Which scan is better depends less on the machines than on where your pain is and who you are. CT generally sees more inside the belly, but ultrasound is the preferred first scan for some common problems, including gallbladder trouble and often kidney stones, and for anyone who should avoid radiation, such as children and pregnant people.1PubMed Central. Abdominal Pain in the Emergency Department: How to Select the Correct Imaging for Diagnosis2PubMed. Right Upper Quadrant Pain: Ultrasound First!3ACOG. Guidelines for Diagnostic Imaging During Pregnancy and Lactation That’s why many doctors start with ultrasound and turn to CT only when the answer is still unclear.
How Accurate Each Scan Is Overall
A CT (computed tomography) scan uses X-rays and a computer to build cross-section pictures of the whole belly. Ultrasound uses sound waves from a handheld probe. CT shows every organ at once. Ultrasound sees best what lies close to the skin and isn’t hidden behind gas-filled bowel. Gas in the bowel and body tissue scatter sound waves and hide whatever lies behind them, but CT isn’t blocked by gas and can follow the bowel from end to end.
For pain that isn’t in one clear spot, ultrasound is generally less accurate than CT.4PubMed Central. Abdominal Pain in the Emergency Department: How to Select the Correct Imaging for Diagnosis Across many belly problems, CT got the diagnosis right about 95% of the time, ultrasound about 83%.5Annals of Medicine and Medical Sciences. Comparative Evaluation of Ultrasonography and Computed Tomography in the Diagnosis of Abdominal Pathologies Ultrasound was especially reliable for liver, gallbladder and kidney problems, while CT was much more accurate for blocked bowel and for lumps or masses inside the belly.6Annals of Medicine and Medical Sciences. Comparative Evaluation of Ultrasonography and Computed Tomography in the Diagnosis of Abdominal Pathologies So your doctor’s guess about which organ is involved largely decides which scan comes first.
Those percentages are averages across many different diagnoses, so they don’t tell you how either scan will do for your particular pain.
The two feel very different. For ultrasound, a tech spreads cool gel on your belly and presses a probe against it, usually for 15 to 30 minutes, with no needle or radiation. Pressing over a tender spot can hurt, though that reaction helps the doctor. For CT, you lie on a table that slides through a large ring, and the scan itself takes only minutes. You may be asked to drink contrast or receive an IV dye.
If the first scan answers the question, you skip a CT and its radiation. When it’s unclear, CT still follows, so little is lost by trying ultrasound first.
Ultrasound First for Gallbladder Pain
For suspected gallstones or an inflamed gallbladder, ultrasound is recommended as the first scan.7PubMed. Right Upper Quadrant Pain: Ultrasound First! It helps separate problems that need surgery from those treated with medicine, and it’s often enough to guide care.8PubMed. Right Upper Quadrant Pain: Ultrasound First! In one community ER, ultrasound caught an inflamed gallbladder more often than CT did.9PubMed Central. Comparing the Sensitivity and Specificity of Computed Tomography and Ultrasound in the Diagnosis of Acute Cholecystitis in a Rural Setting If your pain is under the right ribs, especially after fatty meals, expect ultrasound rather than CT.
You may be asked not to eat for several hours beforehand. Fasting keeps the gallbladder full, which makes it easier to see. Stones show up as bright spots with dark shadows behind them, and the tech may ask you to roll onto your side to watch whether they move.
Besides stones, the tech looks for a thickened gallbladder wall and fluid around it, which suggest inflammation. A tender gallbladder under the probe also points toward inflammation.
CT is still added in some cases: when ultrasound is normal but the pain is severe or unexplained, when doctors suspect pancreas inflammation or a complication, or when the pain may come from the bowel rather than the gallbladder.
Appendicitis Ultrasound Then CT if Needed
Appendicitis often starts as vague pain near the belly button, then moves to the lower right side and sharpens over several hours. Loss of appetite, nausea and a low fever commonly go along with it, though not everyone has all of these. Because the picture is often incomplete, especially in children, older adults and pregnant people, a scan helps confirm what the exam can only suggest.
CT detected about 94% of appendicitis cases, ultrasound about 76%.10PubMed Central. A comparison of the Accuracy of Ultrasound and Computed Tomography in common diagnoses causing acute abdominal pain That means ultrasound misses roughly 1 in 4 cases, so a normal ultrasound doesn’t fully rule it out.
To limit radiation, many experts recommend ultrasound first, then CT if it’s negative or unclear.11ACEP Now. Ultrasound for Appendicitis Some hospitals have radiologists scan everyone with ultrasound and send only the unclear cases on to CT.12PubMed Central. The diagnostic value of the computed tomography scan and ultrasonography in acute appendicitis A bedside scan done by the ER doctor can speed things up: with a structured checklist, diagnosis came in a median 91 minutes versus 162 with usual care.13PubMed Central. The ABCDEFG point-of-care ultrasound protocol for acute non-traumatic abdominal pain: a prospective observational study That approach also cut CT use from about 33% to 16% of patients.14PubMed Central. The ABCDEFG point-of-care ultrasound protocol for acute non-traumatic abdominal pain: a prospective observational study
Often the appendix simply can’t be seen, and that result is unclear rather than normal. Doctors then weigh your exam, blood tests and pain pattern. Pain that gets worse after a normal scan is a reason to go back.
Going straight to CT does happen. Adults with classic signs or larger bodies may skip ultrasound, CT can also show other causes such as diverticulitis, and in a busy ER one definitive scan can win out.
Kidney Stones, Diverticulitis and Pelvic Pain
A stone moving down the ureter typically causes severe pain in the side or back that comes in waves and may spread toward the groin. Blood in the urine, nausea and being unable to get comfortable in any position are common, and urine tests are often checked first. Fever with these symptoms is more worrying, because an infected blocked kidney needs urgent treatment, and imaging helps decide that.
In a large ER trial of adults with suspected kidney stones, starting with ultrasound led to no more serious missed problems than starting with CT.15PubMed. Ultrasonography versus Computed Tomography for Suspected Nephrolithiasis Pain levels, return ER visits and hospital stays were similar, while radiation was much lower.16PubMed. Ultrasonography versus Computed Tomography for Suspected Nephrolithiasis One real-world study did find CT slightly cheaper overall, about €199 less per patient.17PubMed Central. A Real-World Cost-Effectiveness Study Evaluating Imaging Strategies for the Diagnostic Workup of Renal Colic in the Emergency Department
In practice, ultrasound shows swelling of the kidney from a blocking stone but may not show small stones themselves. CT may follow if surgery or a procedure is being planned.
Many small stones pass on their own with pain medicine and fluids. Larger or stuck stones, or a blocked and infected kidney, may need a procedure.
Studies differ on how well ultrasound finds diverticulitis: one found 61% versus 81% for CT.18PubMed Central. A comparison of the Accuracy of Ultrasound and Computed Tomography in common diagnoses causing acute abdominal pain Larger reviews find little difference in simply spotting it.19PubMed Central. Radiological insights into diverticulitis: Clinical manifestations, complications, and differential diagnosis CT is clearly better at showing complications like abscesses, and other problems at the same time.20PubMed Central. Radiological insights into diverticulitis: Clinical manifestations, complications, and differential diagnosis
For sudden pelvic pain in women, ultrasound is the usual first scan, since it shows the ovaries and uterus well. For a twisted ovary, though, ultrasound caught about 3 in 4 cases, so doctors must decide with your exam and pain, not the scan alone.21Surgical Science. Validity of Ultrasound in Patients with Acute Pelvic Pain Related to Suspected Ovarian Torsion A twisted ovary is urgent because its blood supply can be cut off, risking the ovary. A normal-looking ultrasound doesn’t rule it out, and sudden one-sided pain with nausea needs prompt care.
Radiation and Contrast Dye Risks of CT
CT scans done in the US in one year are projected to cause about 103,000 future cancers, including about 9,700 in children.22PubMed Central. Projected Lifetime Cancer Risks From Current Computed Tomography Imaging The risk for any one person from one scan is small, but it’s real across millions of scans. Risk from belly CT is highest in people aged 18 to 20 and lowest in those over 70.23PubMed. Organ dose and lifetime attributable cancer risk from abdominal CT examinations: A multicentre retrospective study in Jordan So avoiding repeated CTs matters most for young people.
Many CTs use an iodine-based contrast dye injected into a vein to make organs show more clearly. It can cause sudden kidney injury, though the risk for most people is low.24National Kidney Foundation. Contrast Dye and Your Kidneys The risk is higher with advanced kidney disease, diabetes, dehydration or serious illness.25National Kidney Foundation. Contrast Dye and Your Kidneys Ultrasound contrast bubbles are unlikely to affect the kidneys.26National Kidney Foundation. Contrast Dye and Your Kidneys
The IV dye often causes a warm flush and a metallic taste for a minute or so, and some people feel as if they’ve wet themselves though they haven’t. Most side effects are mild, such as itching or hives, and staff are trained to treat them. Serious allergic-type reactions are uncommon, which is why staff ask about past reactions before the scan.
Before a CT, mention kidney disease, diabetes or metformin use, any past reaction to contrast dye, and how many CTs you’ve had recently. You can also ask whether contrast is needed for your question. Asking “could ultrasound answer this first?” is reasonable.
Why More Detail Isn’t Always Better
About 20% to 40% of CT scans show at least one incidental finding, something unrelated to why you were scanned.27PubMed. Incidental Findings and Low-Value Care For low-risk people, chasing these can lead to treatment of harmless things.28PubMed. Incidental Findings and Low-Value Care Deciding which findings need follow-up is often uncertain.29PubMed. Incidental findings detected on emergency abdominal CT scans: a 1-year review Checking an odd finding can mean more imaging, blood tests or a specialist visit, and may end with the answer that it was harmless.
If your report mentions an unrelated finding, such as small cysts on the kidneys or liver, which are common and usually harmless, don’t panic. Ask whether follow-up is recommended and when, and keep the report for future doctors.
Children and Pregnancy
US radiology guidelines recommend ultrasound before CT for children with signs of appendicitis.30Pediatrics Nationwide. Reducing Radiation Exposure and Imaging Costs for Children With Appendicitis It has no radiation and costs less, though results depend on the person scanning.31Pediatrics Nationwide. Reducing Radiation Exposure and Imaging Costs for Children With Appendicitis Bedside ultrasound as a first tool can cut radiation in teens without worse outcomes.32PubMed Central. Lesson from a Single Pediatric Emergency Department: Potentially Applicable Radiation-Minimizing Practices for Non-Traumatic Abdominal Pain in Adolescents
It’s fair to ask whether a children’s hospital or an experienced sonographer is available. A small child’s thin body makes ultrasound work well, though CT is still used when ultrasound can’t give a clear answer. When CT is needed, hospitals that treat children often use lower-dose settings sized to the child’s body, since the adult dose would be too much. Doctors usually scan only the area needed and avoid repeat scans of the same area. Parents can ask whether the scan is using a child-sized protocol.
In pregnancy, ultrasound and MRI carry no known risk and are the scans of choice.33ACOG. Guidelines for Diagnostic Imaging During Pregnancy and Lactation MRI accurately shows belly and pelvic disease in pregnant patients.34PubMed. MRI of acute abdominal and pelvic pain in pregnant patients If CT is truly needed, its dose is usually far below levels linked to harm to the baby.35ACOG. Guidelines for Diagnostic Imaging During Pregnancy and Lactation No harm has been seen below 100 mGy, more than typical scans deliver.36PubMed Central. Medical Imaging in Pregnancy: Safety, Appropriate Utilization, and Alternative Modalities for Imaging Pregnant Patients
Body Size and Who Does the Scan
Ultrasound views of organs get worse as body weight rises, even with better probes.37PubMed Central. The application of high-performance ultrasound probes increases anatomic depiction in obese patients Deep organs are often harder to image well with a BMI over 30.38PubMed. Inter-transducer variability of ultrasound image quality in obese adults: Qualitative and quantitative comparisons Ultrasound accuracy also depends strongly on the skill of the person scanning.39PubMed. The association between operator confidence and accuracy of ultrasonography performed by novice emergency physicians In larger adults, doctors may therefore go straight to CT. A scan that can’t see the organ in question is reported as limited, which is not the same as normal.
ER doctors may scan at the bedside to look for specific answers, while a formal radiology ultrasound is more complete. Both performed similarly for kidney stones.
Useful questions to ask: Who will do the scan, a radiologist, sonographer or ER doctor? Would ultrasound likely answer the question in my case? If it’s unclear, what’s next? Is contrast needed?