Radiology Scrabble -- MRI, CT, US?

(c) Warner Bros 1993

I grew up watching the original Star Trek – the one with Bill Shatner & Co. At the time I thought it was pretty cool that Dr. McCoy had that little hand-held scanner gizmo that could tell him exactly what was going wrong with the guy that had just been beamed through a wall. Since then, the idea of a ‘whole-body’ scanner has been a staple of future fiction and has left a lot of us wondering when we’re actually going to have something like that?

The fact of the matter is, probably not for a while. There are actually some really sound reasons that we have so many imaging modalities: first, they don’t all look at the same thing. It’s really like using the right tool for the job. Then there are the side effects, and finally costs also can vary widely and, let’s face it, there’s just no reason to spend more than you need.

In clinical practice I’ve often come across a misconception that there is some kind of progression of ‘betterness’ among the different systems: X-rays aren’t as good as ultrasounds which aren’t as good as CTs which aren’t as good as MRIs. In reality they all have their strengths and weaknesses. Ultrasounds are good for soft tissues and are easy to use in real time. MRIs are also good for soft tissues but are not an ‘as it’s happening’ imaging system and are a lot more expensive, but the level of detail is much better. X-rays are good for looking at more solid structures like bones and kidney stones, and with the addition of radio-opaque markers are quick and easy for things like pulmonary emboli, when a person’s life can be hanging in the balance. CTs are really just fancy X-rays, taken from multiple angles, so the cost is naturally higher simply because more images are being taken, but these can be used to reconstruct three dimensional structures.

There are also limitations based on the systems’ physics: MRIs use powerful magnetic fields and can rip a steel implant right out of you (this is why surgical hardware is usually made of titanium). X-rays and CTs use radiation which must be carefully limited (which is one reason why many times an X-ray on a child will not be ordered: there’s no need to start irradiating them early, they’ll get plenty of that when they're 80).

These are the reasons I don’t feel we’re going to see some kind of ‘all-in-one’ scanner widget any time soon, at least not with our current technology: we’d have to combine these all into one device and then that 26-week fetus would get a dose of radiation more suited to a pneumonia patient, while zinging her mom’s navel piercing across the room like a bullet. And then charge her for it!

So how does the doctor decide what to order? It’s something of an art form, actually, and from my standpoint, if I don’t know right off the top of my head it's always a good idea to talk to the radiologist. I would tell them what I thought I was looking for and they would recommend the best imaging. This may sound a little backward, since one might assume we order scans to see what is wrong, but the actual process is to try to figure out what you are dealing with first (that ol’ ‘History & Physical’ thing), and then order a scan that will either confirm or deny it (take a look at my post ‘Smoking Gun? Or Red Herring?’). And sometimes the educated guess isn’t right anyway, and you find something unexpected that would have shown up better on a different scan. This is why you sometimes get wording in the report to the effect that, ‘a CT with contrast would have been a lot cooler’.

There are whole books written on when and where and why to use this method over that one. Just so we in the trenches can sort it out with a lot less trouble, most of us have found some handy reference that’s not so voluminous (here’s mine):

Rads Guidelines.pdf

Download PDF • 299KB

But then most docs see certain things often enough that they decide pretty quickly what their favorite modalities are.  

It’s all about having the right tool for the job.