When we see an Ultrasound system, we see control panel full of knobs. It is usually said that an Ultrasound system has number of controls as that of a Fighter Jet. The reason is because the system can be used in multiple modes of operation depending on the application and every mode has its own set of controls.
The key modes of ultrasound that can be used to obtain anatomical and physiological information are:
1. 2-D mode or B-mode (Brightness mode) 2. M-mode or motion mode 3. Colour flow Doppler mode 4. Pulse wave Doppler mode 5. Continuous wave Doppler mode
6. Tissue Doppler mode
7. 3D/4D Ultrasound mode
2-D or B-mode:
This is the most used mode in an ultrasound system and is available in any basic (low end) to the most advanced ultrasound machines. Other modes may or may not be available depending on how advanced the machine is.
A 2D cross sectional view of the anatomical structures beneath the scanning plane is obtained from the array of piezoelectric crystals arranged inside the probe. The shape of the 2d image created depends on the type of scanning probe used i.e. Convex, Linear, Cardiac, Endocavity etc.
Number of 2d images are created every second and are displayed on monitor to show a continuous real time image of the underlying structure. Higher the number of frames per second (FPS) the more continuous the image will appear. The FPS depends on how powerful the processing capability of the machine is.
The intensity of the reflected signal depends on the type of tissue it encounters. The bones have high reflectivity and are shown as white; muscles have low reflectivity so they appear grey and water does not reflect any ultrasound, so it appears black on the screen.
Deeper structures are displayed on the lower part of the screen and superficial structures on the upper part.
Fig 1: An example of 2D imaging: Abdominal Cavity showing Liver,
Inferior Vena cava, Aorta and Vertebra (Short Axis View).
The 2-D mode is helpful in determining tissue differentiation, physical anomalies, presence of cysts (liquid filled abnormal growth) and calcification (e.g. Kidney stones), size measurement of organs and many more.
M-mode:
This represents movement of underlying structures over time (as shown in Fig.2).
First a 2D image is acquired over which a single M-mode scan line can be set along the area of interest. The M-mode when activated will then show how the structures intersected by that line move toward or away from the probe with respect to time.
Fig 2: M Mode showing movements of the mitral leaflets over time
The M-mode has good temporal resolution, so it is useful in detecting and recording rapid movements such as heart valves.
M-mode is commonly used for measuring chamber dimensions of heart and calculating ejection fraction, measuring foetal heart rate (Fig.3 show foetal HR using M-mode) etc.
Fig 3: M mode through the left ventricle of foetus
showing movement of the walls over time.
Colour flow mode (Doppler imaging):
In this mode, the velocity and direction of blood flows are shown in a colour map superimposed on the 2-D image.
It uses Pulsed wave Doppler principle to obtain the colour information that shows location and direction of the blood flow.
To obtain accurate colour flow information the adjustment of velocity scale (PRF), Gain and the angle of incidence of the PW signal are very crucial factors.
Fig 4: Schematic diagram of Colour Doppler Principle
The conventional method of interpreting the blood flow is blue showing flow away from the probe and red showing flow towards the probe, although it can be changed (Refer Fig. 4 and 5). Blood flow perpendicular to the probe will appear black. Areas of turbulent flow may be depicted as green or white in colour.
Fig 5: Carotid artery and jugular vein in Colour Flow mode
Colour flow mode helps in localization of blood vessels (as landmarks for imaging) and detect blood flow in various parts of the body (e.g. Thyroid).
The best use of colour flow mode can be seen in detecting leakage of blood through heart valves (e.g. mitral regurgitation shown in fig.6).
It can also be used to guide the accurate placement of the cursor for pulse and continuous wave Doppler.
Estimates of velocity and direction of blood flow are not as accurate as in Continuous wave or pulse wave Doppler.
Fig 6: Four Chamber view of Mitral valve Regurgitation in Colour Flow mode.
Pulse wave Doppler mode (PWD) :
In simple terms PW Doppler is sending pulses of ultrasound waves into the scanning plane and receiving the signals with change in frequency (i.e. Doppler shift) which will be dependent on the velocity and direction of blood flow.
The amount of frequency change will give the velocity of blood and the positive or negative change in frequency will give the direction of blood flow.
The precise location at which the PW Doppler is to be monitored can be set by placing the PW cursor using 2D mode + CF mode (Duplex mode) image as guidance.
Fig 7: Carotid artery in PW Mode
Major drawback of PW Doppler is that it cannot measure higher velocities. Measuring High velocity flow causes “Aliasing” effect. Aliasing occurs when the change in Doppler frequency exceeds ½ of the Pulse Repetition Frequency (PRF). The PRF has limitation for the maximum value because only a single crystal is involved in transmitting and receiving the PW signal.
This limitation is overcomed by CW Doppler because of continuous transmission and reception of ultrasound waves by two separate crystals which allows PRF to be much higher than PW Doppler.
PWD is used to analyse and measure velocities of flow in Superficial, Abdominal, Peripheral and Cardiac areas.
Continuous Wave Doppler (CWD):
Unlike PW Doppler in CW mode simultaneous transmission and reception of ultrasound waves (using two different crystals) is done along a single line that is set on 2D image.
This allows to detect huge Doppler shifts that occur while measuring high velocity blood flows.
Information regarding the location of the blood flow is not possible unlike in PW Doppler.
As all the velocities along the set line are measured, the waveform is filled with overlapping information of velocities (shown in fig.8)
Fig 8: Continuous Wave Doppler of a Tricuspid valve
CW Doppler is mainly used to measure very high blood velocities like regurgitation velocities of Pulmonary, Tricuspid, Mitral, Aortic valve, and velocity of systolic flow through the aortic valve
Tissue Doppler Imaging mode (TDI) :
As PW Doppler is used to study the velocity of blood flow, Tissue Doppler is used to study the velocity of moving tissues (e.g. Heart valves) using the same principle of Doppler shift.
The velocity of moving tissue is much lower than blood velocities.
The sample volume is placed over the area of interest (e.g. Intraventricular Septum shown in Fig.9) using 2D image for placement guidance.
The TDI waveform is very similar to that of PW Doppler waveform.
An audio can be heard in real time of the moving tissue (audio is generated from Doppler data).
Fig 9: Tissue Doppler Imaging of Intraventricular Septum
in Synchronization with Heart rate.
The application of Tissue Doppler Imaging includes measurement of tricuspid and mitral annulus velocities to assess RV systolic function and estimate LA pressure.
3D/4D Ultrasound mode:
· 3D ultrasound uses special type of scanning probe which captures 2d images from multiple angles of the target structure (e.g. Foetal face in Fig.10) with the help of rotating motor inside the probe. The 2d images are then combined to reconstruct a 3D image of the foetus.
· 4D imaging uses the same principle only the image reconstruction is done in real time which allows live 3d image of the foetus to be seen on the monitor.
· Powerful hardware is required for 3D and 4D imaging ultrasound machines as lot of data needs to be processed in short period of time to show a 3D image with high frame rate.
Fig 10: 3D foetus with Live HQ feature
3D ultrasound is useful in characterization of some congenital defects, such as skeletal anomalies and heart issues. Also, 3D ultrasound has important application in Gynaecology for visualization of Uterus anomalies (as shown in Fig. 11)
Fig 11: 3D image of a septate uterus. The coronal view clearly depicts the normal external contour of the uterus and the deeply indented fundal part of the cavity
Role of Ultrasound in COVID -19 diagnosis:
As CT scan is suggested for diagnosis of COVID patients, the equipment may not be readily available in every Emergency Department. In that case Ultrasound can be used as an alternative diagnostic tool says a group of emergency physicians in Italy (One of the severely affected countries due to the pandemic). Physicians affiliated with the Department of Emergency Medicine at Ospedale Guglielmo da Saliceto in Piacenza, Italy says- Lung ultrasound is a surface imaging technique strongly recommended for assessment of acute respiratory failure. Also, lung ultrasound is commonly used in Emergency Departments to diagnose pneumonia right on patients’ bed. Dr. Erika Poggiali, MD, and colleagues observed “strong correlation” between ultrasound and CT findings from an analysis of 12 patients with flu like symptoms and COVID-19 pneumonia.
Fig 12: Sonogram taken under rib cage shows liver (grey) with curved diaphragm-lung border (white). Arrows point to vertical B lines (white) demonstrating diseased lung tissue. The more B lines the worse the disease. Healing is measured by reduction in the number of B lines.
The correlation between CT and ultrasound included bilateral lung involvement with ground-glass opacity. In addition, a "crazy-paving pattern" was noted in 5 of 12 patients, while organising pneumonia was detected in four patients using CT and later confirmed with lung ultrasound.
Fig 13: Multi time point tracking of patient disease progression (Lung CT of Covid-19 patient)
"We are aware that our data are preliminary and further studies are necessary to confirm the role of lung US in the diagnosis and management of COVID-19 pneumonia, but we strongly recommend the use of bedside US for the early diagnosis of COVID-19 pneumonia in all the patients who presented to the emergency department with flu-like symptoms in [the] novel COVID-19 era," wrote Dr Poggiali and colleagues.
References for COVID-19 article: –
Written by:
Karan S. Jejurkar
Former Application Specialist for Ultrasound
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