||This article may be too technical for most readers to understand. (July 2014)|
Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the body. Radiologists use an array of imaging technologies such as X-ray radiography, ultrasound, computed tomography (CT), nuclear medicine, positron emission tomography (PET) and magnetic resonance imaging (MRI) to diagnose or treat diseases. Interventional radiology is the performance of (usually minimally invasive) medical procedures with the guidance of imaging technologies.
The acquisition of medical imaging is usually carried out by the radiographer or radiologic technologist. The diagnostic radiologist then interprets or "reads" the images and produces a report of their findings and impression or diagnosis. This report is then transmitted to the ordering physician, either routinely or emergently. Specialist physicians may also read specific radiologic examinations performed on their own patients. Examples include orthopedic surgeons reading X-rays and MRIs of bones and joints, cardiologists reading cardiac nuclear medicine and performing and reading coronary artery angiograms, obstetricians reading OB ultrasounds, pulmonologists reading chest X-rays and chest CT scans,etc.
The following imaging modalities are used in the field of diagnostic radiology:
Radiographs (or roentgenographs, named after the discoverer of X-rays, Wilhelm Conrad Röntgen) are produced by transmitting X-rays through a patient. The x-rays penetrate the portion of the body to be examined and by controlling the energy and the length of exposure, a high contrast image is formed on silver-impregnated films. Röntgen discovered X-rays on November 8, 1895 and received the first Nobel Prize in Physics for their discovery in 1901.
In film-screen radiography, an X-ray tube generates a beam of X-rays, which is aimed at the patient. The X-rays that pass through the patient are filtered through a device called an X-ray filter, to reduce scatter and noise, and strike an undeveloped film, which is held tightly to a screen of light-emitting phosphors in a light-tight cassette. The film is then developed chemically and an image appears on the film. Film-screen radiography is being replaced by digital radiography (DR), in which the X-rays strike a plate of sensors that converts the signals generated into digital information, which is transmitted and converted into an image displayed on a computer screen.
Plain radiography was the only imaging modality available during the first 50 years of radiology. Due to its availability, speed, and lower costs compared to other modalities, radiography is often the first-line test of choice in radiologic diagnosis. Also despite the large amount of data in CT scans, MR scans and other digital-based imaging, there are many disease entities in which the classic diagnosis is obtained by plain radiographs. Examples include various types of arthritis and pneumonia, bone tumors (especially benign bone tumors), fractures, congenital skeletal anomalies, etc.
Fluoroscopy and angiography are special applications of X-ray imaging, in which a fluorescent screen and image intensifier tube is connected to a closed-circuit television system.:26 This allows real-time imaging of structures in motion or augmented with a radiocontrast agent. Radiocontrast agents are usually administered by swallowing or injecting into the body of the patient to delineate anatomy and functioning of the blood vessels, the genitourinary system, or the gastrointestinal tract (GI tract). Two radiocontrast agents are presently in common use. Barium sulfate (BaSO4) is given orally or rectally for evaluation of the GI tract. Iodine, in multiple proprietary forms, is given by oral, rectal, intra-arterial or intravenous routes. These radiocontrast agents strongly absorb or scatter X-rays, and in conjunction with the real-time imaging, allow demonstration of dynamic processes, such as peristalsis in the digestive tract or blood flow in arteries and veins. Iodine contrast may also be concentrated in abnormal areas more or less than in normal tissues and make abnormalities (tumors, cysts, inflammation) more conspicuous. Additionally, in specific circumstances, air can be used as a contrast agent for the gastrointestinal system and carbon dioxide can be used as a contrast agent in the venous system; in these cases, the contrast agent attenuates the X-ray radiation less than the surrounding tissues.
Interventional radiology (IR or sometimes VIR for vascular and interventional radiology) is a subspecialty of radiology in which minimally invasive procedures are performed using image guidance. Some of these procedures are done for purely diagnostic purposes (e.g., angiogram), while others are done for treatment purposes (e.g., angioplasty).
The basic concept behind interventional radiology is to diagnose or treat pathologies, with the most minimally invasive technique possible. Interventional radiologists diagnose and treat several disorders, including peripheral vascular disease, renal artery stenosis, inferior vena cava filter placement, gastrostomy tube placements, biliary stents and hepatic interventions. Images are used for guidance, and the primary instruments used during the procedure are needles and catheters. The images provide maps that allow the interventional radiologist to guide these instruments through the body to the areas containing disease. By minimizing the physical trauma to the patient, peripheral interventions can reduce infection rates and recovery times, as well as hospital stays. To be a trained interventionalist in the United States, an individual completes a five-year residency in radiology and a one- or two-year fellowship in IR.
CT imaging uses X-rays in conjunction with computing algorithms to image the body. In CT, an X-ray tube opposite an X-ray detector (or detectors) in a ring-shaped apparatus rotate around a patient, producing a computer-generated cross-sectional image (tomogram). CT is acquired in the axial plane, with coronal and sagittal images produced by computer reconstruction. Radiocontrast agents are often used with CT for enhanced delineation of anatomy. Although radiographs provide higher spatial resolution, CT can detect more subtle variations in attenuation of X-rays. CT exposes the patient to more ionizing radiation than a radiograph.
Spiral multidetector CT uses 16, 64, 254 or more detectors during continuous motion of the patient through the radiation beam to obtain fine detail images in a short exam time. With rapid administration of intravenous contrast during the CT scan, these fine detail images can be reconstructed into three-dimensional (3D) images of carotid, cerebral, coronary or other arteries.
CT scanning has become the test of choice in diagnosing some urgent and emergent conditions, such as cerebral hemorrhage, pulmonary embolism (clots in the arteries of the lungs), aortic dissection (tearing of the aortic wall), appendicitis, diverticulitis, and obstructing kidney stones. Continuing improvements in CT technology, including faster scanning times and improved resolution, have dramatically increased the accuracy and usefulness of CT scanning, which may partially account for increased use in medical diagnosis.
The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield at EMI Central Research Labs, Great Britain in 1972. EMI owned the distribution rights to the Beatles music and it was their profits that funded the research. Sir Godfrey Hounsfield and Allan McLeod Cormack (underlying theory and calculations) shared the Nobel Prize for Medicine in 1979 for the invention of CT scanning.
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 and patient body habitus. Larger patients may have a decrease in image quality due to sound wave absorption in the subcutaneous fat layer. This results in fewer sound waves penetrating to organs and reflecting back to transducer, ultimately causing a poorer quality image. Ultrasound is also limited by its inability to image through air (lungs, bowel loops) or bone. Its use in medical imaging has developed mostly within the last 30 years. The first ultrasound images were static and two-dimensional (2D), but with modern ultrasonography, 3D reconstructions can be observed in real time, effectively becoming "4D".
Because ultrasound does not use ionizing radiation, unlike radiography, CT scans, and nuclear medicine imaging techniques, it is generally considered safer. So this modality plays a vital role in obstetrical imaging. Fetal anatomic development can be thoroughly evaluated, allowing early diagnosis of many fetal anomalies. Growth can be assessed over time, important in patients with chronic disease or gestation-induced disease, and in multiple gestations (twins, triplets, etc.). Color-flow Doppler ultrasound measures the severity of peripheral vascular disease and is used by cardiologists for dynamic evaluation of the heart, heart valves and major vessels. Stenosis of the carotid arteries can presage cerebral infarcts (strokes). A deep vein thrombosis in the legs can be found via ultrasound before it dislodges and travels to the lungs (pulmonary embolism), which can be fatal if left untreated. Ultrasound is useful for image-guided interventions such as biopsies and drainages such as thoracentesis. 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 (bleeding inside the body cavity) or injury to the major organs may require emergent surgical exploration and repair.
MRI uses strong magnetic fields to align atomic nuclei (usually hydrogen protons) within body tissues, then uses a radio signal to disturb the axis of rotation of these nuclei and observes the radio frequency signal generated as the nuclei return to their baseline states. The radio signals are collected by small antennae, called coils, placed near the area of interest. An advantage of MRI is its ability to produce images in axial, coronal, sagittal and multiple oblique planes with equal ease. 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 become an important tool in musculoskeletal radiology and neuroradiology.
One disadvantage is the patient has to hold still for long periods of time in a noisy, cramped space while the imaging is performed. Claustrophobia severe enough to terminate the MRI exam is reported in up to 5% of patients. Recent improvements in magnet design including stronger magnetic fields (3 teslas), shortening exam times, wider, shorter magnet bores and more open magnet designs, have brought some relief for claustrophobic patients. However, for magnets with equivalent field strengths, there is often a trade-off between image quality and open design. MRI has great benefit in imaging the brain, spine, and musculoskeletal system. The use of MRI is currently contraindicated for patients with pacemakers, cochlear implants, some indwelling medication pumps, certain types of cerebral aneurysm clips, metal fragments in the eyes and some metallic hardware due to the powerful magnetic fields and strong fluctuating radio signals to which the body is exposed. Areas of potential advancement include functional imaging, cardiovascular MRI, and MRI-guided therapy.
Nuclear medicine imaging involves the administration into the patient of radiopharmaceuticals consisting of substances with affinity for certain body tissues labeled with radioactive tracer. The most commonly used tracers are technetium-99m, iodine-123, iodine-131, gallium-67, indium-111, thallium-201 and fludeoxyglucose (18F) (18F-FDG). 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. The excretory function of the kidneys, iodine-concentrating ability of the thyroid, blood flow to heart muscle, etc. can be measured. 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. With computer processing, the information can be displayed as axial, coronal and sagittal images (single-photon emission computed tomography - SPECT). In the most modern devices, nuclear medicine images can be fused with a CT scan taken quasisimultaneously, so the physiological information can be overlaid or coregistered with the anatomical structures to improve diagnostic accuracy.
Positron emission tomography (PET) scanning, also a nuclear medicine procedure, deals with positrons. The positrons annihilate to produce two opposite traveling gamma rays to be detected coincidentally, thus improving resolution. In PET scanning, a radioactive, biologically active substance, most often 18F-FDG, is injected into a patient and the radiation emitted by the patient is detected to produce multiplanar images of the body. Metabolically more active tissues, such as cancer, concentrate the active substance more than normal tissues. PET images can be combined (or "fused") with an anatomic imaging study (currently generally CT images), to more accurately localize PET findings and thereby improve diagnostic accuracy.
The fusion technology has gone further to combine PET and MRI similar to PET and CT. PET/MRI fusion, largely practiced in academic and research settings, could potentially play a crucial role in fine detail of brain imaging, breast cancer screening, and small joint imaging of the foot. The technology recently blossomed following passing a technical hurdle of altered positron movement in strong magnetic field thus affecting the resolution of PET images and attenuation correction.
Mammography is the radiographic examination of the breast utilizing low energy x-rays and fine detail film-screen or digital imaging. Mammograms may be a screening examination, aimed at detecting early breast cancer or a diagnostic study, to better define abnormalities on a screening, follow-up a previously found abnormality or to evaluate a palpable mass or other symptoms. Early breast cancer may be seen as a mass, a focal asymmetric density or as clustered microcalcifications (< 1 mm).
In the U.S. two views of each breast (cranial-caudal,CC and medial-lateral-oblique, MLO)are obtained while the breast is compressed. The compression while necessary to flatten the breast for optimal imaging and to reduce movement, is the main cause of discomfort and complaint. Mammography is currently in transition from film-screen to FFDM, full field digital imaging. Other newer modalities include breast MRI and breast tomosynthesis examination.
Since 1990, the death rate from breast cancer has decreased 30%. Breast cancer found early has a much better survival rate than later stage breast cancer. Screening mammography recommendations however remain highly controversial with debate as to the frequency of screening (1 or 2 years), age to begin and to end screening, false positive examination stress, risk of radiation induced breast cancer, risk-benefit to individuals and to society,etc.
Teleradiology is the transmission of radiographic images from one location to another for interpretation by a radiologist. It is most often used to allow rapid interpretation of emergency room, ICU and other emergent examinations after hours of usual operation, at night and on weekends. In these cases, the images can be sent across time zones (e.g. to Spain, Australia, India) with the receiving radiologist working his normal daylight hours. However at present, large private teleradiology companies in the U.S. currently provide most after-hours coverage employing night working radiologists in the U.S. Teleradiology can also be used to obtain consultation with an expert or subspecialist about a complicated or puzzling case.
Teleradiology requires a sending station, a high-speed internet connection, and a high-quality receiving station. At the transmission station, plain radiographs are passed through a digitizing machine before transmission, while CT, MRI, ultrasound and nuclear medicine scans can be sent directly, as they are already digital data. The computer at the receiving end will need to have a high-quality display screen that has been tested and cleared for clinical purposes. Reports are then transmitted to the requesting physician.
The major advantage of teleradiology is the ability to use different time zones to provide real-time emergency radiology services around-the-clock. The disadvantages include higher costs, limited contact between the ordering physician and the radiologist, and the inability to cover for procedures requiring an onsite radiologist. Laws and regulations concerning the use of teleradiology vary among the states, with some requiring a license to practice medicine in the state sending the radiologic exam. Some states require the teleradiology report to be preliminary with the official report issued by a hospital staff radiologist.
Radiology is an expanding field in medicine. Applying for residency positions in radiology is competitive. Applicants are often near the top of their medical school classes, with high USMLE (board) scores. The field is rapidly expanding due to advances in computer technology, which is closely linked to modern imaging. Diagnostic radiologists must complete prerequisite undergraduate education, four years of medical school to earn a medical degree (D.O. or M.D.), one year of internship, and four years of residency training. After residency, radiologists may pursue one or two years of additional specialty fellowship training.
The American Board of Radiology (ABR), administers professional certification in Diagnostic Radiology, Radiation Oncology and Medical Physics as well subpecialty certification in neuroradiology, nuclear radiology, pediatric radiology and vascular and interventional radiology. Prior to 2013, "Board certification" in Diagnostic Radiology for M.D.s required passing a Medical Physics written examination in the 2nd year of residency, a Written Board examination after 3 years and an Oral Board examination administered in Louisville, Kentucky in the final month of residency.
The American Board of Radiology certification has completely changed for residents beginning residency in 2010 and thereafter. The Core Exam will be given 36 months into residency. This computer-based examination will be given twice a year in Chicago and Tucson. It encompasses 18 categories. A pass of all 18 is a pass. A fail on 1 to 5 categories is a Conditioned exam and the resident will need to retake and pass the failed categories. A fail on over 5 categories is a failed exam. The Certification Exam, which replaces the Oral Examination, will be taken 15 months after completion of the Radiology residency. This computer-based examination will consist of 5 modules and graded pass-fail. It will be given twice a year in Chicago and Tucson. Recertification examinations are taken every 10 years, with additional required continuing medical education as outlined in the Maintenance of Certification document.
Certification may also be obtained from the American Osteopathic Board of Radiology (AOBR) and the American Board of Physician Specialties.
Following completion of residency training, radiologists may either begin practicing or enter into subspecialty training programs known as fellowships. Examples of subspeciality training in radiology include abdominal imaging, thoracic imaging, cross-sectional/ultrasound, MRI, musculoskeletal imaging, interventional radiology, neuroradiology, interventional neuroradiology, paediatric radiology, nuclear medicine, emergency radiology, breast imaging and women's imaging. Fellowship training programs in radiology are usually one or two years in length.
Some medical schools in the US have started to incorporate a basic radiology introduction into their core MD training. New York Medical College, the Wayne State University School of Medicine, the Uniformed Services University, and the University of South Carolina School of Medicine offer an introduction to radiology during their respective MD programs.
Radiographic exams are usually performed by radiologic technologists, (also known as diagnostic radiographers), who in the United States have a two-year associate degree or four-year bachelor of science degree and, in the UK, a three-year honours degree.
Veterinary radiologists are veterinarians who specialize in the use of X-rays, ultrasound, MRI and nuclear medicine for diagnostic imaging or treatment of disease in animals. They are certified in either diagnostic radiology or radiation oncology by the American College of Veterinary Radiology.
Radiology is a competitive speciality in the UK, attracting applicants from a broad range of backgrounds. Traditionally, applications were accepted only from doctors who had completed higher training in specialities such as surgery, or general medicine. They were usually required to pass a professional exam such as the MRCP or MRCS before they were considered for radiology training. Currently, applicants are welcomed directly from the foundation programme, as well as those who have completed higher training. Completion of professional exams is no longer a prerequisite for application although it can still be helpful during the rigorous interview process.
The training programme lasts for a total of five years. During this time, doctors rotate into different subspecialities, such as paediatrics, musculoskeletal or neuroradiology, and breast imaging. During the first year of training, radiology trainees are expected to pass the first part of the Fellowship of the Royal College of Radiologists (FRCR) exam. This comprises a medical physics and anatomy examination. Following completion of their part 1 exam, they are then required to pass six written exams (part 2A), which cover all the subspecialities. Successful completion of these allows them to complete the FRCR by completing part 2B, which includes rapid reporting, and a long case discussion.
After achieving a certificate of completion of training (CCT), many fellowship posts exist in specialities such as neurointervention and vascular intervention, which would allow the doctor to work as an interventional radiologist. In some cases, the CCT date can be deferred by a year to include these fellowship programmes.
UK radiology registrars are represented by The Society of Radiologists in Training UK, which was founded in 1993 under the auspices of the Royal College of Radiologists. The society is a nonprofit organisation, run by radiology registrars specifically to promote radiology training and education in the UK. Annual meetings are held by which trainees across the country are encouraged to attend.
Currently, a shortage of radiologists in the UK has created opportunities in all specialities, and with the increased reliance on imaging, demand is expected to increase in the future.
After obtaining medical licensure, German radiologists complete a five-year residency, culminating with a board examination (known as Facharztprüfung).
The radiology training program in Italy increased from four to five years in 2008. Further training is required for specialization in radiotherapy or nuclear medicine.
Dutch radiologists complete a five-year residency program after completing the 6-year MD program.
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