Brief Overview of Some of the Types of Medical Imaging

In 1895, Wilhelm Conrad Röntgen discovered X-ray radiation, and less than a month after his discovery, he took the first medical X-ray:  an image of his wife's hand. This event marked the beginning of the field of medical imaging.

X-rays, also known as Röntgen rays, are a form of electromagnetic radiation more energetic than visible light but less energetic than gamma radiation.  X-ray imaging works because the radiation passes through relatively less dense structures, but denser tissues and structures absorb the radiation.  Thus X-rays are useful for producing images of dense tissues such as bones and the dense, wet lungs of pneumonia sufferers.  Adding radio-opaque substances (that is, substances that X-rays cannot penetrate), such as barium in the digestive tract, allows imaging of certain soft tissues.  This procedure is called a contrast study.

Traditional X-ray images show all layers of the three-dimensional body in a single two-dimensional image.  In the 1930s, the first tomographic X-rays were produced.  These are two-dimensional images of a two-dimensional plane through the body.  In 1972, the British music company EMI unveiled a revolutionary new medical imaging technology that allowed the first three-dimensional representations of anatomy.  Their device took a series of tomographic images in planes rotated around the long axis of the patient's body, then combined them using computers, which were newly available at the time.  Accordingly, the new technology was called computed tomography, or CT scanning.

Medical ultrasound imaging was first used in the 1950s.  Ultrasound gets its name from the fact that it uses sound waves at frequencies far higher than those detectable by the human ear.  Ultrasound imaging uses frequencies from 2 to 18 mHz, while humans cannot hear above about 0.02 mHz (20,000 Hz).  This form of imaging operates on the same principle as sonar.  Ultrasonography has many diagnostic applications, the best-known of which are obstetric ultrasounds and echocardiograms.

Other new types of medical imaging machines have been developed in the last three decades.  In 1977, he first nuclear magnetic resonance image, or MRI, of a human marked a major breakthrough in medical imaging technology.  MRI uses magnetic fields to align the hydrogen atoms in the water in the body, and can form high contrast images of soft tissue.

A type of MRI called the functional MRI (fMRI) allows real-time imaging not of anatomy, but of physiological activity.  Brain fMRIs are made possible by the fact that areas of neuronal activity in the brain experience increased levels of oxygenated hemoglobin (compared to deoxygenated hemoglobin).  This physiological phenomenon occurs because neuronal activity demands an increased amount of oxygen.  Oxygenated and deoxygenated hemoglobin appear different under the magnetic resonance signal, so fMRI can detect those regions of increased neuronal activity.

Nuclear medicine is the field of medicine that makes use of the properties of atomic nuclei.  Two major types of medical imaging that fall into the realm of nuclear medicine are PET and SPECT.

The technology behind positron emission tomography (PET) was developed in the 1970s.  Like fMRI, its images represent biological processes ocurring in the body.  In PET imaging, a tracer is injected into the body.  The tracer is molecule used in metabolism which contains a radioactive isotope with a short half-life that decays by emitting positrons (a type of antimatter).  The most common PET tracer is a type of sugar which contains a radioactive isotope of fluorine.  In the body, the isotope decays and emits positrons.  When a positron encounters an electron, they annihilate and emit a pair of photons (light particles) which are detected by the PET machine.  The PET image thus shows where the tracer molecule is found and in what concentration.  Since sugar is used by cells such as neurons when they become active, a PET image shows where cells are active.  Clinically, PET imaging is used in oncology and for detecting certain brain diseases.  PET scans are commonly combined with CT or MRI scans in a single medical imaging system that can show both anatomy and function.

Single photon emission computed tomography (SPECT) works similarly to PET, but its tracers contain molecules that emit gamma radiation rather than positrons, and images are taken using a "gamma camera" that detects the gamma particles.  Like X-ray based CT scanning, SPECT forms true three-dimensional images.  It is used for functional imaging of the heart and brain.

Medical imaging technology is a field of rapidly emerging innovations.  New modalities include optical imaging, which uses low-frequency light (infrared and even radio waves), and photoacoustic imaging, which takes advantage that under certain circumstances, matter can emit a sound when it absorbs light.  This field promises striking new possibilities for both anatomical and functional imaging.