Could a “false negative” actually result from a scan of the abdominal aorta, missing an aneurysm of this large vessel?
If an aneurysm is large enough, it can suddenly rupture and become immediately life threatening.
We’ve all heard of how various imaging modalities miss a life threatening presence, such as a mammogram missing a malignant tumor, or a core biopsy missing cancer of the prostate.
An aneurysm is an abnormal dilation or bulge in a blood vessel. If the bulge gets big enough (the bigger it is, the thinner and weaker its interior wall), it can rupture, causing internal hemorrhaging. The survival rate is low.
In some instances, people at high risk for an abdominal aortic aneurysm are screened (e.g., they have a family history or are a man over 70).
Others may undergo imaging due to suspicious symptoms such as a new-onset throbbing “pulse” in their abdomen.
The three imaging modalities for an AAA are the CT scan, MRI scan and ultrasound (US).
Can an imaging study come back as “false negative” when there is, in fact, an aneurysm?
“Short answer: An AAA will almost always be present on the images, with a few exceptions,” says Brett Mollard, MD, a board certified diagnostic radiologist who specializes in abdominal imaging and nuclear medicine.
“However, aneurysms may rarely be missed, particularly when a radiologist is interrupted one or more times while reading a complicated case.
“A typical CT of the abdomen and pelvis extends from the domes of the diaphragm through the pelvis and uppermost thighs. This includes the entire abdominal aorta and major branch vessels.
“Technically, the aorta starts at the top of the heart and courses down through the chest and abdomen, splitting into the right and left common iliac arteries that account for blood flow to the pelvis and legs.
“In some instances, portions of the aorta may not be visualized — it’s not ‘missed,’ we simply cannot see it.
“This can lead to misinterpretation where, The visualized aorta is normal in caliber’ on US can be misinterpreted as ‘the aorta is normal.”
The key word here is “visualized.” This pertains to the portion of the aorta that’s visible on the image.
“The better way for a radiologist to describe it would be, The visualized aorta is normal in caliber, though the distal [further down] aorta is not well-seen due to overlying bowel gas shadows and was not assessed.”
CT Abdominal Scan
Dr. Mollard explains, “CT will essentially never miss an abdominal aortic aneurysm.
“A radiologist may get interrupted multiple times while reading a case and forget to mention a smaller AAA (say <4 cm) due to the distractions (we’re all human), but it will be present on the CT.”
In this case, the radiologist missed the aneurysm, but the image did not.
MRI Abdominal Scan
“MRI will rarely be able to miss an AAA if there is metallic hardware or surgical clips in the area of the AAA,” says Dr. Mollard.
“Metal causes significant artifact on MRI — essentially a black hole with no information — and you literally can’t see the metal or anything around it.
“For patients with surgical clips near the aorta, we simply can’t see anything.”
Ultrasound of the Abdomen
“US can and does miss abdominal aortic aneurysms. US cannot see through gas, and there is frequently a lot of bowel gas between the ultrasound transducer/probe and the aorta.
“This also creates artifact and prevents us from seeing beyond the gas.
“Larger patients can also be difficult to see the aorta in. The US beam weakens the further it travels, and in morbidly obese patients, we may simply not have enough signal by the time the beam gets to the aorta.”
What about the CT angiogram?
The difference between a CT and a CTA is that the CT doesn’t use contrast dye to “light up” blood vessels, whereas the CTA does. “Angiogram” literally means blood vessel image.
Dr. Mollard explains, “CTA is the modality of choice and the workhorse for following up AAA.
“US is a cheap and relatively good way to screen for AAA (if we can’t see portions on US, CT would be the next step).
“MRI/MRA is also quite good, but CT is generally preferred (higher spatial resolution = better anatomic detail and ability to see smaller things).”
Brett Mollard, MD, completed his residency in diagnostic radiology and nuclear medicine at the University of Michigan where he served as Chief Resident. He subsequently completed a fellowship in abdominal imaging at the University of California, San Francisco (UCSF). He currently works in private practice.
Lorra Garrick has been covering medical, fitness and cybersecurity topics for many years, having written thousands of articles for print magazines and websites, including as a ghostwriter. She’s also a former ACE-certified personal trainer.