Radiology Primer for Non-Radiologists

I’m writing this to try and help anyone who’s in training (or beyond!) have a basic understanding of radiology studies. I feel like radiology bitches all the time about people ordering the wrong study, and we expect people who aren’t radiologists to understand all the intricacies of everything we do. That’s obviously wrong. I can’t take out a gallbladder, intubate a patient, or manage active heart failure. So I don’t know why I should expect you to know the difference between a standard CT Abd/pelvis and a 3 phase study. We are all on the same team, so I’d like to try and help.

Radiographs

A great first line imaging modality. Truly excellent at looking at bones. Due to the physics of how it works, it’s less great at looking at soft tissues. Dense things are white and non-dense things are black.

If you are placing NG tubes, ET tubes, or central lines, you need to get comfortable with looking at your own imaging and assessing placement. Those tubes can cause problems if misplaced, and radiology can have a lag time in looking at them.

Tips:

· NG tubes should course midline and then go below the diaphragm. If it veers left or right at T6-ish, you’ve put it in the airway.

· ET should be 2-5 cm from the carina (honestly everyone has their own preference). Basically, make sure it’s below the clavicles and not in a mainstem bronchus. If you can count ribs, the carina is usually at approximately T6.

· Central lines: Right side lines should go straight down into the SVC/right atrium. Left side lines should cross midline on their way to SVC. If you see a left side line remain on the left side, it may still be venous due to duplicated SVC (or internal mammary vein, but that’s less common). Check a blood gas to see if it’s venous.

· If you are concerned for pneumoperitoneum, make sure to get an upright radiograph. Air is moves up and may not be seen if the patient is supine.

CT

The donut of truth. It’s good at most things, with certain limitations. It functions using xrays, but instead of one image, it does it in cross sections. Again, density is the name of the game and CT is a density map. It’s excellent at looking at bones if you have a questionable finding on XR. It’s also great for evaluating most acute causes of disease, and many chronic causes as well.

One of the most common areas of confusion for non-radiologists is contrast. Contrast is something injected into the vascular system which allows us to highlight certain portions of the vascular system based on what we want to look at.

· CTA: CT angiographs. There are arterially phased studies. The technologist waits until the contrast is at high concentration within the aorta, then triggers the study. In the abdomen and pelvis, this allows for evaluation of the aorta and the proximal portions of its branches. In the head and neck, this allows for assessment of the carotid and vertebral arteries of the neck, and the intracranial arteries of the head. This is primarily used to assess for large vessel occlusion (LVO).

o This can be timed for mesenteric evaluation in GI bleeds and concerns for ischemic bowel.

· Standard CT scans: Performed in the portal venous phase. Typically this allows for roughly equivalent enhancement (fancy term for contrast being present) in the arterial and venous system. Because it’s diluted through the entirety of the vascular system, it’s not great for evaluating distal branches.

· CT PE: The scan is triggered when contrast concentration is maximum at the main pulmonary artery.

· CT venogram: Triggered when the majority of the contrast is in the venous system.

· 3 phase: This includes a non-contrast study, an arterial phase, and a venous/delayed phase.

o Acutely, this is especially good for evaluating for bleed, particularly GI bleed. You’ll see nothing on non-con, a little intraluminal extravasation on the arterial phase, and even more on the venous phase as the amount of blood has increased between scans.

You have to know what you’re looking for and what a scan is good for. For instance:

· CT brain non-contrast: If you already know this, I’m sorry. But it seems like no one knows why we actually do these in stroke patients. These are NOT done to evaluate for stroke. These are done in the setting of clinical stroke to rule out bleed so TPA can be administered. CT brain are terrible for evaluation of stroke. Currently, the earliest we can image ischemic brain is about 6 hours, and it shows up only on restricted diffusion sequences on MRI (deep nerdy stuff). It takes much, much longer to notice on CT (the cells have to die and cause cytotoxic edema). That’s why every inpatient CT brain ordered for altered mental status is negative unless there’s focal deficits. They are a waste of time. Diagnostically anyway. I understand that there’s some medicolegal stuff at play here.

· MRA: This is a good study for evaluation of the vascular anatomy. It is not better the CT angio in almost any situation. At best, it’s equivocal. But people assume MRI=better.

· If you have a soft tissue hematoma and you want to look for active bleeding, don’t get an angio only phased study. This almost certainly won’t show it because it will be too early. Either get a normal phased study (portal venous), or do a multiphase study with angio and delayed.

Those are just some common examples I see. If you don’t know what you’re looking for, then how can you expect to image it?

MRI

This machine is magic. Using the unholy powers of magnets and radiofrequency pulses, MRI allows us to evaluate tissues based on a variety of characteristics. This is excellent for soft tissue evaluation, evaluation of areas that are difficult to see on CT (for instance, the spinal canal which is usually very difficult to interpret on CT due to bone artifact), brains, masses….the list goes on.

As mentioned above, you need to know what you’re looking for. Even more so with MRI than with CT. I’ll give an example:

A provider was concerned with a patient who had lost the sense of taste. They ordered a CTA of head and neck. This study will provide nothing. Loss of taste may be neurologic, but CTA evaluates brain parenchyma (poorly) and shows large vessel occlusions. An LVO is extremely unlikely to cause isolated loss of taste bilaterally. What the provider needed to order, if they were concerned about CNXII if they were concerned about cranial nerves or brain nuclei involved in taste, was a MRI brain. But not any MRI brain. They need a small field of view with a particular sequence which highlights the cranial nerves.

So again. Know what you’re looking for or else the MRI will be useless.

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Nuclear Medicine

Let's talk about nuc med. This is a pretty area of radiology where we use gamma emissions from a radioisotope to image. What we image depends on what pharmaceutical or molecule we tag the radioisotope to. For instance, for a tagged bleeding scan, we take the patient's red blood cells, tag them with something called Tc-99m (this emits gamma radiation which is what we can image), and then place the patient under the gamma camera. If it's positive, we will see signal coming from somewhere outside of the vascular system and in the region of the GI tract. There are many other things we can tag the radioisotope to, and that determines what we can image and evaluate.

Let's talk about some of the more common studies.

HIDA: This uses a pharmaceutical that is taken up by the liver and excreted into the bile ducts. In normal studies, it's excreted into the bile ducts and is then seen in the small bowel and collecting in the gallbladder. If the gallbladder isn't seen, then it's obstructed (no radioisotope can get in) and presumptively diagnosed as acute cholecystitis. This can also be used to diagnose chronic cholecystitis/biliary dyskinesia by administering CCK (which causes gallbladder contraction) after filling of the gallbladder and measuring ejection fraction.

Tagged Red blood cell scan: Discussed above. It is exquisitely sensitive for detecting bleed, even moreso than a CTA. Here's the problem, and why it's not super useful: If you can't see a bleed on CTA, you won't be able to see it on angiography which is what IR uses for embolization. So a tagged red blood scan will confirm active bleeding, but won't be able to help much beyond that if the bleed isn't evident on CTA.

VQ: This is useful if you are concerned for PE but don't want to give the patient contrast. You really need a CXR or other chest imaging within 24 hours for accuracy.

Brain death studies: Radioisotope is injected into the venous system, then imaged for two minutes to see that it goes bilaterally into the common carotids. Static images are taken for a few minutes afterwards. In the setting of brain edema, like what happens in brain death, there is too much intracranial pressure for blood to get in the intracranial space. It's shunted to the ECA. Seeing no radioisotope in the region of the brain is compatible with brain death. Please note it is compatible and not diagnostic. Brain death remains a clinical diagnosis in most (all?) states.

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I'm happy to answer any questions about any studies. I'm also happy to be corrected if you see something I've said which you don't believe to be true.

Finally, if you have any questions about which study you should order, call radiology. It's literally their job to help you. Every specialty has assholes, and it seems like radiology is forgetting that we are a consult service, but we are here as specialists to help you diagnose the patient. So give us a call!

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edit: Added nuclear medicine common studies.

edit 2: It was pointed out to me that CNXII isn't involved in taste, which is true. My bad. I've struck the original phrasing and corrected it.