Correct me if I'm wrong, but aren't X-rays generally much more dangerous than an MRI? I would think that things like 7 tesla MRI technology would be the future of medical imaging, not X-rays, if only for the fact that X-rays are cancer-causing.
Bones don't produce a strong MR signal, and usually show up as black. While people occasionally try to interpret these (e.g., https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3500787/) I'm not confident you'd be able to find a hairline fracture on most scans.
The resolution of a modern CT scanner is also crazy good. Ours goes to 90 µm--you can see individual threads on bone screws--and the scan just takes a few minutes. MRI is usually closer to 500µm-1+mm isotropic voxels and the scans are rather long since you need to average many volumes to get rid of noise.
Finally, the risks are totally different. An X-ray or CT scan definitely exposes you to some radiation, but I don't think there are any absolute contraindications. An MRI is safer in that there's no radiation, but can be incredibly dangerous if some kinds of metals are involved; I would be terrified to scan a kid that ate an unspecified toy, for example. Other metals--especially the ones used in medical implants are safer but create giant artifacts around them that may defeat the point of doing a scan.
For the foreseeable future, I think there will be a place for both kinds of imaging (and others!).
Ultrasound has become the best point of care imaging for a lot of things.
I feel like I didn't do MRI justice there, as it's really a family of techniques. You can tune the pulse sequences to image a lot of different things: a T1 scan of the brain, for example, emphasizes white vs. grey matter; T2* highlights fluids. fMRI uses slight changes in the properties of oxygenated vs. deoxygenated blood to infer brain activity, rather that structure. You can even find the distribution of other chemicals via Magnetic Resonance Spectroscopy. People use this to see if levels of neurotransmitters vary in the brain. However, you need some water/protons to image, which is why this works poorly for bone.
PET uses radioactive tracers that either bind to or replace other substances in the body. For example, in a cancer studies you're given a sugar analog called Fludeoxyglucose, or FDG. Different tissues take up glucose at different rates, and metastatic tumors are particularly hungry for it, so a lot of FDG ends up near the tumor. As it decays into oxygen and sugar, it emits positrons, the antiparticle of electrons. When the positron interacts with an electron in body's own matter, it gives off two photons that fly in opposite directions. By measuring coincidences across a set of detectors, you can figure out where the positron came from and the tracer and the tumor must be near by. FDG is the most common, but you can radiolabel lots of other things too, including drugs.
SPECT is similar, in that it uses a radioactive tracer. The SPECT tracers usually emit gamma radiation directly though, and the detector is a bit different--it doesn't rely on coincidences, which makes it cheaper but a bit lower resolution. An even simpler option is a gamma camera, where you get a single 2D image (like an X-ray vs a CT scan); not sure if this is still used much.
Ultrasound you undoubtedly know from people having babies. It gets used to look at other soft tissue too, especially knees. Ultrasound of the heart are often called echocardiograms.
Photoacoustic techniques are absolutely wild. You shine a light (usually a laser) into the subject. The wavelength of this light is carefully chosen so that it a) passes through stuff that's in the way and b) is absorbed by whatever you want to image. When a substance absorbs light, it heats up and when things heats up, they expand. Rapidly expanding things make pressure waves (i.e., sounds). By detecting those sounds from multiple locations, you can figure out where things of that color are. The color of blood varies slightly depending on how oxygenated it is, so you could potentially use this to figure out what parts of the brain are using more oxygen--and thus more active, just like fMRI. (This sounds terrifying, but the temperature changes are minuscule).
If you were planning a brain surgery, you might want an MRI+CT, which would let you map points on the skull (or things inserted into the brain) onto a more accurate map of the brain. You might even add fMRI to know what these regions do (though this is fairly rare still).
Ditto for MRI+PET or CT+PET, where you might want to see where a tumor is relative to some bone or organ.
CT + MRI is pretty standard, often with both a T1 and T2 type contrast of some sort, e.g. SPGR.
But fMRI is rarely used in practice for this, and may not be specific enough for many needs.
Physical mapping is still very common for function.
For the confused:
FDG is glucose with a F for O substitution. Fluorine radioisotope in it decays into oxygen, converting FDG to sugar.
Isn't magnetic resonance spectroscopy just another phrase for nuclear magnetic resonance or does the term mean something else in healthcare? The NMRs I've seen have cryogenic chambers for holding molecular samples and I just don't see that working very well with a live humans.
Or is it literally just taking the NMR spectrum for a neurotransmitter and targeting its resonance? (that's what MRIs do with water molecules right?)
MRI is superior to x-ray for fracture detection. It’s just vastly too expensive and slow. Bone shows up great, dense cortical bone has little signal but most bone isn’t cortical. When bone breaks it produces oedema which is perfect for MRI. When it’s hard to work out clinically and/or the x-day is inconclusive, patients end up in MR.
Bone tumours and other pathological conditions are also imaged with MRI very effectively.
I’m an MR radiographer.
https://www.researchgate.net/publication/12690778_Diagnosis_...
I try very hard to avoid everything below the neck and our tech was pretty dismissive when I wanted to use it as an impromptu X-ray (but we scan almost exclusively heads).
Is it really better than CT? I can see how anything 3D would beat out a planar X-ray.
The idea that radiation == bad, magnetism == good is overly simplistic. It's not true that an MRI can't damage you, so much as that the design is careful to avoid the damage. Principal mechanisms here are SAR (specific absorbtion) and PNS (peripheral nerve stimulation). Basically you are dumping a bunch of energy into the body; if it gets to high you start to heat tissue too much, and/or induced currents can cause nerves to fire off aggressively.
As a patient you don't need to worry much about this - when you design an MRI you have safety systems in place to limit things like this to safe levels. But your point about high field MRI being "the future" also runs into trouble with the pulse sequences having tighter tolerances, for lack of a better term, than at lower fields, before things like SAR become an issue. So you gain some resolution but some things are harder to do. Also, from a clinical view, higher cost higher resolution scans aren't always what you want. High speed, low cost, lower resolution would be great for a lot of situations.
Another issue that is sometimes a problem is that MRI images are not stereotactically accurate (cf CT).
Also the somewhat confusingly used "contrast" in this context. It is used both to refer to the natural differentiation of tissues by a modality (e.g. bone is bright white and easy to discern in x-ray, but some soft tissues just "look the same") but also for the introduction of a "contrast agents". The latter case is when you introduce a chemical that will highlight certain anatomical structures. There are different ways to do this but for the most part a) the contrast agents are not particularly safe and b) there are different ones for both imaging modality and for type of contrast. So there are tradeoffs there also. See for a common example, angiography.
Finally, siting high field magnets is no joke, which keeps the costs high for capex, and the operational costs are high too.
Probably still safe, but it’s not as clear as I had previously assumed.
When I would put humans in our 7T machine they'd sometimes feel their leg kicking during the acquisition. Because it's actively shielded with a pretty steep magnetic field from 0 to 7T you have to advance them into the scanner very slowly otherwise they can become very uncomfortable and nauseous.
Blood is a conductive fluid. When you move a conductor through a magnetic field there is an electric current induced.
1. https://www.gov.uk/government/publications/ionising-radiatio...
The more important measure is CT, not flat x-ray.
A head CT scan delivers 40 mGy of radiation to an adult brain. The dose of background radiation the average human body gets per year in the US is 3.6 mGy. The average adult brain is 1400 cubic centimeters. The average adult body is 62000 cubic centimeters. 1400/62000 = 0.02258. So the brain is actually only exposed to 2.258% of 3.6 mGy of background radiation per year, which is 0.081288 mGy per year. 40/0.081288 = 492. This means a head CT scan is equivalent to 492 years worth of background radiation blasted into the brain in the span of 10 minutes.
A transatlantic flight exposes your entire body to 0.1 mGy of which your brain only receives 0.002258 mGy. 0.002258/0.081288 = 0.028, and 0.028*365 = 10.22. This means a transatlantic flight exposes your brain to 10 days worth of background radiation over the span of 20 hours.
Also getting secondary cancers years down the line isn’t an issue for palliative type treatments.
So plenty of uses.
The machines are not cheap, but a busy site can churn through scans, so it can end up working out to something fairly reasonable and not at all the most expensive thing going on in US healthcare.