is typically controlled by a knob.
The gain should be adjusted until fluid appears black and soft tissue appears mid-grey with some parts of the image appearing white
Depth measures are shown in cm on the side of the ultrasound monitor.
It is often best to begin deep to orientate yourself and then work more superficially to bring the object of interest into the middle of the screen.
General tips for achieving an optimal view
Some general tips for achieving an optimal view include:
Use lots of gel
Make good contact between the probe and skin (whilst ensuring the patient is comfortable)
Dim the lights to improve your view of the monitor
Ensure the probe is perpendicular to the skin
Measuring the JVP with ultrasound
1. Position your patient as you would when assessing the jugular venous pressure (JVP) in a clinical exam (e.g. supine, head of the bed at 45°, patient’s head laterally rotated to the side not being scanned)
2. Set the gain of the ultrasound to mid-range.
3. Apply gel to the patient’s neck.
4. Place the probe in a transverse orientation within 2cm of the clavicle.
5. Identify the internal jugular vein (IJV) and the carotid artery, assessing the following:
Wall thickness: arteries have thicker more muscular walls than venous structures.
Shape: the carotid will be circular whereas the IJV can be oval or irregularly shaped.
Compressibility: veins are easily compressed (if you only see one vessel, use less pressure as you may have fully compressed the IJV).
Respiratory variability: central venous structures will fluctuate in size with respiration.
Position: the IJV is usually (but not always) lateral to the carotid.
6. Centre the probe so the IJV is centred on the monitor.
7. Slowly rotate the probe keeping the IJV in the centre until a sagittal view is achieved (ensure you are not foreshortening the vein by carefully moving the probe medially and laterally).
8. Locate the point of the initial collapse of the IJV (centre the probe over this point).
9. Measure the JVP height above the sternal angle as normal.
Ultrasound-guided intravenous access
In this section, we will be focusing on peripheral intravenous access, however, similar principles are applied for central venous line insertions.5,6
Indications for ultrasound-guided intravenous access include:
Multiple failed attempts
History of difficult cannulation
Ultrasound-guided IV access should not supplant intraosseous (IO) access in life-threatening situations.
A high frequency (5-12 MHz) linear transducer is typically used as high frequency permits a better resolution of structures close to the surface of the skin
A lower frequency curved probe may be more effective in obese patients.