We commonly ask you to arrive for your pregnancy or pelvic scan with a comfortably full bladder, having asked you to drink 1 to 1.5 litres of water about an hour before your scan appointment. But why do we ask you to do this when it can be uncomfortable, and what is the basic physics behind this simple request?
How does an ultrasound scan produce an image?
An ultrasound probe produces and receives sound waves. Ultrasound waves are produced at a very high frequency that are not audible to the human ear.
When an ultrasound transducer (probe) is placed on or in your body, it transmits sound waves through the ultrasound gel and into your body. The sound waves travel through your body to organs and structures along their path of transmission from the transducer. Sound energy is reflected by different types of tissue, giving an echo as a result, or it can be refracted leading to a change in transmission path.
The ultrasound machine amplifies and processes the reflected echoes and by using a special computer can generate an image of the body part that is being scanned. The computer assesses the echoes, thereby determining the size, shape and consistency of the target organ, and it’s distance from the surface.
Therefore in very simple physics terms, an ultrasound machine functions in a repeating cycle of sound transmission, reception of echoes, analysing and processing of echoes and image display.
The primary objective of every ultrasound scan is to transmit as much sound energy as possible, in order to produce a stronger echo which will in turn, produce better images. The use of ultrasound gel will maximise transmission of sound. Body tissues or organs have their own different inherent tendencies to sound transmission, with bone and calcification producing a strong reflection. Fluid-filled body cavities such as the bladder and amniotic sac in pregnancy have good transmission of sound waves whereas gas filled organs, e.g. the bowel and lungs have poor transmission of sound and therefore weak echoes. So we basically use your full bladder like a window to see the organs behind it.
Sound transmitted through the bladder results in more sound energy at the pelvic organs, e.g. uterus and ovaries, and the pregnancy. This results in stronger echoes and images that are of a higher quality because the sound energy has less resistance to transmission.
Intestinal contents and gas in the gut are detrimental to obtaining a good scan image, because air is a strong reflector and produces artefact in the ultrasound image. This makes it almost impossible to get good transmission of sound to the pelvic organs or baby if the bowel is lying in front because the sound waves will be 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.
The uterus has to be at the correct angle (at right angles to the direction of the ultrasound beam) to be viewed optimally. Most women have an anteverted (tilted forwards) uterus which unfortunately in some cases doesn’t present the ideal angle to the sound transmission pathway. A full bladder tilts the uterus backwards thereby giving a better angle to the transmitted sound energy which results in higher quality images.
If the uterus is retroverted (tilts backwards), it is vital that you have an adequately filled bladder to push the bowel up out of the way to obtain a clear view.
How much should I drink?
We advise women drink a litre of water about an hour before the examination to allow time for the bladder to fill. Bladder capacity and behaviour varies according to each individual, and if you are dehydrated it can take much more to fill your bladder. Conversely, if you are well hydrated generally, you tend to need less fluid to fill your bladder than advised. Ideally, drink as much clear fluid to make your bladder comfortably full. If your bladder is not adequately full it may impact negatively on the examination, and this could also be the case if it is too full and causing discomfort.
What should I drink to fill my bladder?
Any fluid will fill your bladder, but cold still water is ideal. Fizzy drinks should be avoided before an abdominal or pelvic scan because they can increase the amount of gas in the bowel, and this is detrimental to the scan quality.
Should I have a full bladder for every type of scan?
You might not need a full bladder for all ultrasound examinations. Most early pregnancy ultrasound scans after 8 weeks and up to 22-24 weeks will need a full bladder but thereafter usually not.
Early pregnancy and gynaecological scans performed using a transvaginal approach require an empty bladder as the ultrasound probe is much closer to the region of interest, and it can be quite uncomfortable if you have anything in your bladder.