Facts and Rules


I thought it will 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 his or her 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 daily practice. The next chapter 'Knobology' will describe how to acquire and optimise images, but that will not be possible without understanding the contents of this chapter first. 

Some basic scientific facts are essential to know, before one can be able to start using ultrasound machines. The more important ones include:
  • Ultrasound denotes sound wave frequency >20000 cycles / second which is beyond the sonic range
  • One Hertz [Hz] 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
  • Ultrasound velocity = wavelength (mm) x frequency (Hz).
  • Amplitude is the range of vibration measurement from the baseline to the peak. It is measured in decibels (dB).   
  • Diagnostic ultrasound range is 2-20 Mega Hz.

While using an ultrasound machine, and optimising images acquisition, certain knobs are frequently used. They are important for every one to master, and to use as a matter of course. These knobs terminology and functions are given below:

 
Overall gain:

Controls the brightness of the whole image

Dynamic range:


Number of gray shades displayed in an image and helps in tissue identification. It reflects the ratio of the brightest portion of the image to the darkest. 

Output power:

The amount of pressure the sound is producing at tissue level

Time Gain Compensation:


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. It should be changed gradually, as great differences between adjacent knobs may cause stripes in the image.

Distal enhancement: Entails increased reflection from the distal part of the examined structure. A typical example is seen distal to a simple cyst.

Shadowing: Entails the reduction of echoes distal to the organ. A typical example is seen at the distal part or beyond a fibroid. This is more so with calcified ones.
 
 
Resolution
 

Axial resolution

Lateral resolution

  • Indicates the ability to separate two close objects parallel to the sound beam
  • To improve axial resolution increase the pulse repetition rate
  • Indicates the ability to separate two close objects perpendicular to the sound beam
  • Multiple focal zones give better lateral resolution, but slows the frame rate


Higher resolution setup improves image quality, but reduces ultrasound tissue penetration. This is used more frequently with high frequency intracavity, and small parts probes. On the other hand, lower resolution setup allows better tissue penetration, with lower image definition quality. This in turn is used for low frequency transabdominal probes, when thick tissues need to be traversed to reach the target organs.
 
Harmonic imaging

Harmonic imaging is a technique used to optimise image quality, and improve tissue interface, in difficult cases. The ultrasound machine sends and receives singals at two different frequencies, when the function is switched on. As an example, the sent ultrasound frequency may be 5 MHz, yet the machine only reads a higher returning frequency of 10 MHz, as the body tissues reflect sound waves at double the received frequency. This will reduce artefacts, and improve the two objectives mentioned before. The end result would be improved contrast, better resolution and tissue delineation.

Attenuation

Attenuation is the reduction in the amplitude, power and intensity of the beam, while passing through tissues, leading to reduced quality of distant parts of images. This is affected through absorption, scatter and reflection of the sound waves. Reducing the probe resolution helps in tissue penetration by the ultrasound beam, and improves distant image quality. Time gain compensation at the affected area is another way to improve image quality affected by attenuation.

Artefacts

Artefacts indicate anything that is not genuinely part of the scanned area. These may be caused by wrong equipment setting, or transducer positioning. Sometimes, they may result from tissues interaction with sound beams. Very common and well know examples are reverberations, where are parallel linear echoes seen in the near field of the image. The other example is aliasing, which may occur during Doppler scanning, when the pulse peak appears below the baseline.

Auto optimisation

Auto optimisation is a technique where the machine provides the most optimised image, in terms of gain and contrast, after analysis of all tissues in the image. It is a facility provided by most new machines. It can be used to fine tune an image after manual setting of depth, focus, frequency and other parameters.


Further Terminology
  • anechoic or sonolucent indicates a tissue devoid of any echoes [simple cyst],
  • hypoechoic indicates low level echoes like a lymph node,
  • isoechoic means similar to the surrounding tissues,
  • hyperechoic or echogenic means brighter intensity compared to the surrounding,
  • homogeneous indicates smooth gray densities with no focal areas of different echo texture,
  • heterogeneous means mixed densities probably due to different tissue types.

General rules

  • All organs should be examined for location, shape, size, texture, range of motility, relation to other organs and abnormal pathology
  • Each organ should be examined in at least two orthogonal planes
  • Use the gain, power and different setup for different types of tissues and resolution to get the best picture. More information about this subject is given in he next chapter 'Knobology'.
  • Normal and abnormal findings should be labelled and all pictures should include the date, patient's name, date of birth and orientation
  • A permanent record should be kept in the case notes

Safety measures

  • 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.
  • Use the lowest power to start with and change the setting as necessary.
  • Always check the thermal index [TI] and mechanical index [MI] in your machine and keep them to the lowest level possible.
  • Ultrasound scanning should not be   used for social reasons or to create pritty pictures
  • Make sure that the patient is not allegic to latex or the   gel before you start.
  • 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      
  • Identify normal position, size, shape and texture of pelvic organs.
  • Recognise the physiological changes that occur in them during the cycle.
  • Recognise and describe any changes in these normal characteristics.
  • Relate any change to the various known gynaecological conditions.
  • Use the technique to extend the clinical judgment necessary for patients care.
Indications for gynaecological scanning
 
The most common indications for gynaecological scanning are:
  • abnormal uterine bleeding,
  • acute and chronic pelvic pain,
  • ovarian cyst / pelvic mass,
  • fertility scanning including monitoring ovulation and testing the fallopian tube early pregnancy, dating and monitoring of complication,
  • missed intrauterine contraceptive devices.

Legal and Professional obligations

  • Ultrasound scanning should be performed for a proper medical indication,
  • The procedure should be explained beforehand to the patient, or guardian,
  • The patient should be verbally consented,
  • A chaperon should be available in all circumstances. Note that a husband can not act as a chaperon during his wife's examination,
  • Permanent copies of the results should be kept in the patient's record,
  • Proper documentation of findings should include:
    • Name of the patient,
    • Date of the procedure,
    • Proper labelling of findings.

Need for local departmental ultrasound protocol

It is always safer to have a protocol at hand, for all staff members involved with ultrasonography, to follow. This will guard against short cuts and mishaps, and makes auditing easier. Few of the points mentioned below may have been highlighted earlier, but it is important to put everything together in one protocol, as a permanent record.


The document should give salient guidelines agreed in the department. This is very important, if many operators are involved, and there is staff turnover. It is the duty of the head of the department to make sure that such a protocol exists and is followed, especially by new staff members. The following is an example of a protocol I wrote in the past:

  • There should be explicit medical indication for scanning, which should be clearly documented in the patient's notes and ultrasound report
  • Social scans should be avoided.
  • The ultrasound machine should be used as an extra tool to extend the clinical judgement of the gynaecologist.
  • The American Institute of Ultrasound in Medicine (AIUM) encourages more gynaecologists to practice pelvic / gynaecological ultrasonography. The objective is to use pelvic ultrasonography in the same way a physician uses a stethoscope during daily medical practice.
  • The patient's biodata and medical history should be obtained first, for proper interpretation of the scan findings.
  • Ultrasonography should not replace abdominal or pelvic clinical examinations when necessary.
  • For transabdominal scanning the bladder should be at least half full to allow access into the pelvis and adequate visualisation of the pelvic organs. On the other hand the bladder should be empty during transvaginal scanning

  • Before starting pelvic scanning a verbal consent should be obtained after thorough explanation of the procedure, its benefits and limitations. Few patients consider ultrasonography as an ultimate diagnostic tool and expect more information than technically possible from just pelvic scanning. These patients need more information to avoid any disappointment, if the scan gave negative results and proved unhelpful.
  • For certain indications scanning is better done at certain times of the cycle, and attempts should be made to schedule the examination accordingly, to maximise benefit. This is not possible during emergency situations.
  • Using the abdominal or vaginal probe depends on various factors including the patientís age, preference and the indication for the examination.
  • Ideally all pelvic scans should be done transvaginally as it gives better images with high resolution, and improved diagnostic acuity. However, in the presence of a large pelvic mass, large uterus, or advanced pregnancy the abdominal probe will be more appropriate.
  • In some cases both abdominal and vaginal scanning will be indicated.
  • Occasionally transperineal, or even transrectal scanning may be indicated. This is especially so in cases when  a good image could not be obtained through transvaginal scanning due to severe uterine retroversion, pelvic adhesions or ovarian displacement. It is also useful in young and other women who are not sexually active
  • Occasionally a patient may wish to introduce the probe into her own vagina. This should be allowed with the help of a nurse, to prevent personal injury or probe damage. It is better to be done without a male practitioner watching. 

General remarks

  • Never carry an examination without a chaperone and without verbal consent.
  • Ask the patient to get ready behind the screen and make sure she is not overexposed.
  • Always try to allay the patientís anxiety. Be thorough but gentle and respectful. Explain what are you doing.
  • Use national or local guidelines for carrying all examinations, and in documenting the findings. This should include examination of all organs in two planes, and good annotation for future reference.
  • The probe should be cleaned and disinfected according to the manufacturers recommendations. Cover the probe with a condom before carrying any transvaginal scan examinations, and clean it immediately as recommended after finishing the examination.
  • Always wear gloves during examination, and double glove in cases suspected of communicable disease.
  • The paper sheet covering the couch should be disposed of in the designated yellow bag after each examination.
  • Write a report about all the finding and keep a record in the patientís file for future reference.
  • Always clean the machine keyboard and all areas used with disinfectant by the end of the day
  • Switch the machine off and disconnect it at the wall socket, when not in use for long time.
  • Before major cleaning of the probe, disconnect it from the machine and rinse it with warm soap and water solution. Scrub it with a soft sponge to remove all gel and bioresidues and dry it with a soft cloth.
  • Proper disinfection of probes should be done according to the manufacturers recommendation. Most companies recommend chlorhexidine solution, to be followed by thorough rinsing with warm water, before being pat dried with soft cloth or gauze.
  • The ultrasound machine should be serviced every year and as necessary in case of malfunction, by the vendors. This should include probe recalibration which may need more frequent attention.

 
 
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