The definition of imaging is the visual representation of an object. Medical imaging began after the discovery of x-rays by Konrad Roentgen 1896. The first fifty years of radiological imaging, pictures have been created by focusing x-rays on the examined body part and direct depiction onto a single piece of film inside a special cassette. The next development involved the use of fluorescent screens and special glasses to see x-ray images in real time.
A major development was the application of contrast agents for a better image contrast and organ visualization. In the 1950s, first nuclear medicine studies showed the up-take of very low-level radioactive chemicals in organs, using special gamma cameras. This medical imaging technology allows information of biologic processes in vivo. Today, PET and SPECT play an important role in both clinical research and diagnosis of biochemical and physiologic processes. In 1955, the first x-ray image intensifier allowed the pick up and display of x-ray movies.
In the 1960s, the principals of sonar were applied to diagnostic imaging. Ultrasonic waves generated by a quartz crystal are reflected at the interfaces between different tissues, received by the ultrasound machine, and turned into pictures with the use of computers and reconstruction software. Ultrasound imaging is an important diagnostic tool, and there are great opportunities for its further development. Looking into the future, the grand challenges include targeted contrast agents, real-time 3D ultrasound imaging, and molecular imaging.
Digital imaging techniques were implemented in the 1970s into conventional fluoroscopic image intensifier and by Godfrey Hounsfield with the first computed tomography. Digital images are electronic snapshots sampled and mapped as a grid of dots or pixels. The introduction of x-ray CT revolutionised medical imaging with cross sectional images of the human body and high contrast between different types of soft tissue. These developments were made possible by analog to digital converters and computers. The multislice spiral CT technology has expands the clinical applications dramatically.
The first MRI devices were tested on clinical patients in 1980. The spread of CT machines is the spur to the rapid development of MRI imaging and the introduction of tomographic imaging techniques into diagnostic nuclear medicine. With technological improvements including higher field strength, more open MRI magnets, faster gradient systems, and novel data-acquisition techniques, MRI is a real-time interactive imaging modality that provides both detailed structural and functional information of the body.
Today, imaging in medicine has advanced to a stage that was inconceivable 100 years ago, with growing medical imaging modalities:
All this type of scans are an integral part of modern healthcare. Because of the rapid development of digital imaging modalities, the increasing need for an efficient management leads to the widening of radiology information systems (RIS) and archival of images in digital form in picture archiving and communication systems (PACS). In telemedicine, healthcare professionals are linked over a computer network. Using cutting-edge computing and communications technologies, in videoconferences, where audio and visual images are transmitted in real time, medical images of MRI scans, x-ray examinations, CT scans and other pictures are shareable.
See also Hybrid Imaging.

See also the related poll results: 'In 2010 your scanner will probably work with a field strength of', 'MRI will have replaced 50% of x-ray exams by'

(LE) Myocardial late enhancement in contrast enhanced cardiac MRI has the ability to precisely delineate myocardial scar associated with coronary artery disease. Viability imaging implies evaluating infarcted myocardium to see whether there is enough viable tissue available for revascularization. The reversal of myocardial dysfunction is particularly relevant in patients with depressed ventricular function because revascularization improves long-term survival. In comparison to SPECT and PET imaging, myocardial late enhancement MRI demonstrates areas of delayed enhancement exactly in correlation with the infarcted region.
Viability on cardiac MRI (CMR) is based on the fact that all infarcts enhance vividly 10-15 minutes after the administration of intravenous paramagnetic contrast agents. This enhancement represents the accumulation of gadolinium in the extracellular space, due to the loss of membrane integrity in the infarcted tissue. This phenomenon of delayed hyperenhancement has been proven to correlate with the actual extent of the infarct.
MRI myocardial late enhancement can quantify the size, location and transmural extent of the infarct. If the transmural extent of the infarct (region of enhancement on MRI) is less than 50% of the wall thickness, there will be improved contractility in that segment following revascularization. In areas of hypokinesia, if there is a rim of "black" or non-infarcted myocardium that is not contracting well, it indicates the presence of hibernating myocardium, which is likely to improve after revascularization of the artery supplying that particular territory.
The total duration of a myocardial late enhancement MR imaging protocol for viability is approximately 30 minutes, including scout images, first-pass images, cine images in two planes, and delayed myocardial enhancement images. In order to assess viable myocardium, the gadolinium contrast agent is injected at a dose of 0.15 to 0.2 mmol/kg. After about 10 minutes, short axis and long axis views (see cardiac axes) of the heart are obtained using an inversion prepared ECG gated gradient echo sequence. The inversion pulse is adjusted to suppress normal myocardium. Areas of nonviable myocardium retain extremely high signal intensity, black areas show normal tissue.

For Ultrasound Imaging (USI) see Myocardial Contrast Echocardiography at Medical-Ultrasound-Imaging.com.

(PWI - Perfusion Weighted Imaging) Perfusion MRI techniques (e.g. PRESTO - Principles of Echo Shifting using a Train of Observations) are sensitive to microscopic levels of blood flow. Contrast enhanced relative cerebral blood volume (rCBV) is the most used perfusion imaging. Both, the ready availability and the T2* susceptibility effects of gadolinium, rather than the T1 shortening effects make gadolinium a suitable agent for use in perfusion imaging. Susceptibility here refers to the loss of MR signal, most marked on T2* (gradient echo)-weighted and T2 (spin echo)-weighted sequences, caused by the magnetic field-distorting effects of paramagnetic substances.
T2* perfusion uses dynamic sequences based on multi or single shot techniques. The T2* (T2) MRI signal drop within or across a brain region is caused by spin dephasing during the rapid passage of contrast agent through the capillary bed. The signal decrease is used to compute the relative perfusion to that region. The bolus through the tissue is only a few seconds, high temporal resolution imaging is required to obtain sequential images during the wash in and wash out of the contrast material and therefore, resolve the first pass of the tracer. Due to the high temporal resolution, processing and calculation of hemodynamic maps are available (including mean transit time (MTT), time to peak (TTP), time of arrival (T0), negative integral (N1) and index.
An important neuroradiological indication for MRI is the evaluation of incipient or acute stroke via perfusion and diffusion imaging. Diffusion imaging can demonstrate the central effect of a stroke on the brain, whereas perfusion imaging visualizes the larger 'second ring' delineating blood flow and blood volume. Qualitative and in some instances quantitative (e.g. quantitative imaging of perfusion using a single subtraction) maps of regional organ perfusion can thus be obtained.
Echo planar and potentially echo volume techniques together with appropriate computing power offer real time images of dynamic variations in water characteristics reflecting perfusion, diffusion, oxygenation (see also Oxygen Mapping) and flow.
Another type of perfusion MR imaging allows the evaluation of myocardial ischemia during pharmacologic stress. After e.g., adenosine infusion, multiple short axis views (see cardiac axes) of the heart are obtained during the administration of gadolinium contrast. Ischemic areas show up as areas of delayed and diminished enhancement. The MRI stress perfusion has been shown to be more accurate than nuclear SPECT exams. Myocardial late enhancement and stress perfusion imaging can also be performed during the same cardiac MRI examination.

(SPIO) Relatively new types of MRI contrast agents are superparamagnetic iron oxide-based colloids (median diameter greater than 50nm). These compounds consist of nonstoichiometric microcrystalline magnetite cores, which are coated with dextrans (in ferumoxide) or siloxanes (in ferumoxsil). After injection they accumulate in the reticuloendothelial system (RES) of the liver (Kupffer cells) and the spleen. At low doses circulating iron decreases the T1 time of blood, at higher doses predominates the T2* effect.
SPIO agents are much more effective in MR relaxation than paramagnetic agents. Since hepatic tumors either do not contain RES cells or their activity is reduced, the contrast between liver and lesion is improved. Superparamagnetic iron oxides cause noticeable shorter T2 relaxation times with signal loss in the targeted tissue (e.g., liver and spleen) with all standard pulse sequences. Magnetite, a mixture of FeO and Fe2O3, is one of the used iron oxides. FeO can be replaced by Fe3O4.
Use of these colloids as tissue specific contrast agents is now a well-established area of pharmaceutical development. Feridex®, Endorem™, GastroMARK®, Lumirem®, Sinerem®, Resovist® and more patents pending tell us that the last word in this area is not said.
Some remarkable points using SPIO:

See also Superparamagnetism, Superparamagnetic Contrast Agents and Classifications, Characteristics, etc..

T2 weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse. The contrast of a T2 weighted image is predominantly dependent on T2 and the T2 dependence will be increased by using a long echo time.
Fat has a shorter T2 time than water and relaxes or decays more readily than water. Since the amount of transverse magnetization in fat is small, fat generates very little signal on a strong T2 weighted contrast image and appears intermediate to dark. The T2 weighting is stronger with a longer TE. Water has a very high T2 constant, therefore has very high T2 signal and thus appears bright on a T2 contrast image. Cerebral white matter (fat containing) is less intense than grey matter. Flowing blood (flow effects) and haematomas (haemoglobin, haemosiderin) have a variable signal intensity on MR images.
Images created with TR's and TE's to enhance T2 contrast are referred to as T2 weighted images. Both T1 and T2 weighted images are acquired for most medical MRI examinations.