Sound waves are propagated at a speed of approximately 1540 m/sec in soft tissues.

The thickness, size and location of various soft tissue structures in relation to the origin of the ultrasound beam are calculated at any point in time using an 'echo technique'.

The strength of the reflected sound wave depends on the difference in "acoustic impedance" between adjacent structures. The acoustic impedance of a tissue is related to its density; the greater the difference in acoustic impedance between two adjacent tissues the more reflective will be their boundary. Higher frequencyultrasound waves have a longer near field and less divergence in the far field; they permit better resolution of small structures. More energy however is absorbed and scattered by the soft tissues so that higher frequencies have less penetrating ability.

Conversely, a transducer producing lower frequencies will provide greater depth of penetration but less well defined images. Focusing and aperture control technology are often employed to narrow the beam along it's entire path to achieve maximum right-and-left (lateral) resolution. The transducer of a real-time scanner typically contains over 300 crystals( piezo electric devices) arranged in a row where each emit and receives an ultrasound beam in rapid succession to form a sweep. The part of the abdomen under the probe is "swept" about 30 times (frames) a second and a moving picture (a real-time picture) will be formed (not unlike the principle in a movie projector). Beam density and dynamic range control technologies are further being incorporated into each scanner's design to optimize the resultant image.

Ultrasound is a non-invasive method (the patient does not have to be cut open or have anything inserted into him/her) of monitoring foetal development using ultrasonic acoustic waves. It does not employ ionizing radiation, which would be dangerous foetus, as radiation risk is greatest when much cell division occurs, and disruption of DNA would be most harmful when organs are developing.
 

Ultrasound is considered safe. The AIUM (American Institute of Ultrasound in Medicine) in 1977 announced that "no independently confirmed significant biological effects have been demonstrated in mammals" exposed to acoustic intensities of less than 100 mW cm².
Over the last twenty years collected data reveals that this is a conservative upper limit.

• localized temperature rise in tissue,
• cavitation (growth of gas bubbles),
• steady radiation force exerted on tissue structures (which may result in movement),
• streaming agitation in liquids,
• shear stress of objects in the streaming liquid and the
• oscillatory force of the sound field on all structures

but these are only observed at acoustic intensities well above the limit.

here is no room for complacency, though. As with all scanning procedures exposure times and intensities should be kept as low as possible. Patient exposure should be minimized and logged carefully, especially in the case of pregnant women. Output from machines can vary considerably and manufacturer's specifications should be verified. The machine should be regularly checked to ensure that output is 'true'.
 

Pregnancy problems can be detected and general progress quantitatively monitored by using A-scans for accurate measurements and B-scans for general development.

A B-Scan produces a sectional 2-D image. Each point on the monitor represents echo amplitude by 'grey-scale' representation. The brighter the point on the screen the 'louder' the echo. This is used to identify the part of the foetus to be measured. A multiple array of transducers is used to gain the information that builds up into the 2-d representation, whereas an A-scan probe only needs a single transducer..
 

A simple ultrasound probe with a curved transducer array is ideal for obstetric use as it has no moving parts, is comparatively inexpensive and can be used by relatively unskilled operators. A frequency of between 1-3 MHz is suitable for abdominal imaging as low frequency produces low resolution imaging and increasing attenuation occurs at very high frequency.
 

Accurate distances within the eye can be measured using a type of probe designed especially for ophthalmic use. The scanner is either placed in direct contact with the eye or via a water bath (less risk of damage to eye surface) it therefore needs to be small and the transducer head suited to the curvature of the eye. It is used in A-scan mode with a frequency of 8-13MHz

  The anatomical structure within the kidney can be viewed using a common curvilinear (see below) probe with a frequency of 5MHz in B-scan mode. The higher the frequency the better the resolution of the image - 5 MHz will give detail of structure to within 1mm. A simple scan technique is fine for this application.

The angle q needs to be known if v is to be calculated accurately. This is not easy and calculations of v are rarely performed. Rather comparisons are made between v in comparable arteries and veins to detect blockages.

• resolution (better for larger frequency (about 1mm in soft tissue for 5MHz))
• depth of penetration (large frequency cannot probe as deeply).

Patient undergoing a kidney scan                                                                          B-scan of liver and kidney

Ultrasound transducers, often called probes, come in different shapes and sizes for use in different scanning situations. For example, in an obstetric scan the probe used is usually one that looks like a curved soap bar (more accurately known as a 'convex-array' transducer, and the one with a flat surface a 'linear-array' transducer ) which can be slid over the maternal abdomen while maintaining good surface contact to the abdominal surface for the whole width of the probe. In a vaginal scan, the probe has to be a long and slender piece to fit into the vagina. Pulsed ultrasound, because of it's high frequency can be aimed in a specific direction and obeys the laws of geometric optics with regard to reflection, transmission and refraction. When an ultrasound wave meets an interface of 'differing echogenicity' (big difference in acoustic impedance between the media), the wave is reflected, refracted or absorbed. Reflected sound waves are processed. The transducer, though emitting ultrasound in rapid pulses, acts as a receiver most of the time. The ultrasound images can be displayed on an oscilloscope screen or a video monitor (via what is known as a scan converter) and can be recorded on videotape, thermal paper or radiographic film.

B and M mode images of the heart at the level of the mitral valve                         M-mode image of the heart
 
 

NB: M-mode is not on the syllabus neither is the Doppler equation - they are in these notes to extend your background knowledge. Calculations will be restricted to simple echo or simple Doppler equation applications.
 

Useful Websites to visit:

http://www.ob-ultrasound.net/

The following is an extract from this site (taken in March 1999). The site is well worth a visit!

Doppler Ultrasound

The doppler shift principle has been used for a long time in foetal heart rate detectors. Further developments in doppler ultrasound technology in recent years have enabled a great expansion in it's application in Obstetrics, this time in the area of assessing and monitoring the well being of the foetus.
Blood flow characteristics in the foetal blood vessels can be assessed with Doppler 'flow velocity waveforms'. Diminished flow, particularly in the diastolic phase of a pulse cycle is associated with compromise in the foetus. Various ratios of the systolic to diastolic flow are used as a measure of this
compromise. The blood vessels commonly interrogated include the umbilical artery, the aorta, the middle cerebral arteries and the uterine arcuate arteries.
The use of colour flow mapping can clearly depict the flow of blood in foetal blood vessels in a real-time scan, the direction of the flow being represented by different colours. 'Colour' doppler is particularly indispensable in the diagnosis and assessment of congenital heart abnormalities. Another recent development is the Power Doppler (Doppler angiography). It uses amplitude information from doppler signals rather than flow velocity information to visualize slow flow in smaller vessels. A colour perfusion-like display of a particular organ such as the placenta overlapping on the 2-D image can be very nicely depicted.

Doppler blood flow velocity waveforms

The integration of real-time imaging with pulsed wave Doppler allows for the vessel of interest to be identified and as the diameter of the vessel and the angle of interrogation can be measured, it allows for an estimation of velocity and volume of flow. This, nevertheless is not done in Obstetric scans because of the difficulty and inconsistency encountered in non-linear blood vessels such as the umbilical artery. As the foetus is completely dependent on the supply of oxygen and nutrients from the placenta, examination of the blood flow through the umbilical circulation would appear to have great potential for the assessment of foetal health.

In the umbilical artery, there is a relatively high forward velocity during diastole, consistent with blood flow into a low-impedance vascular bed, the placenta. With advancing gestation, there is an increase in end-diastolic flow velocity relative to peak systolic velocity. This is attributed to decreased resistance in the placental circulation with advancing gestation. This change in the pulse velocity waveform can be quantified by the systolic-to-diastolic ratio (A/B ratio). In pregnancies in which the A/B ratio is elevated, there is an increase in intrauterine growth compromise due to a placental circulation that has diminished in volume owing to placental vascular occlusion.
 
 

What about Safety?

It has been over 35 years since ultrasound was first used on pregnant women. Unlike X-rays, ionizing irradiation is not present and embryotoxic effects associated with such irradiation should not be relevant. The use of high intensity ultrasound is associated with the effects of "cavitation" and
"heating" which can be present with prolonged insonation in laboratory situations. Harmful effects in cells of experimental animals or humans however have not been demonstrated in the large amount of studies that have so far appeared in the medical literature purporting to the use of diagnostic
ultrasound in the clinical setting. Findings in one study reporting lower birth-weights in babies exposed to prenatal ultrasound have not been reproducible.
Nevertheless it is general consensus that ultrasound scans should best be performed when there is an indication to do so.

Colour imaging

This is a recent addition to plain 2-D real-time scanning. Also known as "chroma" scans, user-selectable colour hues are assigned to the shades of grey for better visualization of subtle tissue details. This clever enhancement is aimed at better interpretation of the scans. 'Colour scans' do not imply that various
parts of the same picture are depicted in different colours like what we see in a colour photograph.

3-D Ultrasound

3-d Image of the spinal column

3-dimensional ultrasound is slowly moving out of the research and development stages and is very much in the News. Faster and more advanced commercial models are coming into the market. The scans requires special probes and software to accumulate and render the images, and the rendering
time has been reduced from minutes to seconds. A good 3D image is often quite impressive and further 2D scans may be extracted from 3D blocks of scanned information. Volumetric measurements are more accurate and both doctors and parents can better appreciate a certain abnormality or the
absence of a certain abnormality in a 3D scan than a 2D one and there is the possibility of increasing psychological bonding between the parents and the baby. A large volume of literature and documentation is expected to come out in the coming years and the diagnosis of congenital anomalies
could receive revived attention. Present evidence has already suggested that even small defects such as spina bifida, cleft lips/palate, and polydactyl may be more lucidly demonstrated. Other more subtle features such as low-set ears, facial dysmorphia or clubbing of feet can be better assessed, leading to
more effective diagnosis of chromosomal abnormalities. The study of foetal cardiac malformations is also receiving attention. The ability to obtain a good 3D picture is nevertheless still very much dependent on operator skill, the amount of liquor around the foetus, it's position and the degree of maternal obesity, so that a good image is not always readily obtainable.

Other experts in this field have not considered that 3D ultrasound will be a mandatory evolution of our conventional 2D scans, rather it is an additional piece of tool like doppler ultrasound. Whether 3D ultrasound will provide unique information or merely supplemental information will only wait to be seen. It's greatest potential is still in research and particularly in the study of foetal embryology.