4 ms·

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 t

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).
  - mattkrause 7y ago
    These all have different strengths and weaknesses, so combining them is pretty common. 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.
    - ska 7y ago
      More likely to see DTI than fMRI today in practice for this sort of work. CT + MRI is pretty standard, often with both a T1 and T2 type contrast of some sort, e.g. SPGR.
      - mattkrause 7y ago
        DTI identifies white matter pathways, but it can't (alone) tell you much about function, can it?
        ska 7y ago
        True, at least not directly (you can segment tractography to infer this to some degree). 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.
  - labawi 7y ago
    > As [FDG] decays into oxygen and sugar, ... For the confused: FDG is glucose with a F for O substitution. Fluorine radioisotope in it decays into oxygen, converting FDG to sugar.
  - akiselev 7y ago
    > You can even find the distribution of other chemicals via Magnetic Resonance Spectroscopy. 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?)
lostlogin 7y ago
> 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. 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_of_scaphoid_fracture_and_dedicated_extremity_MRI https://www.researchgate.net/publication/12690778_Diagnosis_...
- mattkrause 7y ago
  Interesting... 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.
- jmhmd 7y ago
  MR is not always superior to x-ray for fracture detection, in fact, in the acute phase MR is often normal as marrow edema has not yet developed. Resolution is far worse than CT or plain film xray, making subtle non-displaced fractures impossible to see. MR can be good for occult fractures that are not visible on xray, but usually better after at least a few days so that marrow edema will have developed.
  - lostlogin 7y ago
    Thanks for this. In keeping with what you say, the type of patient we see is nearly always a week post injury with pain but a negative x-ray. They want to work or travel or something and are after a definitive answer. Another key benefit to MRI, I suspect, is that if it isn't a fracture a CT isn't that helpful, while any soft tissue injury is likely well demonstrated on MR.

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).
- mattkrause 7y ago
  These all have different strengths and weaknesses, so combining them is pretty common. 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.
  - ska 7y ago
    More likely to see DTI than fMRI today in practice for this sort of work. CT + MRI is pretty standard, often with both a T1 and T2 type contrast of some sort, e.g. SPGR.
    - mattkrause 7y ago
      DTI identifies white matter pathways, but it can't (alone) tell you much about function, can it?
      - ska 7y ago
        True, at least not directly (you can segment tractography to infer this to some degree). 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.
- labawi 7y ago
  > As [FDG] decays into oxygen and sugar, ... For the confused: FDG is glucose with a F for O substitution. Fluorine radioisotope in it decays into oxygen, converting FDG to sugar.
- akiselev 7y ago
  > You can even find the distribution of other chemicals via Magnetic Resonance Spectroscopy. 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?)

lostlogin 7y ago

> 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. 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_of_scaphoid_fracture_and_dedicated_extremity_MRI https://www.researchgate.net/publication/12690778_Diagnosis_...

mattkrause 7y ago
Interesting... 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.
jmhmd 7y ago
MR is not always superior to x-ray for fracture detection, in fact, in the acute phase MR is often normal as marrow edema has not yet developed. Resolution is far worse than CT or plain film xray, making subtle non-displaced fractures impossible to see. MR can be good for occult fractures that are not visible on xray, but usually better after at least a few days so that marrow edema will have developed.
- lostlogin 7y ago
  Thanks for this. In keeping with what you say, the type of patient we see is nearly always a week post injury with pain but a negative x-ray. They want to work or travel or something and are after a definitive answer. Another key benefit to MRI, I suspect, is that if it isn't a fracture a CT isn't that helpful, while any soft tissue injury is likely well demonstrated on MR.