Mrs. P. wrote The Informed Parent with an interesting question that seemed to be a good topic for this month’s article. Her 22-month-old daughter was diagnosed with a brain tumor. The surgeons removed a piece of the tumor for a biopsy to make the diagnosis of a craniopharyngeoma. They did not, however, remove the tumor. Mrs. P. did significant research about this type of tumor in children and really did not have questions about this specifically. Her main question was about the differences between a MRI scan and a CT scan, and the different dyes that are injected in these two studies.
Radiology has come a long way since Wilhelm Roentgen, a German scientist, first used ionized radiation to take the first x-ray in 1895. He essentially pointed a beam of ionized rays through a part of the body and this recorded an image on a plate. Since that time, there have been steady advances in the field of radiology. But the past 30 years has seen a virtual explosion in the technological discoveries in this field. The CT scan, MRI scan and PET scans have revolutionized the practice of medicine.
The CT scan, or computerized axial tomography, is a specialized x-ray (therefore, it uses ionized radiation) whereby the patient is placed in a tube and the x-ray source rotates 360 degrees around the patient directing a precise narrow x-ray beam through the body. A computer then interprets the information to construct an image that is a cross section of the body part to be examined. By moving the patient up or down in the scanner, the radiologist can get serial “cuts” or “slices”, and basically get a picture of what is inside the body. As a general rule, whereas the typical x-ray is good for highlighting dense structures like bone, the CT is good for soft tissues such as the chest or abdomen.
The MRI, magnetic resonance imaging, is a newer modality that uses a special energy source other than ionized radiation. This is nuclear magnetic resonance. During a MRI, powerful electromagnets create a magnetic field, which is over 30,000 times stronger than the Earth’s own magnetic field. This influences the protons of the hydrogen atoms inside the body to align a certain way. Then a radio wave is emitted at around 25 times per second, and this knocks the protons out of alignment. When the radio wave stops, the protons realign within milliseconds. The MRI can use these differences in alignment to create a clear image or scan. The MRI has become the preferred imaging modality to diagnose disease of the brain or central nervous system.
Both the CT scan and the MRI have their strong points and their differences. The CT scan does use radiation, whereas the MRI does not. The CT scan is a quicker test, and is more accessible in less metropolitan areas. The CT scan is less costly than the MRI. As was already mentioned, the MRI is the preferred test for evaluating the brain and central nervous system.
Often times the radiologist will inject a contrast material to enhance the images created by the CT or MRI scans. For CT scans, the basic contrast materials used are of an iodine base. The MRI scans use a material called gadolinium. Both of these greatly enhance the pictures recorded by the respective scanners, and the differences are subtle. As a general rule, the gadolinium is safer than the iodine based contrast materials as it causes less allergic or hypersensitivity reactions. The gadolinium contrast materials are much safer in an individual who might have decreased kidney functions. Each has its place when used by proper hands in the proper setting.