Radiology is the medical specialty that utilizes imaging technologies to diagnose and sometimes treat diseases. Traditionally it was the branch of medical science dealing with the medical use of X-rays emitted by X-ray machines or other such radiation devices for the purpose of obtaining visual information as part of medical imaging. Radiologists now also use other imaging technologies (such as ultrasound, computed tomography (CT) and magnetic resonance imaging) to diagnose or treat disease. Interventional radiology is the performance of (usually minimally invasive) medical procedures with the help of medical imaging.

 

Outside of the medical field, radiology also encompasses the examination of the inner structure of objects using X-rays or other penetrating radiation.

 

 

As a medical specialty, radiology can be classified into subfields.

 

Diagnostic radiology is concerned with the use of various imaging modalities to aid in the diagnosis of disease. Radiology can be used for therapeutic purposes as well. Interventional radiology uses imaging to guide therapeutic procedures. Radiation therapy uses radiation to treat diseases such as cancer. While originally encompassed within radiology, radiation oncology is now a separate field.

 

 

Commonly used imaging modalities include plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and nuclear imaging techniques. These techniques are usually non-invasive. Each of these modalities has strengths and limitations which dictate its use in diagnosis.

 

Radiographs (or Roentgenographs, named after the discoverer of X-rays, Wilhelm Conrad Roentgen (1845-1923)) are often used for evaluation of bony structures and soft tissues. Fluoroscopy and angiography are special applications of X-ray imaging, where a fluorescent screen or image intensifier tube is connected to a closed-circuit television system, which allows real-time imaging of structures in motion or augmented with a radiocontrast agent. Radiocontrast agents are administered, often swallowed or injected into the body of the patient, to help delineate anatomy such as the blood vessels, the genitourinary system or the gastrointestinal tract. Specific radiocontrast agents are used for specific types of evaluations; for example, barium in a suspension is administered into the gastrointestinal tract and imaged with fluoroscopy or radiography. Radiocontrast agents, which strongly absorb X-ray radiation, in conjunction with the real-time imaging allows demonstration of dynamic processes, such as peristalsis in the digestive tract or blood flow in arteries and veins.

 

 

CT imaging uses X-rays in conjunction with computing algorithms to image a variety of soft tissues in the body. CT is acquired in the axial plane, while coronal and sagittal images can be rendered by computer reconstruction. Radiocontrast agents are often used with CT for enhanced delineation of anatomy. Intravenous contrast allows 3D reconstructions of arteries and veins. Although radiographs provide higher resolution for bone X-rays, CT can generate much more detailed images of the soft tissues. CT exposes the patient to more ionizing radiation than a radiograph.

 

 

Medical ultrasonography uses ultrasound (high-frequency sound waves) to visualize soft tissue structures in the body in real time. No ionizing radiation is involved, but the quality of the images obtained using ultrasound is highly dependent on the skill of the person (ultrasonographer) performing the exam. The use of ultrasound in medical imaging has developed mostly within the last 30 years. The first ultrasound images were static and two dimensional (2D), but with modern-day ultrasonography 3D reconstructions can be observed in real-time; effectively becoming 4D.

 

Because ultrasound does not utilize ionizing radiation, unlike xrays, nuclear medicine and CT scans, it is generally considered safer and therefore the modality plays a vital role in obstetrical imaging. Fetal development can be thoroughly evaluated allowing early diagnosis of fetal anomalies or confirmation of a normal gestation. Color Flow Doppler well documents the severity of peripheral vascular disease. Stenosis of the carotid arteries can herald cerebral infarcts or strokes. Intraluminal clots in the venous systems of the extremities may result in pulmonary embolism, which is often fatal if undiagnosed and untreated. Ultrasound is useful for image-guided interventions like biopsies and drainages. It is also used in the treatment of kidney stones/renal lithiasis via lithotripsy. Small portable ultrasound devices now replace peritoneal lavage in the triage of trauma victims by directly assessing for the presence of hemorrhage in the peritoneum and the integrity of the major viscera including the liver, spleen and kidneys. Extensive hemoperitoneum or injury to the major organs may require emergent surgical exploration and repair.

 

 

MRI uses strong magnetic fields to align spinning atomic nuclei (usually hydrogen protons) within body tissues, then disturbs the axis of rotation of these nuclei and observes the radio frequency signal generated as the nuclei return to their baseline states. MRI scans give the best soft tissue contrast of all the imaging modalities. With advances in scanning speed and spatial resolution, and improvements in computer 3D algorithms and hardware, MRI has great potential for development in the next few years. One disadvantage is that the patient has to hold still for long periods of time in a noisy, cramped space while the imaging is performed. Recent improvements in magnet design like wider, shorter magnet bores and more open magnet designs, have brought some relief for claustrophobic patients. MRI has great benefit in imaging the brain, spine, and musculoskeletal system. The modality can be contraindicated for patients with pacemakers, certain types of cerebral aneurysmal clips or metallic hardware due to the strong magnetic fields. Areas of potential advancement include functional imaging, cardiovascular MRI, as well as MR image guided therapy.

 

 

Nuclear medicine imaging involves the administration into the patient of substances labeled with radioactive tracers which have affinity for particular tissues. The heart, lungs, thyroid, liver, gallbladder, and bones are commonly evaluated for particular conditions using these techniques. While anatomical detail is limited in these studies, nuclear medicine is useful in displaying physiological function. As such, processes such as the growth of a tumor can often be monitored, even when the tumor cannot be adequately visualized using any of the other modalities. The principal imaging device is the gamma camera which detects the radiation emitted by the tracer in the body and displays it as an image. Often the information is converted into a series of slices through the body. In the most modern devices Nuclear Medicine images can be fused with a CT scan taken at the same time so that the physiological information can be overlayed on the anatomical structures to improve diagnostic accuracy.

 

The applications of nuclear medicine can include bone scanning which traditionally has had a strong role in the work-up/staging of cancers. Myocardial perfusion imaging is a sensitive and specific screening exam for reversible myocardial ischemia, which when present requires angiographic confirmation and potentially life-saving balloon angioplasty, stenting or cardiac bypass grafting. Molecular Imaging is the new and exciting frontier in this field.

 

 

Radiography is the creation of images by exposing a photographic film or other image receptor to X-rays. Since X-rays penetrate solid objects, but are weakened by them depending on the object's composition, the resulting picture reveals the internal structure of the object. Medical radiography is undertaken by a specially trained professional called a radiographer.

 

 

X-rays are the second most commonly used medical tests, after blood tests. Bone and some organs (such as lungs) especially lend themselves to X-ray imaging. It is a relatively low-cost investigation with a high diagnostic yield, although CT scans or other more specialized technologies may be necessary to delineate diseases. Ultrasound, by comparison, requires more expertise to perform. Therapeutic radiography is a key treatment in oncology departments, with 6 out of 10 patients treated with radiotherapy for cancer cured. This involves application of a prescribed dose of ionizing radiation to specific targeted tissue, whilst limiting damage to the surrounding healthy area.

 

Diagnostic radiography can be divided into four main classes:

 

·        Dental - for teeth. A dentist may examine a painful tooth and gum using X-ray equipment. These examinations tend to give only a very small radiation dose.

·        Mammography - X-ray examination of female breasts and other soft tissues. This has been used on older women to screen for breast cancer, but implants designed to enlarge the breasts reduce the ability of mammography to observe changes within the breast. The radiation used for mammography tends to be softer (has a lower photon energy) than that used for the harder tissues. Often a tube with a molybdenum anode is used with about 30 000 volts (30 kV), giving a range of X-ray energies of about 15-30 keV. Many of these photons are "characteristic radiation" of a specific energy determined by the atomic structure of the target material (Mo-K radiation).

·        Hard tissues such as bone. After an accident, a patient's bones may be examined for breaks using X-rays. For this type of work a higher energy photon source is needed, and typically a tungsten anode is used with a high voltage (50-70 kV) to generate braking radiation. Depending on the part of the body which needs to be examined the dose can either be low or high. A chest x-ray is thought to be about the same as smoking one cigarette.

·        Double contrast technique. To examine the digestive system, a substance which is opaque to X-rays (barium sulfate) is fed to the person in food or as an enema. Barium sulfate coats the walls of the digestive tract (positive contrast), which allows the shape of the digestive tract to be outlined on an X-ray after the introduction of air (negative contrast). The barium meal is an example of a contrast agent swallowed to examine the upper digestive tract. Note that while soluble barium compounds are very toxic, the insoluble barium sulfate is non toxic because its low solubility prevents the body from absorbing it.

·        A number of substances have been used as positive contrast agents: silver, bismuth, cesium, thorium, tin, zirconium, tantalum, tungsten and lanthanide compounds have been used as contrast agents. The use of thoria (thorium dioxide) as an agent was rapidly stopped as thorium causes liver cancer. Most modern injected radiographic positive contrast media are iodine based. Patients who suffer from allergy to shellfish may be allergic to iodine, and should consult their physician regarding pre-medication to lessen risk of allergic reaction.

·        Negative radiographic contrast agents are air and carbon dioxide (CO2). The latter is easily absorbed by the body and causes less spasm. It can also be injected into the blood, where air absolutely cannot.

 

 

Radiosensitivity is the relative susceptibility of cells, tissues, organs or organisms to the harmful effect of ionizing radiation. Cells are least sensitive when in the S phase, then the G1 phase, then G2 phase and the most sensitive in the M phase of the cell cycle.

 

 

AP	Anteroposterior (view in X-rays).
BA	Barium.
CAT	Computed axial tomography.
cGy	Centi Gray (unit of radiation).
C-spine	Cervical spine.
CT	Computerized tomography.
CXR	Chest X-ray.
DI	Diabetes insipidus.
DSA	Digital substraction angiography.
FX	Fracture.
Gy	Grays (units of radiation).
IVP	Intravenous pyelogram.
KUB	Kidneys, ureters, and bladder.
LAT	Lateral (view in x-rays).
LS	Lumbar spine.
PA	Posteroanterior (view in X-rays).
PET	Positron emission tomography.
rad	Radiation absorbed dose.
SPECT	Single photon emission computed tomography.
UGI	(see GI) upper gastrointestinal.
US	Ultrasound.
VQ	(Scan), ventilation/perfusion scan.
XRT	Radiotherapy.