A history of medical imaging
Medical Imaging began with radiography after the discovery of x-rays in 1895 by Wilhelm Röntgen, a German professor of physics. X-rays were put to diagnostic use very early, before the dangers of ionizing radiation were discovered. Upon travelling through the tissues, the radiation from X-rays is absorbed differentially depending on the density of the tissue being penetrated. The radiation piercing the tissues produces on a photographic film or a fluorescent screen the different densities within the body, thus creating an image. The limiting factor in this method of diagnosis is the similarity between the densities of adjacent soft tissues within the body, making it a very interesting imaging technique for bone or foreign objects in the body but not for soft tissue pathologies.
The application of pharmaceutical contrast agents between 1906 and 1912 to help visualize organs and blood vessels was a major development in radiography. The harmless contrast agents were administered orally or via vascular injection and allowed to visualize blood vessels, digestive and gastro-intestinal systems, bile ducts and gall bladder for the first time.
By the 1960's, this made possible a new radiological application: angiography. This is a medical imaging technique used to visualize the inside or lumen of blood vessels and organs of the body, with particular interest in arteries, veins and the heart chambers. Usually a radio-opaque contrast agent is injected into the blood vessel and imaged using X-ray based techniques such as fluoroscopy.
Nuclear Medicine became possible in the 1950s. In nuclear medicine procedures, radionuclides are combined with pharmaceutical compounds, to form radiopharmaceuticals. Once these radiopharmaceuticals have been administered to the patient, they converge in specific organs or in cellular clusters that are more 'active' than others. This allows nuclear medicine to image the extent of a disease process in the body, based on the cellular function and physiology, rather than relying on physical changes in tissue anatomy. In nuclear medicine the recorded radiation is emitted from within the body rather than generated by an external source as is the case with X-rays. This emitted radiation is then registered by gamma cameras. In some diseases nuclear medicine studies can identify medical problems at an earlier stage than other diagnostic tests.
In the 1960's the principals of sonar (developed extensively during the second world war) were applied to diagnostic imaging. The technique is similar to the echolocation used by bats, whales or dolphins. The ultrasound scanner transmits high-frequency sound waves into a body by use of a probe or transducer. The pulses or waves penetrate into the body and bounce off the organs. The return wave vibrates the transducer; the transducer turns the vibrations into electrical pulses that are sent to the ultrasonic scanner where they are transformed into an image. Ultrasound has become a very popular imaging technique, as it has no adverse bio-effects.
In the 1970's G.N. Hounsfield and A.M. Cormack were awarded the Nobel Prize in medicine for the invention of Computed Tomography. This technique uses computer-processed X-rays to produce tomographic slices of specific areas of the body. It provides a better insight into the pathogenesis of the body, thereby increasing the chances of recovery.
Hounsfield's original CT scan took several hours to acquire one single slice of image data and more than 24 hours to reconstruct this data into an image. Today's CT systems can acquire a single image in less than a second and reconstruct the image instantly.
In 1971 Raymond Damadian showed that nuclear magnetic relaxation times of tissues and tumors differed, motivating scientists to use MRI to study disease. Many scientists over the next 20 years contributed to the development of Magnetic Resonance Imaging. With MRI radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are sent through the body. This strong magnetic field causes the alignment of particles, called protons, which are found naturally within the body, mostly in hydrogen atoms. As these protons move back into their original positions, they send out radio waves of their own. The scanner picks up these signals and a computer turns them into a picture. These pictures are based on the location and strength of the incoming signals. Different protons send out different signals, depending on which tissue the proton can be found in. A traditional MR scanner has a moveable table upon which the patient is placed, and which slides into the hollow cylindrical magnet. The magnetic field usually lies between 0,5 and 7 Tesla and is generated by super conducted magnets, which are being cooled by helium.
PACS & DICOM
The revolution in digital imaging has been accompanied by the adoption of a Picture Archiving and Communication System (PACS), which provides electronic storage, retrieval, distribution, and presentation of images. PACS facilitates the handling of digital radiology images so that they can be readily accessed and viewed by a variety of medical professionals in different locations and settings. DICOM (Digital Imaging and Communications in Medicine) is a standard for storing and transmitting information in medical imaging. It includes a file format definition and a network communications protocol. DICOM enables the integration of scanners, servers, workstations, printers, and network hardware from multiple manufacturers into a picture archiving and communication system (PACS) and ensures the easy transmission of information between the different users.
Radiotherapy or radiation oncology is concerned with prescribing radiation, and is different from radiology, the use of radiation in medical imaging for diagnostic purposes. Radiation therapy is commonly applied to a cancerous tumor because of its ability to control cell growth. Ionizing radiation works by damaging the DNA of exposed tissue, leading to cellular death. To spare normal tissues (such as skin or organs which radiation must pass through to treat the tumor), shaped radiation beams are aimed from several angles of exposure to intersect at the tumor, providing a much larger absorbed dose here than in the surrounding, healthy tissue. Current research focuses on even more precise targeting of the tumor, for instance by collimator adaptation, gating or dose painting. Radiation dosimetry is the measurement and calculation of the radiation dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation.