Medical Imaging: CT Scans, MRI, and More
Medical imaging is the set of techniques used to view the human body and create a picture of it: internally or externally. Doctors use these images to not only diagnose diseases but also plan and execute their treatments. In Part I of the medical imaging series, we saw that from the 1850s to the 1970s, doctors became more accustomed to using 2D imaging but started to see many of its limitations. 2D photographs and X-rays were only so detailed and had low resolution. While ultrasound offered more visibility of internal organs, there wasn’t a way to see how internal organs function, for instance.
In this part of the history of medical imaging series, we’ll discuss some of the medical imaging tools implemented in the 1970s and afterward. We’ll see how the advancement of technology helped develop CT, MRI, and PET scanning and how they have changed the way doctors can diagnose problems.
Let’s continue on our journey through the history of medical imaging.
What is a CT Scan?
With the computers arriving on the scene, the techniques used for medical imaging, one of which is CT scanning, became much more advanced. CT, or computed tomography, was invented in 1967 by British engineer Godfrey Hounsfield and physicist Allan Cormack (they were later jointly awarded the Nobel Prize in Medicine in 1979 for their invention). CT scans are also referred to as CAT scans, or computerized axial tomography.
What does a CT scan show?
A CT scan creates cross-sectional images of the area being scanned using X-rays and a computer. An analogy of this would be cutting a loaf of bread into slices to see what’s inside, where the slices of bread are the images taken in a CT scan. Before CT, imaging of the head only consisted of plain films, which showed just bones or certain blood vessels. Essentially, there was no way to directly capture images of the brain prior to the invention of CT scans. CT was a major advance in medical imaging since it could take multiple, successive tomographic images (slices), a capability which did not exist previously.
Well how does CT work exactly? While the patient lies on a bed, a motorized X-ray source rotates around the circular opening of the scanning machine. In one rotation, a series of 2D image slices is taken and the bed slowly moves for the next rotation. This process is repeated until the desired number of slices are obtained. Generally, the thickness of the tissue in each slice ranges from 1-10 millimeters. These 2D image slices can either be looked at individually or stacked to create a reconstructed 3D model of internal structures, a process where 2D slices are merged and a 3D model is created by interpolating the bony structures depicted in the CT scan.
CT scans became more popular and started getting installed more in 1974. By 1980, three million CT scans had been performed, and by 2005, that number had grown to over 68 million CT scans annually. One of the first CT models took half a day to process the image slices, but CTs used today can get many sub-millimeter slices in a fraction of a second. A CT scan covering the entire body can be performed in about 20 seconds (more detailed scans can take up anywhere from 15 to 45 minutes). A CT scan is often the first test administered in an ER. Because of its great capability, CT became widely available as a popular tool for surgeons to use since it allows them to better understand their patients’ hard tissue and more effectively plan major procedures.
Since its invention, CT scanning has become a staple for diagnosing medical diseases and is still widely used today.
Introduction of Magnetic Resonance Imaging: MRI
Concurrent with the evolution of CT, MRI, or magnetic resonance imaging, emerged during the 1970s. In 1973, Paul Lauterbur showed that the spinning atom effect, or nuclear magnetic resonance (NMR), could be used to create images by changing the strength of the magnetic field. Using this knowledge, British scientist Peter Mansfield developed a mathematical process which helped turn MRI into an imaging tool. Lauterbur and Mansfield were awarded the Nobel Prize in Medicine in 2003. The use of MRI for medical diagnoses was discovered by American doctor and scientist Raymond Damadian in 1971. Damadian noticed that cancer tumors emitted different signals than healthy cells, a different which could be captured by MRI. Because of this, Damadian built the first whole-body MRI machine in 1977.
CT Scan vs MRI
Unlike CT which uses X-rays, MRI does not generate radiation and instead uses strong magnetic fields to change the spin of atoms in our bodies, which is virtually harmless. These tiny changes are measured by radio signals, which MRI computers use to create images of soft tissue inside the body. The images produced from an MRI are slices, similar to the those of CT imaging. It was clear early on that MRI was much better than CT at distinguishing different types of soft tissue. For instance, MRI can help doctors see inside joints, cartilage, muscles, and also other internal body structures, which is important for diagnosing illnesses earlier on. However, CT scans are more widely used than MRIs and are less expensive.
An extension of the MRI is the functional MRI, or fMRI. An fMRI is a way to visualize functional activity in the brain by measuring the changes in blood flow to different parts of the brain. For example, if you tap your finger, the part of the brain that controls finger tapping gets more blood, which shows up in the fMRI. Doctors can use fMRIs to evaluate any damage to the brain from, say, a head injury or Alzheimer’s disease. fMRI has been a revolutionary tool, especially for studying the brain and its various functions.
During an MRI, the patient lies on a bed that can slide into the doughnut-shaped machine. The patient is asked to remain as still as possible since any movement can distort the images. While the patient can’t feel the magnetic field, there is often loud thumping or tapping noises during the scan (headphones are usually given to block out the sound). Sometimes, a contrast solution or dye is given to the patient, so that specific structures in the body are highlighted that may not be seen otherwise. The scan can take anywhere between 30 to 60 minutes, depending on the area of the body being scanned. An important limitation to note is that any kind of metal in the body, such as a pacemaker, cochlear implants, artificial joints, etc. can be hazardous in an MRI since the strong magnets used can move or heat up any metal in the body.
Much like CT, MRI has become a popular method to captures internal images of the body. Over 60 million MRI scans are carried out annually nowadays. Magnetic resonance imaging has helped doctors for years to create more informed diagnoses and get information at a cellular level.
PET Scans: What They Are and What They Show
Another type of imaging which can get images at a molecular level is PET scanning. PET scans, or positron emission tomography, are a newer extension of nuclear testing, discussed in Part I of our Medical Imaging History series. A PET scan is a good way to evaluate the chemical activity in different parts of the body. This type of imaging can help identify various conditions, such as heart disease, different types of cancer, and brain disorders. In the 1970s, American scientist Michel Ter-Pogossian and his colleagues at Washington University discovered the use for PET scans in medicine. Inspired by the power of CT scanners, Ter-Pogossian developed the first computerized PET scanner in 1975.
Unlike CT or MRI which create images of internal structures, PET scanning produces images of the body’s functions, including blood flow and parts of the brain responsible for certain processes. Older forms of nuclear medicine would emit gamma rays, but PET uses isotopes that emit positrons, or positively charged electrons.
Patients either ingest or are injected with safe, radioactive materials, or tracers, before going into the PET machine. During the scan, detectors scan and track the tracers around the body. This information is captured by computers and an image of the body is created. Most PET scans use fluorine (F-18), a positron-emitting isotope, along with a certain type of glucose. Since glucose uptake is increased in most cancers, PET scans are commonly used to diagnose various types of cancers.
In the early 1990s, PET was combined with CT scanning, a process known as PET-CT, to find tumors more effectively. PET-CT incorporates the advantages of both PET and CT to find the localization of cancer: information on the body’s functions from PET scans and the high spatial resolution (highly detailed) of CT. The combination of PET with CT allows doctors to get a more detailed, precise understanding of a patient’s illness. With PET-CT, doctors can see metabolic and anatomical information simultaneously. The combination of these two tools has been an incredible advance for cancer treatment planning. By 2005, approximately 90% of PET scanners were actually PET-CT fusion imaging scanners.
Better Medical Imaging, Yet Still an Incomplete Field
With the introduction of computers in medicine, medical imaging technology has transformed dramatically. CT, MRI, and even PET scans all utilize the power of computers to provide doctors with imaging methods more detailed and all-encompassing than ever before. No longer do doctors need to perform exploratory surgery to get a better look inside a patient’s body to diagnose an illness; this technique has been replaced with noninvasive CT, MRI, and PET imaging. Now, we can see the internal hard and soft tissues of patients, and get both anatomical and metabolic information together.
However, even with this abundance of imaging techniques, we lack imaging methods for accurately documenting external soft tissue. In the next and final installment of the history of medical imaging, we will explore the introduction of 3D imaging and see just extensive the field of medical imaging has become.