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I thought it would be useful to start with some definitions of few basic terms used during scanning to allow the candidate to improve his or her understanding of the subject and improve technique. This section is not meant to give a comprehensive description of the physics involved. It will be useful to dwell in each heading and get as much information out of it as possible and use it to improve your practice.
Some basic scientific facts to know
- Ultrasound denotes sound >20000 cycles / second [>20000 Hz] which is beyond the sonic range
- One Hertz is the number of sound wave cycles or pulses / second
- Wavelength is the time it takes to complete one cycle
- Frequency is the number of cycles in a fixed period of time
- The average speed of sound in soft tissue 1540 m/s
- Amplitude is the range of vibration measurement from the baseline to the peak
- Attenuation is the reduction in the amplitude and intensity of the beam when passing through tissues
- Diagnostic ultrasound range is 2-20 Mega Hz
Terminology and Function
| Overall gain: |
Controls the brightness of the whole image |
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Dynamic range: |
Number of grey shades displayed in the image and helps in tissue identification |
| Output power: |
The amount of pressure the sound is producing at tissue level |
| TGC: |
Time gain compensation varies the gain up or down in certain depths of the 2D image and compensates for attenuation of brightness in the selected area |
| Distal enhancement: |
Entails increased reflection from the distal part of the examined structure |
| Shadowing: |
Entails the reduction of echoes distal to the organ |
Thermal Index
The ratio of the power used over the power required to produce a temperature rise of 1o C. It is an indication of the relative potential for thermal tissue damage.
Mechanical Index
It is a measure of the maximum amplitude of the pressure pulse in the exposed tissues. It is calculated by the equation= peak negative pressure over the square root of frequency. It is an indication of the relative potential of non thermal damage e.g. cavitations. Mechanical index >0.3 indicates risk of capillary bleeding and >0.7 indicates risk of cavitations.
Resolution
Axial resolution
Indicates the ability to separate close objects parallel to the sound beam. To improve axial resolution, increase the pulse repetition rate.
Lateral resolution
It indicates the ability to separate close objects perpendicular to the beam. Multiple focal points give better lateral resolution but slow the frame rate
Further Terminology
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anechoic or sonolucent indicates a tissue devoid of any echoes [simple cyst]
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hypoechoic indicates low level echoes like a lymph node
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isoechoic means similar to the surrounding tissues
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hyperechoic or echogenic means brighter intensity compared to the surrounding
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homogeneous indicates smooth grey densities with no focal areas of different echo texture
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heterogeneous means mixed densities probably due to different tissue types.
General rules
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All organs should be examined for location, shape, size, texture, range of motility, relation to other organs and abnormal pathology
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Each organ should be examined in at least two orthogonal planes
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Use the gain, power and different setup for different types of tissues and resolution to get the best picture.
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Normal and abnormal findings should be labelled and all pictures should include the date, patient's name, date of birth and orientation
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A permanent record should be kept in the case not
Safety measures
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You must be aware of the guidelines for the safe use of diagnostic ultrasound equipment published by the British Medical Ultrasound Society and any new upgrades when applicable.
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Use the lowest power to start with and change the setting as necessary.
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Always check the thermal index [TI] and mechanical index [MI] in your machine and keep them to the lowest level possible.
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Ultrasound scanning should not be used for social reasons or to create pretty pictures
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Make sure that the patient is not allergic to latex or the gel before you start.
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The term ALARA should be in the back of your mind at all times to keep the power level and exposure time as low as reasonably achievable.
Objectives of gynaecological scanning
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Identify normal position, size, shape and texture of pelvic organs
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Recognise the physiological changes that occur in them during the cycle
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Recognise and describe any changes in these normal characteristics
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Relate any change to the various known gynaecological conditions
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Use the technique to extend the clinical judgment necessary for patients care.
Indications for gynaecological scanning
The most common indications for gynaecological scanning are:
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abnormal uterine bleeding
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acute and chronic pelvic pain
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ovarian cyst / pelvic mass
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fertility scanning including monitoring ovulation and testing the fallopian tubes
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early pregnancy dating and monitoring of complications
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missed intrauterine contraceptive devices |