Selection of Ultrasound Equipment for Hospitals

Introduction
In the last 20 years ultrasound imaging has level-oped from laboratory experiments to one of the most commonly performed diagnostic examinations. Because of its usefulness, cost effectiveness in major hospitals, regional and country hospitals and private practices.
The rapid advances made in the design of ultrasound equipment have led to bewildering choice facing the prospective purchaser. It is important to seek independent advice to avoid the selection of inappropiate units.

Type of equipment

1. Static ultrasound scanners
These may be articulated arm scanners or water bath types where the transducers are immersed in water, giving a large field of view, sectional, static images in a variety of planes. Static scanners are no longer produced and have been replaced by real-time scanners and should not to be used in a modern department.

2. Real-time scanners
Real-time scanners produce moving, sectional images similar to X-ray fluoroscopy. Their major disadvantage is that the field of view is limited, depending on the transducer design.
Mechanical sector scanners produce high resolution images from relatively small transducers with small contact areas. Their major disadvantages are a small field of view (particularly superficially)and reliability problems due to mechanical wear. Sophisticated annular array mechanical sector scanners are capable of producing the best quality images at present available.
Linear array transducers are composed of multiple small elements in linear alignment and electronically fired in small groups. They can range from the cheapest available real-time units with minimal focusing and relatively simple transducer to sophisticated, dynamically focused transducers capable of producing high quality images.
The relatively large contact area of the transducers is their biggest disadvantages, but the lack of mechanical parts gives them superior reliability.
Electronicallyi steered phased arrays consist of multiple small elements wihtin the transducer which are electronically steered and focused to produce a real-time sector image. They are small, reliable and prodeuce excellent quality images but are veryrelatively expensive.
A recent development has been convex (curved) linear arrays which overcome the disadvantage of the long, flat linear arrays without losing their inherent reliability. With an increased number of elements in the array and improved focusing techniques, convex arrays are now capable of producting high quality, consistent images. They are available in a wide range of sives, curvatures and frequencies, and comprise the predominant type of transducer now used of general purpose ultrasonography.
M (motion) mode ultrasound was widely used for measurement techniques in echocardiography. It has now been largely replaced by real-time to examine the dynamics of the valves and chambers of the heart> Real-time units for echocardiography require small transducers (either mechanical or passed arrays) with high frame rates to compensate for the rapid movements of the heart.
Doppler ultrasound units measure the change in frequency of the returning ultrasound signal to peripheral blood vessels. They range from simple continuous wave units, used to detect the feotal heart, to sophiticated range-gated pulsed doppler used to detect vascular diseases.
Doppler colour flow imaging has recently been developed which allows real-time colour coding of the blood flow in the heart, abdominal and peripheral blood vessels. Although relatively expensive, the technique is proving so useful that most manufacturers are now providing this facility on all but their low cost units. The technology is still in a state of rapid development and care should be taken in the selection of this equipment as there is a wide range in the performance of various units. Sensitivity and their filter requirement are different for echocardiography, abdominaml and peripheral colour flow Doppler imaging, and equipment sholud be selected considering its major proposed use.
Colour Doppler power imaging based on the amplitude rather than the frequency shift generated by moving particles is now available on many units. It is extremly sensitive in detecting low flows and intersitial perfusion.
Specialised intracavity transducers are now available for transvaginal, transrectal, transoesophageal, intra-arterial and operative techniques.
Transoesophageal echocardiography is providing improved images of the heart but at present is only used in specialised cardiology units.
Transrectal scanning of the prostate is slowly gaining acceptance, but because of frequent need for simultaneous transrectal biopsy its use will probably remain limited to hospitals and urological clinics.
Transvaginal ultrasound is now widely used and is the method of choice for the detction of early pregnancy and ectopic pregnancy.

Choisce of equipment
The introduction of machines capable of using high density cenvex and linear arrays, mechanical sector and intracavity transducers has made the all purpose unit areality. The type of examinations to be performed and image quality required should be carefully assassed. The transducer used for particular examinations depend to some extent on personal preference, but as general guide:
Real-time sector transducer (mechanical or phased array) are best suited for examinations which need a small windows such as echocardiography, neonatal heads and intercostal abdominal scanning. they are also suitable for general abdominal and pelvic imaging;
Convex array transducers are suitable for most pelvic, obstetric and abdominal examinations;
Small, high frequency high resolition linear arrays are suitable for thyroid, breast, testicular musculoskeletal ultrasound. They are the transducer of peripheral Dopller colour flow imaging.
The cheapest linear array only unit have a very limited diagnostic capability. For small hospitals and regional centres which can only purchase a single unit, a midrange convex array would be most suitable.
Care should be taken to ensure the equipment is capable of being upgraded if necessery and that adequate service and spare parts are available.
Any machine purchased should have a standard an alphanumeric keyboard (for patient identification) and electronic calipers (preferebly dual). Touch sensitive control allow easy cleaning. Cineloop, allowing review and selection of images before printing, is useful option for difficult cases and vascular work.

Choice of transducer frequency
The frequency of the ultrasound transducer determines the depth of penetration of the sound wave and the axial resolution of the image.
Any ultrasound unit should be ordered with the range of transducers.
As a general guide :
3.5 MHz medium to long focus transducer for obese abdomens and advanced pregnancies; 3.5 to 5 MHz medium to long focus transducer for routine abdomens and second trimester pregancies; 5 MHz short to medium focus transducers for early pregnancy and pelvic examinations; 7.5 to 10 MHz short focuse for breath, teste, vascular, musculoskeletal and intacivity examinations; 2.25 to 3.5 MHz medium to long focus transducer for echocardiography.
Many manufacturers now offer swithable dual or triple frequency transducers, or wide frequency transducers which use a range of frequencies.

Image recording
A variety of methods are available.
Videotape recorder are useful for recording dynamics but can be time consuming to review.
Optimal images are recordered on laser cameras using infrared sensitive film or multiformat cameras using high resolution flat face monitors with single-sided emulsin film. the most reliable of the miltiformat cameras use a multiple fixed lens system. The monitorface and lenses must be cleaned regularly to maintain images quality.
Polaroid film is expensive and the image quality is inferior to that obtained in multiformat cameras.
Heats sensitive paper recorders are used for M-Mode Echocardiography.
Multiformat thermal printer are available available for recording of Doppler colour flow images.

Coupling media
Coupling media are necessery to exclude air between the patient and ultrasound transducer, with watersoluble aqueous gels being most commonly used. Care should be taken to ensure that the coupling gel is recommended by the manufacturer of the equipment as some gels and lotions have been known to damage transducer membranes. In cold climates some methodes of warming the gel is desirable. Stand-off gel pads are available for imaging of superficial structures.

General considerations
Examination couches should be at a height that allows for comfortable operation by the sonographer. Motor driven couches that allow independent raising and lowering of the ends of the couch are useful for vascular examinations.
Indirect wall mounted lighting with dimmer switches allows comfortable visualisation of monitors.
Adequate change cubicles, showers and toilets should be available for the patients' convenience.

Care of equipment
All probes should be cleaned after use. Vaginal & rectal probes should be covered by a protective shealth or condom prior to inserton.Following the examination the shealth should be discarded and the probe cleaned in an antimocrobial solution. The type of solution & amount of time for cleaning depends on the manufacturers & local infectious disease/quality control guidelines.

Quality assurance
1. Ultrasound equipment must be regularly serviced and maintained, & a service contract with the manufacturer or other qualified provider should be enterd into.
2. Adequate documentation is essential for high quality patient care.
3. A register of ultrasound examinations performed including clinical indications for the study should be kept. There must be a permanent record of the ultrasound examination & its interpretations. Imaging of all appropiate areas both normal and abnormal should be recordered in an imaging or storage format. Images are to be appropriately labelled with the examination date, patient identification, and body part examined.
4. A written and signed report should be issued on all ultrasound examinations y the responsible medical practitioner.

Reviewed by:
Dr Johan Hoe, Singapore

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