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Chapter: Basic Radiology : Scope of Diagnostic Imaging

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Conventional Radiography

Conventional Radiography
Conventional radiography refers to plain radiographs that are generated when x-ray film is exposed to ionizing radiation and developed by photochemical process.

CONVENTIONAL RADIOGRAPHY

Conventional radiography refers to plain radiographs that are generated when x-ray film is exposed to ionizing radiation and developed by photochemical process. During develop-ment, the metallic silver on the x-ray film is precipitated, ren-dering the latent image black. The amount of blackening on the film is proportional to the amount of x-ray radiation ex-posure. Plain radiography relies on natural and physical con-trast based on the density of material through which the x-ray radiation must pass. Thus, gas, fat, soft tissue, and bone produce black, gray-black, gray, and white radiographic im-ages, respectively, on film (Figure 1-1).


Although other image modalities such as CT, ultra-sonography, and MR imaging are being used with increasing frequency to replace plain radiographs, conventional radiog-raphy remains a major modality in the evaluation of chest, breast, bone, and abdominal diseases.

Computed radiography (CR) or digital radiography is presently replacing conventional screen-film combination techniques. The most common CR technique, photostimu-lable phosphor computed radiography (PPCR), uses a phosphor-coated plate to replace the film-screen combina-tion. When a cassette containing the phosphor-coated plate is exposed to x-rays, the phosphor stores the absorbed x-ray energy. The exposed cassette is then placed in a PPCR reader that uses a laser to stimulate release of electrons, re-sulting in the emission of short-wavelength blue light. The brightness of the blue light is dependent on the amount of absorbed x-ray photon energy. This luminescence generates an electrical signal that is reconstructed into a gray-scale image, which may be displayed on a monitor or printed as a hard copy. Digital images generated from PPCR are capable of being transmitted through a picture archiving and com-munications system (PACS), similar to other digital images acquired from CT or MR facilities. PPCR is better than plain radiography in linear response to a wide range of x-ray exposure. However, PPCR provides less spatial resolution than plain radiography. Another CR technique that is being developed uses an amorphous selenium-coated plate, which directly converts x-ray photons into electrical charges.

 

Fluoroscopy uses a fluorescent screen instead of radi-ographic film to view real-time images generated when an x-ray beam penetrates through a certain part of the body. An image intensifier absorbs x-ray photons and produces a quantity of light on the monitor. The brightness of the image is proportional to the number of incident photons received. Fluoroscopy is a major modality used to examine the gas-trointestinal tract. For example, fluoroscopy can be used to follow the course of contrast materials through the gastroin-testinal tract, allowing the evaluation of both structure and function. Spot filming or video recording may be used syn-chronously with fluoroscopy to optimally demonstrate pathology. Fluoroscopy is also used to monitor catheter placement during angiography and to guide interventional procedures. In recent years, digital detectors (such as charge-coupled devices, CCDs) have begun to replace video cameras on fluoroscopy units.

 

Conventional tomography produces an image of one in-tended area by blurring structures superimposed on both sides of a focus plane. This technique, however, has been largely replaced by CT.

Mammography uses a film-screen combination tech-nique to evaluate breast lesions for the early detection of breast carcinoma. A mammographic unit is installed with a special x-ray tube and a plastic breast-compression device. A standard mammogram obtains views in two projections, producing craniocaudal (CC) and mediolateral oblique (MLO) images of the breast. Additional images of the breast in other projections, such as mediolateral (ML) views, and using diagnostic techniques such as magnification and/or spot compression views may also be obtained to further char-acterize potential pathologic findings. Ultrasonography (US) is also used in breast imaging as a complementary modality to further characterize breast pathology. Several image-guided breast interventional procedures, such as preoperative needle placement for lesion localization and core needle biopsy using stereotactic ultrasound or MR guidance, are widely available.

 

Contrast Studies

 

Contrast materials are used to examine organs that do not have natural inherent contrast with surrounding tissues. Contrast media are commonly used to evaluate the gastroin-testinal tract, the urinary tract, the vascular system, and solid organs. Contrast media used in MR imaging are described in the MR modality section.

 

Barium suspension is still used daily in the examination of the gastrointestinal tract. Barium suspension is a safe contrast media that provides high imaging density on upper gastrointestinal (UGI) series, small-bowel studies, and evaluation of the colon. Both single-contrast and double-contrast techniques may be used to evaluate the gastroin-testinal tract (Figure 1-2). In the single-contrast study, barium suspension is administered alone. In the double-contrast study, both barium and air are introduced to de-lineate the details of the mucosal surface, which facilitates the identification of superficial lesions in the bowel lumen. In the UGI double-contrast study, air is introduced into the bowel lumen by administering oral effervescent agents. For double-contrast evaluation of the lower GI tract with bar-ium enema, air is introduced into the bowel lumen via di-rect inflation with a small pump through a rectal catheter. Small-bowel contrast studies include peroral, enteroclysis, and retrograde techniques. The peroral small-bowel study is performed by feeding barium suspension to the patient and recording the progress of contrast through the small bowel. Enteroclysis is performed by placing a catheter in the proximal jejunum and infusing barium suspension through the catheter. Enteroclysis is preferred for evaluat-ing focal small-bowel lesions or the cause of small-bowel obstructions. Retrograde small-bowel examination is per-formed by retrograde reflux of barium suspension into the small bowel during barium enema or via direct injection through an ileostomy.


Figure 1-2. A single-contrast retrograde colonic enema in the left posterior oblique view demonstrates an annular lesion representing a cecal carcinoma (arrows). Bilateral hip prostheses are an incidental observation.


Water-soluble contrast media are commonly used for an-giography, interventional procedures, intravenous urogra-phy, and enhancement of CT. All water-soluble contrast media are iodinated agents that are classified as high or low osmolar, ionic or nonionic, and monomeric or dimeric in chemical nature. The iodine atoms in contrast medium absorb x-rays in proportion to the concentration in the body when radiographed. The most common water-soluble contrast media are the high osmolar ionic contrast agents (diatrizoate and its derivatives). Low osmolar contrast media include ionic (meglumine ioxaglate) and nonionic (iohexol, iopami-dol, ioversol, iopromide) monomers, as well as nonionic dimers (iodixanol). Low osmolar contrast media have an overall lower incidence of adverse reactions, including nephrotoxicity and mortality, than high osmolar ionic agents; however, lower osmolar agents are also three to five times more expensive.


The occurrence and severity of adverse reactions after ad-ministration of iodinated contrast material are unpre-dictable. These reactions are categorized as mild, moderate, or severe based on degree of symptoms. Mild adverse reac-tions include nausea, vomiting, and urticaria that do not re-quire treatment. The incidence of mild adverse reactions may be less if using a lower osmolality contrast agent. Moderate reactions include symptomatic urticaria, vasovagal events, mild bronchospasm, and/or tachycardia that requires treat-ment. Severe and life-threatening reactions, such as severe bronchospasm, laryngeal edema, seizure, severe hypotension, and/or cardiac arrest, are unpredictable and require prompt recognition and immediate treatment.

 

Contrast-induced nephropathy (CIN) is characterized by renal dysfunction after intravenous administration of iodi-nated contrast material. There is no standard definition of CIN. Findings with CIN include percent increasing serum creatinine from baseline (such as 20% to 50%) or increasing absolute serum creatinine above baseline (such as 0.5 to 2.0 mg/d) within 24 to 48 hours (or in 3 to 5 days). The inci-dence of CIN is variable. Patients with renal failure or under-lying renal diseases are several times more likely to develop CIN than those with normal renal function following the administration of iodinated contrast material.

 

Water-soluble contrast agents are used in the gastroin-testinal tract when barium suspension is contraindicated, when perforation is suspected, when surgery is likely to fol-low imaging, when confirmation of percutaneous catheter location is necessary, and when gastrointestinal opacifica-tion is required during abdominal CT evaluation. Unlike barium suspension, water-soluble contrast agents are readily absorbed by the peritoneum if extraluminal extravasation occurs, but provide less image density. High osmolar water-soluble contrast agents may cause severe pulmonary edema if aspirated. High osmolar contrast agents may also cause fluid to shift from the intravascular compartment into the bowel lumen, resulting in hypovolemia and hypotension, which is less likely to occur with low osmolar water-soluble contrast media.

 

Intravenous urography (IVU) uses ionic or nonionic water-soluble contrast agents to evaluate the urinary tract. Renal excretion/concentration of intravenously administered iodinated contrast material opacifies the kidneys, ureters, and bladder approximately 10 minutes postinjection. Intra-venous urography has been largely replaced over the past decade by unenhanced helical CT evaluation. IVU, however, remains useful for the evaluation of subtle uroepithelial neo-plasms and other diseases of the renal collecting system, and it can provide additional information that complements data from cross-sectional image modalities. Additional contrast-enhanced imaging examinations of the genitourinary system include cystography, voiding cystourethrography, and retro-grade urethrography to evaluate the bladder and urethra.

 

Hysterosalpingography is primarily used to evaluate the patency of fallopian tubes and uterine abnormalities in pa-tients with infertility. Hysterosalpingography is also used for postsurgical evaluation and to define anatomy for reanasto-mosis procedures.

 

Hysterosalpingography is performed by inserting a catheter into the uterus and subsequently injecting water-soluble contrast medium (some institutions prefer oil-based iodine contrast) to delineate the uterine cavity and the patency of the fallopian tubes. A fluoroscopic spot image is taken once contrast medium fills the uterus and fallopian tubes, but before spillage into the peritoneum. A second image is takenafter fallopian tube spillage appears. A transcervical recanal-ization of obstructed fallopian tube has been introduced to improve the fertility rate.

 

Angiography is the study of blood vessels following intra-arterial or intravenous injection of water-soluble contrast agents. A series of rapid exposures is made to follow the course of the contrast medium through the examined blood vessels. Angiographic images are recorded by standard or dig-ital imaging, and/or stored digitally.

 

Thoracic aortography is performed when there is suspi-cion of traumatic aortic injury, dissection (Figure 1-3), or atherosclerotic aneurysm, and to evaluate cerebral and upper extremity vascular disease. Multidetector CT has largely re-placed conventional aortography as the initial modality to evaluate aortic trauma (Figure 1-4). Conventional aortogra-phy, however, remains important in specific settings, such as planning endovascular stent graft therapy and assessing small branch vessel injuries in stable patients. Abdominal aortogra-phy is used to evaluate vessel origins in vascular occlusive disease or prior to selective catheterization. Abdominal aortography is also used for vascular mapping prior to aneurysm repair or other intra-abdominal surgery. Coronary angiography is most commonly performed to evaluate coro-nary occlusion. Pulmonary angiography is used in patients who are suspected of having pulmonary embolus, especially in the setting of equivocal results on ventilation-perfusion imaging. Inferior venacavography is performed to evaluate for caval occlusion from venous thrombosis, obstruction or compression by retroperitoneal lymphadenopathy, or fibro-sis. Inferior venacavography is also performed to evaluate the configuration of the inferior vena cava before filter place-ment. In recent years, conventional angiography has been replaced by CT angiography and MR angiography.

 

Less commonly used contrast studies include myelogra-phy (evaluating disk herniation and spinal cord compres-sion), fistulography (sinus tracts for abscesses and cavities), sialography (evaluating the salivary glands for ductal ob-struction or tumor), galactography (assessing the breast duc-tal system), and oral cholecystography, cholangiography (evaluating the biliary tree), and lymphangiography (assess-ing lymph nodes and lymph channels for malignancies).

 

Computed Tomography

 

Computed tomography, an axial tomographic technique, produces source images that are perpendicular to the long axis of the body (Figure 1-5). Attenuation values generated by CT reflect the density and atomic number of various tis-sues and are usually expressed as relative attenuation coeffi-cients, or Hounsfield units (HUs). By definition, the HUs of water and air are zero and –1,000, respectively. The HUs of soft tissues range from 10 to 50, with fat demonstrating negative HU. Bone is at least 1,000 HU. The contrast resolu-tion of vascular structures, organs, and pathology, such as hypervascular neoplasms, can be enhanced following intra-venous infusion of water-soluble contrast media. The type, volume, and rate of administration as well as the scan delay time vary with specific study indication and protocol. Addi-tionally, oral contrast material, namely, water-soluble agents or barium suspensions, can be administered for improved bowel visualization. Artifacts may be produced by patient motion or high-density foreign bodies, such as surgical clips.


Figure 1-5. Contrast-enhanced CT image of the upper abdomen demon-strated two low-attenuation areas (M) confirmed as multiple hepatic metas-tases from a gastrointestinal stromal tumor.



Variety Scanners

Conventional CT scanners have traditionally operated in a step-and-shoot mode, defined by data acquisition and pa-tient positioning phases. During the data acquisition phase, the x-ray tube rotates around the patient, who is maintained in a stationary position. A complete set of projections are ac-quired at a prescribed scanning location prior to the patient positioning phase. During this latter phase, the patient is transported to the next prescribed scanning location.

The first helical (spiral) CT scanner was introduced for clinical applications in the early 1990s. Helical CT is charac-terized by continuous patient transport through the gantry while a series of x-ray tube rotations simultaneously acquire volumetric data. These dynamic acquisitions are typically ob-tained during a single breath hold of about 20 to 30 seconds. Higher spatial resolution can be achieved with narrower col-limations. The advantages of helical CT technology include reduced scan times, improved speeds at which the volume of interest can be adequately imaged, and increased ability to detect small lesions that may otherwise change position in non-breath-hold studies. In addition, gains in scan speed permit less contrast material to be administered for the same degree of vessel opacification.

 

The evolution of multidetector CT (MDCT) scanners has resulted from the combination of helical scanning with multi-slice data acquisition. In this CT system, a multiple-row detec-tor array is employed. Current state-of-the-art models are capable of acquiring 64, 128, or 256 channels of helical data simultaneously. For a given length of anatomic coverage, multidetector CT can reduce scan time, permit imaging with thinner collimation, or both. The use of thinner collimation (0.4 mm to 2 mm) in conjunction with high-resolution re-construction algorithms yields images of higher spatial reso-lution (high-resolution CT), a technique commonly used for evaluation of diffuse interstitial lung disease or the detection of pulmonary nodules. Multidetector CT offers additional ad-vantages of decreased contrast load, reduced respiratory and cardiac motion artifact, and enhanced multiplanar recon-struction capabilities. These innovations have had a signifi-cant impact on the development of CT angiography (CTA). Multidetector CT has replaced conventional angiography as a primary modality in patients with acute aortic injuries.


CT Angiography

 

CT angiography protocols combine high-resolution volu-metric helical CT acquisitions with intravenous bolus ad-ministration of iodinated contrast material. Using an MDCT scanner, images are acquired during a single breath hold, en-suring that data acquisition will commence during times of peak vascular opacification. This has permitted successful imaging of entire vascular distributions, in addition to mini-mizing motion artifact and increasing longitudinal spatial resolution, thus potentially lowering administered contrast doses. The time between the start of contrast injection and the commencement of scanning can be tailored in response to a particular clinical question, permitting image acquisition during the arterial, venous, and/or equilibrium phases. Ex-quisite anatomic detail of both intraluminal and extralumi-nal structures is revealed using this technique, including detection of intimal calcification and mural thrombosis. CT angiography has become an important tool for assessment of the abdominal and iliac arteries and their branches, the thoracic aorta, pulmonary arteries, and intracranial and extracranial carotid circulation (Figure 1-6).


Figure 1-6. 3D reformatted image from CT angiography of brain shows a 16-mm aneurysm (arrow) arising from the left lateral aspect of the mid basilar artery. 

CT Colonography

 

CT colonography (virtual colonoscopy), introduced in 1994, is a relatively new noninvasive method of imaging the colon in which thin-section helical CT data are used to generate two-dimensional or three-dimensional images of the colon. This technology has been used primarily in the detection and characterization of colonic polyps, rivaling the traditional colonoscopic approach and conventional barium enema examinations. These images display the mucosal surface of the colon and internal density of the detected lesions, as well as directly demonstrating the bowel wall and extracolonic abdominal/pelvic structures.

 

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