Thursday, September 23, 2010

3..5 Secrets of Body Imaging - X-ray & CT, MRI, Ultrasound, EEG

Physician's Notebooks 3 - - See Homepage

 X-ray & CT, MRI, PET Ultrasound and EEG - Update 10 December 2017

The following descending column in order as each appears in text may be used for reading a topic by search & find  or scroll down.
Computer Tomography (CT)
PET Scans
Magnetic Resonance Imaging (MRI - T1, T2, FLAIR)
Interventional Imaging (Balloons, Stents, Coils)
Ultrasound Imaging (US, Echo)
Other Imaging SPECT, functional MRI, Radioactive Imaging 
EEG (Electroencephalography)

Until the 1970's for body imaging we relied on x-ray. Each time you get x-ray, you have a small risk of cancer later in life. (Risk based on procedure, technique and machine) The younger, the more risk, so x-ray of infant (or worse, pregnancy) is riskiest. (Depends on body part; neck is high risk because of thyroid cancer) We get x-ray if doctor says we need it, because its benefit is in the now and its risk is in a far future. But now, ultrasound or MRI can give better imaging with no damaging radiation risk (but sometimes, MRI, at much higher cost). And for examination of your brain, a yearly EEG is a noninvasive inexpensive screen.
   When you get x-ray, the emitting machine is a cathode tube that shoots electrons at a metal piece that emits x-rays in a spray but mainly directed into your body for imaging. Most x-rays pass through a body and strike the photo plate behind it. (Today in 2017, electronic screen replaces the plate) What develops is the partial shadow of your body tissues. X-rays pass through air completely so air-filled lung gives blackened image. But various tissues block x-rays. Denser tissue allows less x-rays in and densest tissue like bone gives an image that is white.
   The X-ray’s high- wave frequency and short wavelength combined with lack of electrical charge allows it to penetrate deeply into body tissue and into its atoms and when it does, it may knock one or more electrons out of orbital and that creates a high-energy ion (Hence, ionizing radiation), resulting in tissue damage; also it may penetrate the nucleus and knock electrons from the atoms of the DNA, which leads to risks of mutation to cancer or, in a young woman, to risks of malformation of her future babies in later pregnancies.
   “Getting x-ray” (with bad effects that go with it) is not just chest x-ray, or barium x-ray, or kidney dye IVP x-ray; it may also be computed tomography (CT).  A CT is x-ray “in spades”, because it packs more body ionizing radiation than all other types of X-ray.
  Why do we still need x-ray?
  Ultrasound (US) cannot image bone well; also an organ like the lungs gives a poor US image. And MRI is expensive (machine & test) and complicated so x-ray and its CT still has a place. 
   An x-ray exam should be accurate, inexpensive and low-risk. After trauma accident, the x-ray may give useful info.
   Where you get x-ray and who interprets it is important. Best is certified radiologist in clinic or hospital. Radiologist should supervise x-ray but it is normally done by a technician. This prevents unneeded x-ray, assures that x-ray machine meets rules for low radiation, and improves diagnosis.
   Chest x-ray checks bony thorax, lungs and heart. In 1930, a chest x-ray for TB was necessary; today x-ray for TB still may need to be done but first the skin test should show TB positive. Chest x-ray also is useful when heart disease suspected. An anterior-posterior (AP) outlines heart size and shape, and estimates its function based on degree of lung congestion. Today  much of chest x-ray for heart disease has been obsoleted by the ultrasound echocardiogram,  which gives no radiation and more information but is more expensive.
   Abdominal x-ray (in specialized views, KUB, or Kidney Urinary Bladder) is front-back view of abdomen that still has place with acute abdominal pain that lasts to emergency room visit.
   Suspected fracture or joint damage requires x-ray supervised by orthopedist (bone doctor).
   In head trauma, the simple skull x-ray remains valuable at its small risk. (Head trauma simple x-ray showing skull fracture has up to 20 times the predictive accuracy for brain damage) MRI is best for detecting a tumor or stroke. But with acute brain stroke, the CT is still first choice for many hospital emergencies because it may be better than MRI to discover early acute brain bleed. Also, especially in poor countries, the hospital may not have an MRI machine.
   Mammography (MM) is x-ray used to screen breast for pre-cancer. Screening is for breast with no obvious tumor. With suspicious breast lump or suspicious mammography, proceed to a top breast cancer diagnosis and treatment center for ultrasound-directed needle biopsies and other diagnostic imaging. See Breast Cancer chapter in Notebooks 10 for advice on mammography.
   Gastrointestinal (GI) Barium Dye x-ray, Gallbladder x-ray (Cholecystography), Kidney x-ray (Intravenous Pyelography) and Bladder dye study were frequently done procedures that now are much less frequent because of superior information, lower risk, and lesser expense of UltraSound and the much higher accuracy of MRI scans.

Computer-Tomography:   The CT is well defined by its 2-words. Tomography is a rapidly shot series of body x-ray focused on millimeters-thick slices of the body at various levels. And then the x-rays are integrated in computer to make virtual 3-D image slice of the body organ or part being imaged. If a large area of the organ (brain tumor or hemorrhage) is being looked at, many such slices are made at successive levels. CT was developed in the early 1970s from computer application of the already existing x-ray tomography. The first commercial CT was in 1975, a few years before MRI. The CT represented a big advance; it made a virtual image dissection of the human body, in horizontal slices. Quickly it was associated with radio-opaque dye. (Dye absorbs x-ray; appears as white contrast to darker tissue shadow on x-ray screen) But CT in 2017 is being bested by MRI, which for most views or organs does everything CT can do but does it better, with no dye or less risky dye, and without the ionizing radiation. 
    CT machines consist of a “gantry” (hollow metal cylindrical corridor, not closed off) that the patient lies inside of on an examining table. Now, electron beam CT (EBCT) has come into use and cut time to a sixth of the mechanical rotation x-ray tube CT. The CT still retains a slight advantage over MRI in very acute brain hemorrhage because it detects small degrees of blood spill into the brain or its cavities a little earlier than MRI. However, this will change within a few years; essentially MRI is superior to CT in accuracy and earliness of picking up disease.

PET Scans are Positron Emission Tomography; it combines the CT scan with biologically radioactive tracer injection. In a PET scan a biologically active substance identified with a particular brain disease, like the tau protein in Alzheimer's Disease, is labeled with an unstable radioactive isotope atom (replacing a normal atom in the protein) and then is injected into the blood and, in the brain, it zeros in on the Alzheimer pathology. The unstable isotope releases a positron (1+ charge anti-electron) which immediately is annihilated by a nearby electron and the annihilation gives off opposite direction traveling gamma waves which are detected by the PET scanner and the sum of the many reactions are expressed as a CT scan that shows the Alzheimer Disease. The following figures help understanding.

Emission of gamma rays in PET scans:
The nucleus of an unstable radionuclide emits a positron. The positron travels a certain distance before it collides with an electron and is annihilated, emitting two gamma rays that travel in precisely opposite directions. The site of positron annihilation that is imaged may be a few millimeters from the site of origin. For example, the average distance between the site of origin and annihilation is 2 mm for 18F (Fluorine atom radioactive isotope) and 3 mm for 15O (Oxygen atom radioactive isotope). The distance between the emitting nucleus and the site where the positron is annihilated is an absolute limit on the spatial resolution of PET scan images.

PET scanners contain an array of gamma ray detectors encircling the subject's head.
Only gamma rays that are detected simultaneously by diagonally placed detectors are recorded.

Image not available.

PET image.
PET produces an image showing the areas of heightened neural activity as revealed by the radionuclides.

Image not available.

 Because of the rather high radiation exposure (PET exposes a patient to 100 times more radiation than a chest x-ray and 3 times more than the usual CT) PET has a limited though important use. Also it is a relatively slow procedure because it involves injecting the body with radioactive tracer and waiting up to 1 hour before the CT shows the metabolic effects of the tracer in the tissue being studied. Also it is expensive. But PET scanning has found use in staging the advance of cancer, in looking for metastases and in making early diagnosis of Alzheimer's and other difficult-to-diagnose brain diseases.

Magnetic Resonance Imaging (MRI - T1, T2, FLAIR)
    What Actually Does MRI Do and How? It is useful to compare it to CT because of similarities and differences.
   Both CT and MRI produce computerized image of millimeters-thin body slices as virtual 3-dimension. An advantage of MRI is that, practically, CT only produces horizontal body slices (like horizontally bread-slicing your head sideways ear to ear starting from top) but MRI also gives longitudinal up-down slices. This makes MRI ideal imaging for taking a look at the whole spinal cord, because in one image you see length of cord.
   Both CT and MRI involve patient in whole-body housing but the full form of MRI involves the patient inside a fully enclosed capsule, which may be frightening and practically impossible for young children or the mentally unstable. (Recently, an unenclosed MRI solves that problem but has a slightly lower image quality because of the lesser need for the very strong magnetic field of the original enclosed MRI)
   CT costs more than MRI but both are relatively expensive and take longer than a usual x-ray.
   The enclosed MRI capsule machine is more massive than the CT machine. This adds to cost and, as implied above, produces isolation and anxiety.

Most important, the CT uses x-ray with its invasive potential damage while MRI does not involve x-rays
   What about MRI's shooting electromagnetic wave into body? Is not that harmful? In answering questions and describing how MRI actually works why don’t I guide you through getting one done?
   First we show up at hospital imaging office at 15 minutes before appointed test time. Instructions may or may not say to eat or drink after .
   We will be asked to remove clothes and body objects and change into patient gown. That brings us to entering the MRI body test area. (Recently, the open MRI machines are less massive and less frightening than the older enclosed capsule)  We lie down on a stretcher that is pushed into the opening at one end of MRI capsule and (inside of a closed MRI) it is closed off; and dark, quiet and scary.
   Why the shape, size and need for the enclosed or semi-enclosed body test area? While an MRI is going on, for highest quality imaging, we need to be subjected to magnetic field 100,000 times more powerful than Earth’s magnetic field.  It is produced by circular coils surrounding the imaging space, and needs cooling to 4 K. (Kelvin, zero K is absolute zero where all molecular motion stops, circa minus 273 C, or minus 4600 degrees Fahrenheit) Yet we don’t feel cold! Why? Because of thick insulation around coils. Thus the thick body-enclosing capsule.
   How about the affect of the powerful magnetic field? It involves important warnings, some of which we read on pre-MRI instruction sheet and others I give you here. If you have implantable electronic device (pacemaker, defibrillator), metallic implant or metal shrapnel or bullet or fragment embedded in body, it is a reason to stay away from MRI. A related, lesser known and rarely considered but potentially serious problem is for operators of machine tools who may acquire unnoticed small metal fragment in orbit of eye and under powerful MRI magnetic field the fragment can do damage. Best to discuss ahead with doctor. There is a story of a physician assisting a patient in vicinity of MRI capsule having his metal instruments drawn from pockets and forcibly striking the patient. (Check
   Other than magnetically attracted object, no report yet of immediate body damage or cancer from MRI. One long-term effect is increased eye cataracts in offspring born to animals subjected to MRI. It means a No-No for MRI in pregnancy. Another reason to avoid it in pregnant woman is that the ultra powerful magnetic field may affect the migration of brain and spinal neurons that is going on in the unborn baby during pregnancy and that could cause retardation or mental illness in offspring. (No evidence for this but it has not yet been subject to test) And since, if there is an effect, that would be terrible, it is another reason for No No for MRI in pregnancy (unless an overwhelming, life-saving medical need for the MRI).
   How is the MRI image made? The 100,000 times greater magnetic field does not produce the MRI image but it provides the necessary environment for it. This powerful field affects the orientation of hydrogen ions (H+) in such a way that a certain percentage of the ions line up in a direction in which their spin is oriented to the MRI north pole (in parallel with Earth's north pole). In that state, a series of radio frequency pulses (like the electromagnetic waves that carry cell phone, radio and TV signals) are shot into part of body being imaged and excite the magnetized H+ ions to tip over for the instant of the pulse, and, when the pulse passes, the H+ ions will relax and tip back into their original orientation and in so doing each one emits a radio frequency signal. Millions of these signals from millions of H+ ions tipping over and then relaxing after repeated bursts of the radio signal, are fed into a computer and after completion of the MRI scan, a 3-Dimensional image is produced of body part being examined. In MRI of the head, the series of slices are reconstructed into a picture of the individual brain.
   Beyond the already cited advantages of MRI is its sensitivity: For the enclosed capsule high-magnetic field MRI, as small as 1 mm diameter brain tumors or blood vessel abnormalities can be picked up by simple MRI compared to a lower limit of 10 mm for the equivalent CT scan with dye.
   The disadvantages of MRI are being resolved with time but still remain blocks to MRI usage: MRI is more expensive and for certain techniques (noninvasive coronary angiography), or very early detection of bleeding in brain; and certain organs or types of tissue (lung and bones) may not be as revealing as the EBCT scan. Thus, despite the MRI technical superiority over CT and its greater safety because of CT's x-ray damage, the CT may sometimes need to be done before an MRI is done (Acute brain hemorrhage). So, if your doctor suggests you get CT scan, ask in a nice tone Why CT?
   Types of MRI - T1, T2, FLAIR and Diffusion Tensor Imaging, or DTI:  "T1-" and "T2-weighting" are slightly differing techniques of MRI with specific time constants for proton relaxation. "T1" makes fluid appear dark and makes concentrated tissue like the underlying white matter in brain appear lighter than the neuron-dense gray matter of brain in the cerebral cortex while "T2" does the opposite (Looking at an MRI image of brain or spinal cord you can always spot T2 technique because the fluid in the brain/spinal images appears bright white). The T2 -FLAIR (fluid attenuated inversion recovery, usually just "FLAIR") suppresses the bright fluid signal of T2 images in order to help interpretation of brain tumors. The separate DTI further improve the imaging by identifying axonal bundle tracts in the brain's white matter. A dye - Gadolinium - is sometimes injected intravenously just before doing an MRI scan in order to enhance brain tumor detection but it has caused occasional severe allergic and kidney reaction, so a patient about to get MRI should pay particular attention to the need for the dye in his signed permission for the procedure.
For more about special MRI noninvasive imaging for artery blood vessel circulation in brain and in heart called MRA (magnetic resonance angiography and click on 9.24b Secrets of Neuroimaging).

Interventional Imaging and Treatment (Balloons, Stents, Coils) are becoming frequent and important. In Notebooks 6, non-invasive and invasive coronary angiography with stenting is discussed. The non-invasive CT or MRI coronary angiography (CTA, MRA) should be done when diagnosis and not intervention is the reason to do. Interventional artery balloon and stent is useful for opening up a narrowing in the carotid and cerebral arteries in treatment or prevention of strokes, especially after a TIA (transient ischemic attack - a fleeting stroke symptom) It is discussed in Notebooks 9.  
  The other important interventional procedure is small-coils embolization to stop hemorrhaging from arteries that are either difficultly accessible for the surgeon or to delay the need for risky surgery in a bleeding patient in poor condition. In coils embolization, a catheter artery-angiography is first done to outline the artery and localize the bleeding site. Then the catheter is advanced to just before the bleeding point and many tiny metal coils are released to clog the damaged artery area and stop the bleeding. In experienced hand it is low risk and may be organ-saving or life-saving. Especially in bleeding brain aneurysm and in hemorrhages of the spleen where normal surgery is very risky.

Ultrasound Imaging (US, Echo) signal is high frequency sound waves sent from external transmitter. Its return-reflection, depending on density of tissue is put together on computer TV screen to show image of the anatomy. An organ like liver, dense enough to reflect much of the US signal, is whiteness on screen, while an organ like a urine-filled bladder not very dense shows up dark. Combination of white and dark gives good image of organ. The liver appears as dense white mass surrounded by less dense dark. If liver has cyst, the less dense area of cyst is darker and surrounded by white of liver giving real-life size measurement of cyst. Or if liver is seeded with tumor from cancer metastasis, the denser tumor tissue stands out as whiter area within darker normal liver tissue and can be measured
   The US is useful in showing real-time action - such as unborn baby moving in womb, or heart beating, or large blood vessel pulsing or blood flowing in artery imaging the artery wall.
   All studies show US imaging to be without harmful effect and because of low expense, it can be repeated rapidly unlike with x-ray, CT or MRI and allowing serial study, increasing its diagnostic power.
   Ultra-Sound cannot pass through bone, so not as useful in bone disease; also not good for lung examination or, in adult, to view brain.
   The echocardiogram that uses Doppler ultrasound color imaging for the heart has evolved into the best imaging of heart shape and function, not only in accurate numerical and anatomic data it gives but also in safety and convenience; and it is less invasive and low cost.
  Doppler US is also very useful noninvasive imaging the carotid arteries in neck to prevent the worst brain strokes. If a severe carotid artery narrowing is diagnosed by Doppler, an interventional angioplasty can prevent the previously inevitable stroke.
   Liver, gallbladder, spleen, pancreas, kidneys, bladder, ovaries and uterus can be visualized in single setting by a US abdominal scan; and in prenatal diagnosis the US can determine beating fetal heart and show whether the fetus is a boy or girl from 10-week pregnancy on. It is gold standard for monitoring unborn child in womb.
   From self-help standpoint, the best thing about US is safety and  simplicity to layperson self use. Already, women are using ultrasound kits to check heart rate of the unborn baby, and in not far future, non-medical person will visualize internal organ at home and with instruction.
   In order to get best US, be sure a radiologist does it in HMO or hospital setting. Expertise of examiner prevents overuse.

Other Imaging SPECT functional MRI. Radioactive Imaging  The Single Photon Emission Computed Tomography (SPECT) is variations of radioactive tracers combined with CT and mostly research. Functional MRI lights up the brain based on oxygen and other paramagnetic elements in molecules. It causes no radioactivity and is proving useful in diagnosing early strokes but still is mostly used in advanced research. Full radioactive imaging for coronary heart disease is dealt with in Notebooks 6 under tests of heart and coronary heart disease.

EEG (Electroencephalogram) is, strictly speaking, not imaging but instead it is a measurement of the electrical field created by the brain activities in the small space just over the scalp. It is easy to do, inexpensive and noninvasive. The information it gives is less specific than body imaging but because it is rapidly repeatable and does tell much about the brain, it may prevent getting a CT or MRI. It is in Notebooks 9 chapter on epilepsy 9.19 Epilepsy and Seizures and the EEG.

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