6 ms·

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

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.

TheSpiceIsLife 7y ago
A chest x-ray is about 5x less radiation exposure then a transatlantic flight.[1] 1. https://www.gov.uk/government/publications/ionising-radiation-dose-comparisons/ionising-radiation-dose-comparisons https://www.gov.uk/government/publications/ionising-radiatio...
- sosilkj 7y ago
  I think that is an interesting comparison but I'm not sure it's really applicable. A brain MRI can take up to 45 minutes or so. To be honest, I'm doubtful you could image someone's entire brain in detail via X-ray (as the article seems to imply) by using the sort of "one-and-done" X-ray normally used to look for run-of-the-mill bronchial infections, etc.
  - klipt 7y ago
    Right, the 3D equivalent is a CT scan which is a much higher radiation dose than a regular x-ray. On average every 1000 CT scans done causes one new case of cancer.
    - stilley2 7y ago
      Source?
  - mattkrause 7y ago
    A brain CT doesn't take 45 minutes though--and a brain MRI doesn't use x-rays.
- ebg13 7y ago
  > A chest x-ray is about 5x less radiation exposure then a transatlantic flight.[1] The more important measure is CT, not flat x-ray.
- FriendlyNormie 7y ago
  Let’s assume this new X-ray technique is not actually an ordinary X-ray but really is a modification to a CT scan, since otherwise there’s no reason for them to compare it to an MRI. 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.
- Cantbekhan 7y ago
  Always relevant http://xkcd.com/radiation/ http://xkcd.com/radiation/
deleted 7y ago
[deleted]
mattkrause 7y ago
Depends on what you want to image (and why). 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/ 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!).
- sorenjan 7y ago
  Thanks for the informative answer. What other kinds of scans are there? I know of PET scans, how are those used? Do people combine different kinds of scans into one visualization?
  - hashmymustache 7y ago
    Emissions based scans like PET/SPECT use positrons/gamma rays released by an infused tracer for imaging things like hungry tumors (labeled glucose) but can also be used to image other molecules or receptors. Very low resolution. Usually overlayed on CT scans. Ultrasound has become the best point of care imaging for a lot of things.
  - mattkrause 7y ago
    tl;dr? Skip to the photoacoustic stuff at the end, which is amazing. 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).
ben_w 7y ago
IIRC, for fields >5T the movement of your blood is enough to induce currents you can detect as a weird tingling in your tongue. Probably still safe, but it’s not as clear as I had previously assumed.
- mattkrause 7y ago
  You sure it's the blood? I would have guessed it affects nerves directly.
  - ben_w 7y ago
    The more I introspect, the less confidence I have in my own knowledge in this regard. At best, I forgot the important lesson of Gell-Mann Amnesia.
- hashmymustache 7y ago
  I'm not sure what you mean by movement of blood induces currents. The magnetic gradients are what induce current in the body, but usually more peripherally where the gradients are steepest and in big nerve bundles like sciatic. 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.
  - phkahler 7y ago
    >> I'm not sure what you mean by movement of blood induces currents. Blood is a conductive fluid. When you move a conductor through a magnetic field there is an electric current induced.
    - cma 7y ago
      what about rotating a ring on its axis of symmetry? Blood is in a loop, though not perfectly smooth and unchanging flow, as valves and stuff are squeezing at different time.
SketchySeaBeast 7y ago
Long term, sure, but right now there's a limited number of very expensive MRI machines, X-rays are much more ubiquitous. Even if this involved a new machine, I imagine it would be much cheaper than an MRI.
- Merrill 7y ago
  Liquid helium to cool superconducting magnets in MRI machines is also at a premium. This may use a much less expensive machine.
- roywiggins 7y ago
  In the US the cost of an MRI varies wildly and doesn't much correlate with the cost of providing them. 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.
epanchin 7y ago
MRIs are expensive. X-Rays are dangerous, but if you’re about to give someone radiotherapy I would think negligible. Especially if it means you can better target the treatment dose. Also getting secondary cancers years down the line isn’t an issue for palliative type treatments. So plenty of uses.
ska 7y ago
Others have answered part of this well so I won't repeat, but there are some other factors that aren't obvious so I'll mention them. 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.
Perenti 7y ago
Those of us with certain implants can't have MRI without either our chests being torn apart or burned by red-hot metal from the inside. X-Rays are much safer in my case.

TheSpiceIsLife 7y ago

A chest x-ray is about 5x less radiation exposure then a transatlantic flight.[1] 1. https://www.gov.uk/government/publications/ionising-radiation-dose-comparisons/ionising-radiation-dose-comparisons https://www.gov.uk/government/publications/ionising-radiatio...

sosilkj 7y ago
I think that is an interesting comparison but I'm not sure it's really applicable. A brain MRI can take up to 45 minutes or so. To be honest, I'm doubtful you could image someone's entire brain in detail via X-ray (as the article seems to imply) by using the sort of "one-and-done" X-ray normally used to look for run-of-the-mill bronchial infections, etc.
- klipt 7y ago
  Right, the 3D equivalent is a CT scan which is a much higher radiation dose than a regular x-ray. On average every 1000 CT scans done causes one new case of cancer.
  - stilley2 7y ago
    Source?
- mattkrause 7y ago
  A brain CT doesn't take 45 minutes though--and a brain MRI doesn't use x-rays.
ebg13 7y ago
> A chest x-ray is about 5x less radiation exposure then a transatlantic flight.[1] The more important measure is CT, not flat x-ray.
FriendlyNormie 7y ago
Let’s assume this new X-ray technique is not actually an ordinary X-ray but really is a modification to a CT scan, since otherwise there’s no reason for them to compare it to an MRI. 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.
Cantbekhan 7y ago
Always relevant http://xkcd.com/radiation/ http://xkcd.com/radiation/

deleted 7y ago

[deleted]

mattkrause 7y ago

Depends on what you want to image (and why). 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/ 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!).

sorenjan 7y ago
Thanks for the informative answer. What other kinds of scans are there? I know of PET scans, how are those used? Do people combine different kinds of scans into one visualization?
- hashmymustache 7y ago
  Emissions based scans like PET/SPECT use positrons/gamma rays released by an infused tracer for imaging things like hungry tumors (labeled glucose) but can also be used to image other molecules or receptors. Very low resolution. Usually overlayed on CT scans. Ultrasound has become the best point of care imaging for a lot of things.
- mattkrause 7y ago
  tl;dr? Skip to the photoacoustic stuff at the end, which is amazing. 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).

ben_w 7y ago

IIRC, for fields >5T the movement of your blood is enough to induce currents you can detect as a weird tingling in your tongue. Probably still safe, but it’s not as clear as I had previously assumed.

mattkrause 7y ago
You sure it's the blood? I would have guessed it affects nerves directly.
- ben_w 7y ago
  The more I introspect, the less confidence I have in my own knowledge in this regard. At best, I forgot the important lesson of Gell-Mann Amnesia.
hashmymustache 7y ago
I'm not sure what you mean by movement of blood induces currents. The magnetic gradients are what induce current in the body, but usually more peripherally where the gradients are steepest and in big nerve bundles like sciatic. 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.
- phkahler 7y ago
  >> I'm not sure what you mean by movement of blood induces currents. Blood is a conductive fluid. When you move a conductor through a magnetic field there is an electric current induced.
  - cma 7y ago
    what about rotating a ring on its axis of symmetry? Blood is in a loop, though not perfectly smooth and unchanging flow, as valves and stuff are squeezing at different time.

SketchySeaBeast 7y ago

Long term, sure, but right now there's a limited number of very expensive MRI machines, X-rays are much more ubiquitous. Even if this involved a new machine, I imagine it would be much cheaper than an MRI.

Merrill 7y ago
Liquid helium to cool superconducting magnets in MRI machines is also at a premium. This may use a much less expensive machine.
roywiggins 7y ago
In the US the cost of an MRI varies wildly and doesn't much correlate with the cost of providing them. 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.

epanchin 7y ago

MRIs are expensive. X-Rays are dangerous, but if you’re about to give someone radiotherapy I would think negligible. Especially if it means you can better target the treatment dose. Also getting secondary cancers years down the line isn’t an issue for palliative type treatments. So plenty of uses.

ska 7y ago

Others have answered part of this well so I won't repeat, but there are some other factors that aren't obvious so I'll mention them. 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.

Perenti 7y ago

Those of us with certain implants can't have MRI without either our chests being torn apart or burned by red-hot metal from the inside. X-Rays are much safer in my case.