Additional views may include parasternal short axis and apical 4 chamber.
2.) Obtain one static representative image.
Medical Necessity: Evaluate for gallstones and dilated common bile duct and sonographic signs or cholecystitis, such as “positive sonographic Murphy’s sign,” thickened gall bladder wall, and pericholecystic fluid
Structures studied: Gallbladder (GB), common bile duct
Short Axis, Long Axis, Common Bile Duct
HOW TO SCAN - RUQ
1.) Obtain a video clip of the GB in it’s entirety, in both the longitudinal and transverse planes.
2.) Obtain a static image of the GB in the transverse plane with a measurement of the anterior GB wall.
3.) Obtain a static image of the Common bile duct (CBD).
COMMON BILE DUCT
<5mm-normal in patients up to age 50 (Add 1 mm for every decade of life thereafter)
Note: The left lateral decubitis, in addition to the supine position, is generally superior for the GB and CBD visualization.
Medical Necessity: Evaluate for intraperitoneal or pericardial fluid
Structures studied: Hepatorenal space, splenorenal recess, pericardial space, and pelvic views.
3 Components to a FAST Exam (include each as clinically indicated) : Abdomen, Cardiac, Thoracic
RUQ - include hepatorenal space, sub-diaphragmatic space, and inferior edge of the liver
LUQ - include splenorenal space, sub-diaphragmatic space, left paracolic gutter
Pelvis - transverse and sagittal views
Obtain any one of the Parasternal Long Axis, Apical 4 Chamber, or Subxiphoid Views
Anterior Chest - Right and Left
Lateral Inferior Chest, Coronal View - Right and Left
Visualize Diaphragm and Spine. These can be obtained concurrent with abdominal RUQ and LUQ views
HOW TO SCAN - FAST
1.) Video in the longitudinal plane, fanning through Morrison’s pouch (hepatorenal space) to include the inferior tip of the liver/inferior pole of right kidney.
2.) Video clip fanning through the bladder in both the longitudinal and transverse planes.
3.) Video clip in the longitudinal plane scanning through the spleen and kidney. Must include diaghram, spleno-renal space, and inferior tip of the spleen/lower pole of left kidney.
4.) Video clip of the subxiphoid cardiac view in the transverse plane.
5.) If subxiphoid view is techniquely inadequate, a video clip of the parasternal long axis will suffice.
6.) Obtain one static representative image.
Medical Necessity: Confirm intrauterine pregnancy (IUP) and evaluate for signs of ectopic pregnancy.
Structures studied: Uterus and its contents, vesicouterine space, and rectouterine space (Pouch of Douglas).
REQUIRED IMAGES: Transabdominal and Transvaginal Views
Transabdominal Views - Transverse and Sagittal Views of the Uterus
Transvaginal Views - Transverse and Sagittal Views of the Uterus
If Intrauterine Pregnancy Visualized, Document FHR and Crown Rump Length
HOW TO SCAN - EARLY PREGNANCY
1.) Obtain a video clip fanning through the uterus in its entirety in the longitudinal and transverse planes. Assess for uterine lie, presence or absence of free fluid and evidence of an IUP.
IF THE TRANSABDOMINAL SCAN IS NON-DIAGNOSTIC FOR AN IUP, PERFORM A TRANSVAGINAL SCAN.
1.) Obtain a video clip fanning through the uterus in both the longitudinal and transverse (coronal) planes.
2.) Obtain one static representative image.
To be diagnostic for an IUP, the uterus must be scanned in it’s entirety in 2 orthogonal planes showing a minimum of a gestational sac (GS) containing a yolk sac. The sac must be confirmed to be located within the endometrial cavity. This is best determined by demonstrating continuity between the endometrial cavity containing the gestational sac and the endocervical canal. Assessment for myometrial mantle all around the sac in 2 orthogonal planes must be confirmed. *If the myometrium is <6 mm, this is concern for a corneal/interstitial or ectopic pregnancy.
If evaluating only for pneumothorax bilateral anterior chest clips are sufficient. M-mode can also be utilized.
If evaluating only for pleural effusion bilateral lateral inferior chest views that demonstrate the diaphragm are sufficient.
If evaluating for parenchymal disease 10 zones should be assessed: Right and left anterior superior, anterior inferior, lateral superior, posterior inferior, lateral inferior.
Obtain clips demonstrating compression of the vein. Another option is to utilize dual mode and obtain side-by-side still images of each site uncompressed and compressed.
HOW IT WORKS
Ultrasound describes a sound wave with frequency above what humans can hear. The human ear operates in a frequency range of 20Hz to 20kHz. Anything above that range can be classified as ultrasound. The frequency range of diagnostic ultrasound varies from 1MHz to 20MHz. Similar to all sound waves, ultrasound requires a medium through which to transmit energy. The way in which this occurs depends on several acoustic variables:
Wavelength (λ) : the length (distance) of one complete cycle
Frequency ( f ) : The number of cycles per second
Period (T) : the time per wavelength (1/f)
Velocity (v): Speed at which the wave travels (v=fλ). AKA Propogation Speed
Power (P) : strength of the sound wave. Proportional to amplitude² (measured in watts)
The most important concept to understand in terms of medical ultrasound is velocity, as this varies dependent on the material through which sound is propagating. Ultrasound machines use this property to produce an image by using pulsed ultrasound. This means that the probe intermittently produces sound waves and has a ‘listening period’, during which it listens to the echo that returns from an object and uses the time difference and change in energy to make a judgment on the object's distance and composition. It can use the change in propagation speed (v) in different tissues to create a visual representation of the underlying material.
The ultrasound probe contains a piezoelectric crystal that vibrates at a specified frequency when current is applied to it. This causes a sound wave of that frequency to be produced from that probe. When this current is pulsed at specific intervals, it propagates sound waves through a given tissue. As this sound wave travels through the tissue, waves are either:
Absorbed: energy absorbed to tissue, usually through heat.
Reflected: as the sound wave approaches a boundary between tissue of 2 different densities, some waves bounce back toward the source
Scattered: sound waves which bounce back in direction not directly toward source
Refracted: path of sound wave is ‘bent’ as it travels between interface of two different densities
All of these cause attenuation, or weakening of sound energy as the wave passes through tissue. Each type of tissue has a theoretical amount of attenuation associated with it.
Note: this only applies to sound travelling in uniform tissues, and also depends on the frequency of the initial wave.
For our purposes in medical ultrasound, penetration is a more relevant measurement. This refers to the distance a wave travels through a tissue before attenuation causes enough loss of energy to prevent image formation. In general, the higher the frequency of the the initial wave, the lower the penetration.
Different tissues in the body also have different impedance values. This refers to the tissue’s resistance to sound propagating through it. It is directly related to density. This is important and essential to the use of ultrasound because the reflection of sound waves is dependent on the relative impedance between the two materials at an interface.
Generally, when the impedance (density) is very different between two tissues more sound is reflected.
For example: placing an ultrasound on a truly uniform substance yields a uniform appearing image. However, in areas with multiple different impedances (i.e: biological tissue) the reflections of sound waves at these interfaces allow you to ‘see’ these deep tissues as interfaces. In areas where the difference in impedance is very high (i.e: lung/tissue) almost all of the sound waves are reflected
Solids such as bone have high impedance, whereas liquids (fluid filled structures, as well as ‘solid’ organs such as liver/kidney, effectively function as liquids for US purposes) have low impedance values. Air has the lowest impedance, but propagation in air is complicated by high attenuation.
Importantly, the sound waves which are reflected to the source (in this case, the probe) are absorbed by the piezoelectric crystal, during the ‘listening period’. The crystal then vibrates, producing an electric current which is relayed back to the machine. This data is interpolated into an image. Since the reflections of sound waves from deeper structures take a longer time to travel to and return to the probe, we are able to determine the relative depth of different structures. These are displayed as a cross sectional image, with structures closest to the probe towards the top of the screen, and structures further from the probe lower on the screen.
Resolution refers to the ability to determine two closely spaced objects as two distinct points. There are multiple types of resolution in ultrasound:
Axial Resolution: The ability to differentiate objects close together depending on depth. This is better with higher frequency.
Lateral Resolution: ability to differentiate objects which are close together ‘side by side’. This is dependent on the width of the ultrasound beam, as well as the number of crystals in the ultrasound probe.
Temporal resolution-ability to track moving objects over time. Related to number of frames/sec. Higher frame rate allows for smoother image acquisition.
It is important to balance these properties in choosing the appropriate probe for a particular exam. A high frequency probe will have better axial resolution but poorer penetration. A probe with smaller footprint will generally have worse lateral resolution.
Note improved temporal resolution with higher frame rate on the right (same patient)
Ultrasound has specific terms used to describe the images in terms of relative appearance, or echogenicity
Echogenic/Hyperechoic: refers to tissue which appears brighter than surroundings
Hypoechoic: refers to tissue which is darker than surroundings
Anechoic: Appears black (such as in a simple fluid)
Solid/Reflective: no visible echogenicity behind object, casts a shadow (such as bone, or calcified material)
Homogenous: uniform in echogencity
Heterogenic: not uniform in echogenicity
In a perfect world, using ultrasound would produce a beautiful image every time. However, sometimes, the way information is gathered by the probe and interpreted by the ultrasound machine is not perfect. This leads to imaging artifacts.
This occurs when there is a mismatch between assumptions made by the ultrasound machine algorithm and the behavior of sound waves which return to the probe. Knowledge of these artifacts is useful in interpreting what type of structure is potentially producing them.
Shadowing: Reduction in reflected echoes deep to a strongly reflecting or attenuating structure
Example of shadowing artifact from gallstones
Posterior acoustic enhancement: Occurs when a fluid structure is juxtaposed near normal tissue. The sound is less attenuated through fluid than through surrounding tissue, causing the tissue behind (or deep to) the fluid structure to appear brighter, or more echogenic
Lateral Cystic shadowing (edge artifact): The reflection angle at the edges of a spherical structure do not allow reflection back towards the probe. Therefore this information is lost and appears as shadowing posterior to the lateral edges of these structures.
KNOW THE MACHINE
Run over probe cords
Hang probes from non-designated holders
Clean the probes and keypad before and after each use
Put machines where they belong in the department
Plug in machines following use
The University Hospital ED currently employs 2 different machines
It is important to care for the machines and their components!
These are the true workhorses of the ultrasound machine. Each probe contains piezoelectric crystals which vibrate under electric current. When these crystals vibrate, they produce sound that penetrates into tissues. The probe also acts as a receiver for returning echo, which in turn, cause vibration of the crystals that the probe converts to current, that is later interpreted as data by the ultrasound machine.
While there are many types of probes available for POC (point-of-care) ultrasound, there are 4 that are encountered most commonly.
Phased array: often referred to as the ‘cardiac probe’ this is a low frequency (1-5MHz) probe which is scanning the abdomen, retroperitoneal and even soft tissue structures. Given the low frequency, you can expect this to have good penetration, at the expense of resolution. Not great for visualizing small superficial structures. The phased array probe also has a small footprint (area of probe surface) which makes it ideal for getting between the ribs in cardiac studies.
Curvilinear or Curved Array Probe: often referred to as the ‘abdominal probe’. Also low frequency (2-5MHz). Functions very similarly to phased array probe except with a larger footprint, which allows for better lateral resolution.
Linear array probe: High frequency probe (5-15MHz). Unlike the above probes, the array is parallel, allowing for high lateral resolution, as well as maintaining resolution with depth. Due to this, will only see structures directly below probe. Ideal for superficial soft tissue evaluation, evaluation of neurovascular structures, and procedural use.
Endocavitary Probe: Moderate frequency (8-15MHz). Curved array. Ideal for Intraoral or trans -vaginal use.
A-mode: Rarely used. Plots echo as function of depth where x-axis is depth and y-axis is amplitude. Primarily useful in ophthalmologic/retinal ultrasound and for use of ultrasound beam therapy
M-mode: takes a small section of horizontal beam (as would be seen in B-mode) and displays it over time. Useful to track motion/dynamic changes in a specific point of an image over a period of time. Vertical axis remains depth, while horizontal axis is time. (used in cardiac valve imaging and fetal heart rate measurements)
2D or B-mode: Standard scanning mode. Spatially oriented- Structures are seen as function of brighness (echoic vs anechoic), depth, and width. Provides a cross sectional view of underlying structures. Able to see structures in real time.
Power Doppler: Similar to color doppler, but without assigning color to direction. Instead utilizes shades of color to represent level of flow. Useful for low flow states.
Color Doppler: Uses ‘doppler shift’, an induced change in frequency of returning echo to determine movement. Superimposes color to image to show flow either toward or away from probe, as indicated by a color key. By convention, movement towards probe is displayed red, and movement away as blue.
Color doppler used to show flow across mitral valve
Power doppler used to show low flow state in IVC/hepatic vein
Spectral doppler: Either via pulse-wave (alternating) or continuous (simultaneous transmission and reception). Allows for assessment of flow velocity at a specific point in the image which is then displayed as a function of time. Useful for echocardiography.
Spectral doppler used to quantify flow velocity across mitral valve
GETTING READY TO SCAN
Scanning Planes: Ultrasound images are a 2-D representation of the underlying structure. Just as in other imaging modalities such as CT scanning or MRI, the image is displayed as a cross section in a particular plane. However, ultrasound can be more complicated in that the plane of section is determined by the sonographer. Just as one can obtain axial images, one can also obtain images in oblique planes.
Transverse plane - AKA axial plane. Perpendicular to the long axis of the body. Separates top from bottom.
Sagittal plane - Parallel to the long axis of the body, separating left from right. Midsagittal plane refers to the plane in the midline as opposed to parasagittal which is either left or right from midline.
Coronal plane - Parallel to long-axis of the body, perpendicular to sagittal plane. Separates anterior from posterior.
Oblique plane - Not at right angles to any of the above.
When an object or organ of interest does not lie in a standard plane, we often refer to it in terms of its axis.
Long Axis: the plane which extends parallel to the maximum length dimension of an object
Short Axis: the plane which is perpendicular to the long axis of an object
Probe indicator: There is always a dot, ridge or groove on one side of the probe which functions to assist with orientation. This allows one to determine which side of the probe corresponds to which side of the screen. By convention the indicator on the screen is a blue dot located on the top left of the screen image. By convention, the probe indicator has a specific position based on the imaging plane:
Transverse plane: Probe indicator towards the patient’s right side
Sagittal and Coronal planes: Probe indicator towards the patient’s head
HANDLING THE PROBES
It is important to have stable grasp on the probe, as the acquisition of a good image is often dependent on minute adjustments. In order to be able to make fine adjustments with the probe hand, it is best to hold the probe in the dominant hand with the thumb and index finger, using the other 3 fingers and ulnar aspect of the palm as a sort of ‘tripod’ to brace against the patient’s body.
Sweep: performed by sliding the entire probe in direction of its short axis over the surface of the body with a 90 degree angle of insonation-generally used to find a good imaging window.
Sliding: performed by sliding the entire probe in direction of its long axis over the surface of the body with a 90 degree angle of insonation- generally used to find a good imaging window.
Rock: similar to fanning, but in the left-right dimension (long axis) of the probe. Allows sonographer to extend imaging area to desired target not immediately below probe.
Fanning: performed by adjusting the angle of the probe body to the skin in anterior-posterior dimension (short axis) of the probe. This allows the ultrasonographer to scan through different planes of an organ from one imaging window.
Pressure/Compression: Pressure/Compressive Force on the probe into the body- used to manipulate vessels/tissues in areas of interest.
Rotation: performed by turning the probe on its long axis. This can allow the provider to obtain an image in an area with small imaging window (i.e ribs), or to obtain appropriate plane of section given variances in anatomy (i.e cardiac views).
Modes: Machines often preloaded with imaging modes for particular organs/indications which optimize settings for study of interest (e.g cardiac, vascular, abdominal, etc.)
Depth: Adjust depth of field presented on screen
Focus: Improve resolution at a particular depth in image (not available on all machines)
Gain: Adjusts the strength of the signal returning to the probe- visualized on screen as contrast
Time gain compensation: Adjusts the gain for a specific depth in the image
AutoGain: allows the machine to calculate optimal gain settings
Calipers: used for measurement of distance