Why full bladder and ultrasound: What you need to know

Please allow me to kindly remind you to present for your ultrasound examination with a “full bladder” thanks.

This is a common phrase from the receptionist whenever one books for a pregnancy or gynaecological ultrasound examination. What is the medical science behind this simple but sometimes uncomfortable request?

How ultrasound machines function:

Ultrasound equipment generates and receives sound waves. Ultrasound waves are at very high frequency that can’t be heard by the human ear.

Once the ultrasound probe is placed on or in your body, it transmits sound waves through your body. The sound waves travel through your body to organs and structures along their path of transmission. Sound energy and light energy have lots of similarities. Both can be reflected and in the case of sound, giving an echo as a result or it can be refracted leading to a change in transmission path.

Sound waves which are reflected or bounced back (echo) are very important in ultrasound examinations. We rely on the echo to make a diagnosis. The ultrasound machine can amplify and process these echoes and by using a special computer can generate an image of the body part that is getting scanned on a video screen which can be saved e.g. baby parts, uterus, ovaries etc. Once the computer has assessed these echoes it is possible to determine the distance, size, shape and consistency of the target organ.

In summary, an ultrasound machine functions in this cycle of: sound transmission, reception of echoes, analysing and processing of echoes and image display.

What is the Advantage of a Full Bladder:

The primary objective of every examination is to transmit as much sound energy as one can, so that we receive a good echo which may lead to better images. The use of ultrasound gel helps to maximise transmission of sound. Body tissues or organs have their own different inherent tendencies to sound transmission. Fluid filled body cavities have good transmission of sound whereas gas filled organs e.g. bowel and lungs have poor transmission of sound. In fluid filled cavities, there is better transmission and good echoes and in gas filled organs there is poor transmission and weak echoes.

A full bladder helps the sonographer in several ways:

  1. Sound transmitted through the bladder results in more sound energy at the target organs e.g. baby, cervix, uterus ovaries etc. This results in good echoes and better, more crisp images on display as the round trip of sound energy has little resistance to transmission.
  2. Air is a very strong ultrasound beam reflector. Bowel tends to fill up with intestinal contents and gas. This makes it almost impossible to get good transmission of sound to target organs if bowel is along the pathway as almost all the sound will get reflected before reaching the region of interest. A full bladder helps by pushing bowel out of the pelvis allowing transmission of ultrasound to target organs.
  3. Most women have an anteverted (tilted forwards) womb which unfortunately in some cases doesn’t present the ideal angle to the sound transmission pathway. A full bladder tilts the womb backwards hence presenting a more favourable angle to the transmitted sound energy which results in better images.

How much fluid should one take:

Bladder capacity and behaviour varies with individuals. Ideally, drink as much fluid to make your bladder comfortably full. If your bladder is not adequately full or is very full and causing great discomfort, both may impact negatively on the examination.

Depending on the service provider you may not need a full bladder for all examinations. Most early pregnancy ultrasound scans and up to 22-24 weeks will need a full bladder but thereafter possibly not. Gynaecological scans performed using the vaginal approach generally don’t need a full bladder as the ultrasound probe is much closer to the region of interest. When booking for an ultrasound scan, the reception staff will advise you accordingly depending on the indication for the scan on your referral.

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