Medical Image Acquisition: Static to Digital World

Overview:

Early radiology was embedded in morphology, namely skeletal morphology. The change towards image of physiology of the human body began with nuclear medicine. With this transformation approach the ability to not only display presence of diseases but also the mechanism of disease and the biology of treatment. In the midst of the excitement brought about by the Roentgen’s discovery, Becquerel discovered radioactivity in the early 1896. Thus began the dawn of the nuclear age. Similar to the discovery of x-rays, the discovery of phosphorescence was accidental. Becquerel had placed some photographic plates in a drawer with some crystals of uranium. Upon retrieving the plates, he found that the plates had been exposed. He deduced that exposure must have been from rays of a radioactive source i.e; the uranium crystals. Over the years numerous scientists such as the Curies and Rutherford had contribute to the advancement of nuclear medicine. The use of single-photon emission computed tomography (SPECT) and to a greater extent positron emission tomography (PET) to display functional abnormalities not detected by other imaging tools have made assessment of treatment feasible.

Background:

In the early years, radiographs were initially made onto glass photographic plates which were coated with emulsion only on one side. In 1918, Eastman introduced film coated with emulsion on two surfaces. Radiograph at this time was focused on imaging of extremities, mainly to detect fractures and to localize position of bullets. This was due to fact that bone, soft tissue and dense foreign bodies provided the only contrast between materials. In 1910, orally administered contrast medium (bismuth nitrate later replaced by barium sulphate) was used to image the gastrointestinal system. Further development brought about an intravenous contrast agent marketed for urinary tract radiograph in 1927.

The next development involved the use of fluorescent screen, an x-ray tube, and x-ray table and red goggles and required the radiologist to stare directly into the screen so that x-ray images could be displayed in real time. This was a rather primitive method as the fluorescence emitted was very dim. The first iodine-based contrast arteriogram in a patient was reported in 1929 by Dos Santos, approximately 34 years after the discovery of x-ray. However without the benefit of the image intensifiers at this time, arterial access was obtained via a blind tranlumbar puncture. The emergence of image intensifiers gave a much-needed boost to this flagging enterprise. Greater steps were taken when Seldinger introduced a safer, simpler and more effective method of accessing the femoral artery. Despite the advent of the Seldinger technique, real advances in diagnostic angiography were still stunted, as fluoroscopy remains primitive. In the late 1980’s and early 1990’s however, two essential technologies have greatly impacted the evolution of angiography; moveable multiple-angle C-arm fluoroscopy and digital image acquisition.

The Power of Three:

By the 1970’s ultrasound (US) and computed tomography (CT) had arrived displacing angiography as the supreme imaging tool in radiology. By the 1990’s duplex ultrasound, CT angiography and Magnetic Resonance (MR) angiography began to replace diagnostic arteriography for the direct study of vascular pathology. In most radiology departments today, catheter-based angiography is reserved mainly for diagnosis of atherosclerotic vessels and as an adjunct to interventional procedure. Early imaging studies were projections of 3-Dimensional (3D) body parts displayed as if a steamroller as in our favorite cartoons had flattened the human body. This results in much overlap of the body parts making interpretation of disease difficult. The emergence of three powerhouses imaging tools namely ultrasound, computed tomography and magnetic resonance imaging have revolutionized the care of patients across the continuum of medicine and surgery. Radiology is now often referred to as ‘imaging’ reflecting the fact that it is no longer dependant on x-rays alone. Over the years, ultrasound has stood the test of time proving to be a safe, reliable, portable and cheap imaging modality. In 1972 the cross-sectional imaging became a catch phrase; this was attributed to the invention of computed tomography (also known as computed axial tomography or CT scan). The earliest CT scanners were limited to imaging of the head, by 1976 the technology had evolved to whole body scanners, and by the 1980’s CT Scans had gained worldwide acceptance. Today there are an estimated 600,000 locations around the world where this diagnostic tool is in use. The prototype CT Scanners took roughly four minutes of lapsed time to acquire a single image. Currently, modern units produce images in less than 0.5 seconds. The advent of CT had an enormous effect on our ability to ‘SEE’ inside the body and immediately changed the practice of medicine; the momentum created by CT scanners fueled the commercial development of MRI systems. In its infancy, many thought that MRI would have a limited impact because of its high cost, the technical difficulties associated with it and the belief the CT scanning was a superior method of imaging. MRI has quickly become the primary imaging method for brain and spine imaging as well as functional imaging of the heart.

Inflowing the Digital World:

Computers and the digital world have impacted the science of Radiology bringing it to what it is today. The advancement of artificial intelligence in the last 25 years has created an explosion of diagnostic imaging technique. These techniques have now been developed for digital rather than photographic recording of conventional radiographs. In the early days, a head x-ray would require up to 11 minutes of exposure time but now digital radiographic images are made in milliseconds while reducing the radiation dose to as little as 2% of what was used for the 11 minutes head examination 10 years ago.

The resolution achievable by the different imaging methods may be classified as spatial, contrast or temporal. Spatial resolution is the ability of a system to resolved anatomic detail. Contrast resolution is the ability of the system to differentiate different tissue especially to distinguished normal from pathological tissue. Temporal resolution is the ability of the modality to reflect either changing physiological events such as cardiac motion or disease remission or progression as a function of time. Each imaging modality has its strength and weaknesses much to frustration of hospital administrators no single method will solve all diagnostic problems and the fusion of knowledge gleaned from different modalities would serve our patient best.