Ultrasound is a medical imaging technique that uses high-frequency sound waves to create live pictures of the inside of the body. These sound waves reflect off tissues and organs, and the resulting echoes are processed by a computer to form an image. The clarity and accuracy of these images depend heavily on how well the sound waves travel through the body. Because air and fat strongly interfere with sound transmission, preparation instructions regarding what you drink are essential for an effective scan. Instructions vary significantly based on the specific organ or area being examined.
Preparing for Pelvic and Obstetric Scans
A full bladder is often required for transabdominal pelvic ultrasounds, which examine organs like the uterus, ovaries, and prostate, and for early-stage obstetric scans. The fluid in the bladder serves two primary purposes. First, it acts as an acoustic window, providing a clear path for sound waves to travel through the lower abdomen without being scattered by air or gas. Second, a full bladder physically displaces gas-filled loops of the bowel upward, clearing the view for the sonographer and allowing for better visualization of pelvic structures.
To achieve the necessary fluid volume, patients are typically instructed to drink 24 to 32 ounces (about one liter) of clear liquid, such as still water or a clear electrolyte drink. This volume should be consumed one hour before the scheduled appointment time. It is important to refrain from emptying the bladder after drinking this fluid until the scan is complete.
If the bladder is not adequately full, the images may be less distinct, potentially requiring a delay or a repeat of the procedure. If the bladder is too full, it can cause significant discomfort, though a partial voiding may sometimes be allowed by the sonographer to relieve pressure while maintaining sufficient fluid for image quality.
Guidelines for Abdominal Ultrasounds
Preparation for ultrasounds focused on the upper abdomen (liver, gallbladder, pancreas, or kidneys) requires fasting, often for six to twelve hours prior to the scan. Fasting is necessary because food and liquid introduce air and gas into the stomach and intestines, which can completely obscure the organs being examined. Furthermore, eating, particularly fatty liquids, causes the gallbladder to contract and empty its contents. If the gallbladder is contracted, it cannot be properly evaluated for gallstones or other pathology, potentially requiring a repeat procedure.
For abdominal scans, the only universally permissible liquid is plain, still water, limited to small sips. Patients should only drink enough water to take essential medications. Any other liquid intake must be avoided to maintain the fasted state and minimize intestinal gas.
Some facilities may permit black coffee or tea without milk or sugar, but plain water is the safest choice for the clearest images. Following these restrictive guidelines ensures the digestive system is quiet, the gallbladder is distended, and the organs are not obscured by gas or digestive activity.
Liquids to Avoid Before Any Scan
Certain liquids interfere with image clarity and should be avoided before any type of ultrasound scan, regardless of preparation requirements.
- Carbonated beverages: Soda, sparkling water, and beer are prohibited because carbonation introduces gas bubbles into the digestive tract. This air acts as a barrier that hinders the transmission of ultrasound waves.
- Dairy products: Milk, cream, or milk-based shakes must be avoided, especially before scans requiring fasting. The fat content can trigger gallbladder contraction, and the liquid density may interfere with sound waves.
- Caffeine and alcohol: These substances are diuretics, meaning they increase the rate of urination. This diuretic effect counteracts the required preparation of retaining fluid for a full bladder, making it difficult to complete the scan successfully.