Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualised within the human 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.
Acquisition of radiological images
The following imaging modalities are used in the field of diagnostic radiology:
Projection (plain) radiography
Radiographs (or roentgenographs, named after the discoverer of x-rays, Wilhelm Conrad Röntgen) are produced by transmitting x-rays through a patient. A capture device then converts x-rays into visible light which then forms an image for review and diagnosis. The original – and a still common imaging procedure uses silver-impregnated films.
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 by placing a device called x-ray filter, made of aluminium 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. Now replacing Film-Screen radiography is Digital Radiography (DR), in which x-rays strike a plate of sensors that converts the signals generated into digital information and an image that is 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.
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 injection into the body of the patient to delineate anatomy and functioning of the blood vessels, the genitourinary system, or the gastrointestinal 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-ray radiation, and 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. 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 (abbreviated 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 pathology, 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, biliarystents andhepatic interventions. Images are used for guidance and the primary instruments used during the procedure are needles and tiny tubes called catheters. The images provide road 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 time as well as shorten 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 Interventional Radiology.
Computed tomography (CT)
CT imaging uses X-rays in conjunction with computing algorithms to image the body. In CT, an X-ray generating 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, while coronal and sagittal images can be rendered 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 Multi-detector CT uses 8, 16, 64 or more detectors during continuous motion of the patient through the radiation beam to obtain much finer detail images in a shorter exam time. With rapid administration of IV contrast during the CT scan these fine detail images can be reconstructed into 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 which funded the research. Sir Hounsfield and Alan McLeod McCormick shared the Nobel Prize for Medicine in 1979 for the invention of CT scanning. The first CT scanner in North America was installed at the Mayo Clinic in Rochester, MN in 1972.
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 less sound wave 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. 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 use ionizing radiation, unlike radiography, CT scans, and nuclear medicine imaging techniques, it is generally considered safer. For this reason, 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 diseaseand is used by Cardiology for dynamic evaluation of the heart, heart valves and major vessels. Stenosis of the carotid arteries can presage cerebral infarcts (strokes). DVT 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 like biopsies and drainages such as thoracentesis). Small portable ultrasound devices now replaceperitoneal lavage in the triage of trauma victims by directly assessing for the presence of hemorrhage in theperitoneum 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.
Magnetic resonance imaging (MRI)
MRI uses strong magnetic fields to align atomic nuclei (usually hydrogenprotons) 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 that 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, in magnets of equal field strength 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 modality 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 the body is exposed to. 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 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 18F-FDG. The heart, lungs,thyroid, liver, gallbladder, and bones are commonly evaluated for particular conditions using these techniques. Whileanatomical 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 (SPECT images, single-photon emission computed tomography). In the most modern devices Nuclear Medicine images can be fused with a CT scan taken quasi-simultaneously so that the physiological information can be overlaid or co-registered with the anatomical structures to improve diagnostic accuracy.
Positron emission tomography (PET), scanning is a nuclear medicine procedure that 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 Fludeoxyglucose (18F), is injected into a patient and the radiation emitted by the patient is detected to produce multi-planar 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 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.
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 are often sent across time zones (i.e. to Spain, Australia, India) with the receiving radiologist working his normal daylight hours. Teleradiology can also be used to obtain consultation with an expert or sub-specialist about a complicated or puzzling case.
Teleradiology requires a sending station, high speed Internet connection and high quality receiving station. At the transmission station, plain radiographs are passed through a digitizing machine before transmission, while CT scans, MRIs, Ultrasounds and Nuclear Medicine scans can be sent directly as they are already a stream of 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 states 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 has become increasingly competitive. Applicants are often near the top of their medical school class, 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, 4 years of medical school, one year of internship, and 4 years of residency training. After residency, radiologists often pursue one or two years of additional specialty fellowship training.
To be certified by the American Board of Radiology (ABR), the radiology resident must pass a multiple choice medical physics board exam during training (usually taken after the second year of radiology residency) covering the science, technology and radiobiology of ultrasound, CTs, x-rays, nuclear medicine and MRI. A second multiple choice exam (“written board”) on clinical aspects is then subsequently taken usually one year later. After passing these two tests and being in good standing, the resident is eligible to take the oral examination or eligible to “sit for the Boards”, taken about a month before graduating from the radiology residency. To complete the oral section of the ABR certification, a radiologist must pass all eleven sections. An applicant who passes fewer than eight sections has failed and must re-take the entire exam. An applicant who passes at least eight of the eleven sections of the ABR oral boards is considered “conditioned” and can retake the last three or fewer sections again at a later date to become ABR certified. Once successful in passing all sections, the physician then becomes a diplomate of the American Board of Radiology. Beginning with the residency class that started in 2010, the ABR’s board examination structure will be changed to include two computer-based exams, one given after the third year of residency training covering both physics and clinical aspects with increasing emphasis on images, and the second exam to be taken 15 months after graduating from residency. The current oral examination is eliminated. Recertification examination is taken every 10 years with additional continuing medical education as outlined in the Maintenance of Certification (MOC) document.
Certification may also be obtained from the American Osteopathic Board of Radiology (AOBR) and the American Board of Physician Specialties (ABPS).
Following completion of residency training, radiologists may either begin practicing or enter into sub-specialty training programs known as fellowships. Examples of sub-speciality 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 1 or 2 years in length.
Several medical schools in the US have started to incorporate radiology education 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 integrated radiology curriculum 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 2-year Associates Degree or 4 year Bachelors of Science Degree and, in the UK, a 3 year Honours Degree.
Veterinary radiologists are veterinarians that 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.
Chiropractic Radiologists are doctors of chiropractic that have completed a minimum of 3 years of residency training, and are board certified Diplomates of the American Chiropractic Board of Radiology (ACBR). Chiropractic radiology includes, but is not limited to, diagnostic interpretation of plain film radiography, digital radiography, fluoroscopy, computed tomography, ultrasonography, radioisotope imaging and magnetic resonance imaging. Some chiropractic radiologists also complete post-graduate fellowships to specialize in a specific area of radiology. Chiropractic radiologists practice and teach in a variety of institutions including chiropractic schools, medical schools, hospitals, group practices, imaging centers, and private practices.