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Medical Is MRI or CT better?

  1. Jul 30, 2015 #1
    Hello guys,

    I have few questions

    1. I know MRI is best for soft tissue masses? What does soft tissues mean? I searched on the internet and the definition is vague. Is it simply all the tissue apart from bone in the body, which means its includes organs such as liver. Or is it all supporting tissue in the body such as connective tissue.

    2. If soft tissue includes organs such as liver, pancreas. Does this mean since MRI is best for soft tissue masses, it is always the best investigation for liver pathologies when given the choice of performing CT or MRI. I know this depends on the disease and in some cases biopsy, USS may be better options but I'm just asking if given the choice between CT and MRI.

    3. I know CT is good for bones. But I read somewhere MRI spine is better than CT spine when detecting spinal fractures.

    4. Also is MRI or PET scan better when finding for metasates of certain cancers.

    5. Also is CT or MRI better for bones.

    Thanks a lot :)
  2. jcsd
  3. Jul 30, 2015 #2


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    Hi sameeralord,

    1. Soft tissue is all tissue with a low density, and in out bodies that is all squishy stuff apart from bones.
    This is important only for CT as it's imaging capabilities is influenced by density. So bones are easy to look at through CT.

    2. MRI has a lot of features to view different physiological functions in living tissue of which: https://en.wikipedia.org/wiki/Magnetic_resonance_imaging#Specialized_applications
    While MRI has a generally low resolution compared to CT, the ability to view individual organs on the basis of their different water content is a very powerful tool.

    3. Not sure. No medic. But spinal fractures might not have bone fractures, that can be easily seen with CT, and have physiological markers like swelling which can be viewed by MRI.

    4. No ideea.

    5. For bone fractures i will go with CT, as it is a very high resolution imaging technique. For other bone problems, maybe cancer or who knows MRI would still be better, even with it's low resolution.
  4. Jul 30, 2015 #3
    CT images depend on electron density of the material. In so far as the electron density is affected by the disease process so the CT will be useful. Solid tumors tend to be more dense for example.

    MRI is sensitive to density too but it mainly depends on the relaxation time of the magnetic moment of hydrogen atoms which are affected by the local magnetic field within the tissue.which is affected by its chemical environment. MRI is more flexible than CT while both can use contrast materials injected into the body to enhance the disease process or delineate anatomical structures, MRI is capable of monitoring physiological processes and organ motion. It is also capable of specifically visualizing blood vessels without the contrast agent needed by CT.

    MRI using no ionizing radiation is considered safer than CT.

    As far as resolution is concerned I believe they are comparable. For high res CT you need more radiation exposure (slower or longer scans) for less noising images so unless you know you need the high resolution you will settle for faster scans with lower resolution. The resolution can also be compromised by organ and patient movement during the scan.

    CT and MRI are considered complementary moralities. .CT is better for bone issues. CT is generally less expensive also.

    PET depends on a radioactive material preferentially going to the disease process so it can be more sensitive than CT or MRI. Resolution is generally not of too much concern with PET when you have good sensitivity.
  5. Jul 30, 2015 #4


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    There's no global "better" or "worse" when comparing imaging modalities. The best imaging modality usually depends on the specific question you need an answer to. And within the modality itself, there are often different ways that it can be applied or used to ask different questions.

    With respect to CT, one could argue that it's a "one trick pony" in a sense. You pass low energy ionizing radiation through the body volume you're intersted in, measure the attenuation, and use that information to reconstruct an image. The image is directly related to the radiological properties of the materials in the volume at the energies used (density and effective atomic number run the show). You get large contrast between volumes that have large differences in these properties - so it's easy to see bones for example. Detecting differences between "soft tissues" (which are exactly what they sound like - anything in your body that's a tissue that's not hard) can be difficult because the differences in density and effective atomic number are often subtle. Major advantages of CT include that It can be done very quickly - scans only take a few seconds and that the images tend to be relatively free of spatial distortion. One of the big downsides is that the patient receives a dose of ionizing radiation.

    MRI allows you to extract a lot more information out of the volume in question because the images are based on protons spinning inside a magnetic field and emitting radiofrequency (RF) signals. The different techniques for interrogating and measuring the signal allow you to measure different properties such as the proton density, spin-lattice relaxation (T1), spin-spin relaxation (T2), chemical content through MR spectroscopy, diffusion and perfusion, etc. These properties vary considerably between different types of soft tissue and so the MRI images allow radiologists to detect different conditions, such as the presence of cancer with more certainty than on CT. On the other hand, MRI scans can often take a long time (10 - 20 minutes) and some patients may not be able to lie inside a 1.5 - 3.0 T magnetic field. The images are also susceptable to artefacts and are less reliable with respect to spatial distortion (which can be very important when you're trying to plan a radiotherapy treatment, for example). Further, in answer to your third and fifth question, it's very difficult to get any signal from bone at all.

    PET allows you to extract physiological information. So the patient will be administered a substance, for example a type of sugar labelled with a positron emitting isotope, and then scanned. The positrons will generate coincident photon emissions that can be traced back to the location of the emission. Because the sugar will be taken up in the regions of the body that are metabolically more active, you can get a picture of the regions where the sugar is being used. Some places are naturally more metabolically active than others, but when you find a spot that's normally quite dormant that's using up a lot of sugar, that implies that the cells in that area are proliferating and suggests the presence of a cancer. Modern PET scanners are coupled with a CT scanner and so you tend to get a PET image superimposed on a CT image. Disadvantages of PET are that it take a lot of time for a scan (the patient has to lie in a quiet room for an hour or so before the scan even begins), you have to ingest or inject a radioactive substance, and the last I cheched there was a resolution limit on the order of about 4 mm. On the other hand it can give you information about other physiological processes as well.

    EDIT: Looks like Gleem beat me to it while I was typing.
  6. Jul 31, 2015 #5
    Thanks a lot for all 3 replies :) I have a better understanding now. Thanks for your time guys :)
  7. May 5, 2016 #6
  8. May 5, 2016 #7


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    Are CT scans loud? Louder than an MRI? I've been in an MRI multiple times. How do they compare?
  9. May 5, 2016 #8
    No. There is no banging or loud knocking just sort of wirring sound of an electric motor as it rotates a rather large ring with an x ray tube on a track around the patient.
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