690 consolidation, free air in the abdomen). 4 Suggest a differential diagnosis: Develop a structured approach so as not to miss any maj or abnormality. Try not to jump immediately to one diag- nosis, and develop your differenti al based both on radiologic findings and patient history/physical findings. 5 Treat the patient, not the movie. The chest movie See Figures 18.1 \200\22318.7, and PLATES 1 \200\2238 Before you start Orde r the best possible exam : Upright PA and lateral views are the standard initial imaging of choice for almost all cardiopulmonary disease. These com- plementary views are particularl y important when localization of a lesion/ pathology is required. An ideal PA view is taken with the patient in an upright position with full inspiration, and it demonstrates no rota tion or motion and minimal overly- ing osseous structures. However, this ideal is rarely se en in chest x-rays taken in ICU and emergency settings. y g o o d a R l i The interpretation \212 \212 Double-check the name on the movie . Comment on the quality of the movie ( \200\234RIPE \200\235): Rotation : The spinous process of T3 should be midway between the clavicles. Inspiration : 5 \200\2237 ribs anteriorly or 9 \200\22310 ribs posteriorly are visible in f ull inspiration. Position of the patient : Entire thorax from larynx to costophrenic angles should be visible. Exposure : Pulmonary vessels and thoracic vertebra should be visible through the heart. \212Look thoroughly and systematically . Examine all the structures, including visible por tions of the abdomen, bones, mediastinum, lungs together (to judge symmetry), and lungs sep arately Age and progression of find- Compare to old examinations when possible ings can be crucial to diagnosis and treatment. For example, a calcified nodule that has remained un changed over years may merely be a calcified granuloma. \212 Lines and hardware: All lines and hardware need to be identified and exam- ined for correct location and potential complications. Remember, all the hardware visible on CXR may not be inside the patient (e.g., electrocardio- gram [ECG] leads, oxygen lines, etc.). Examples o f common hardware in- clude central venous catheters (check for tip location and for pneumo tho- rax), endotracheal tubes (typically, the tip should be above the carina to avoid endob ronchial intubation), chest tubes, feeding tubes (nasogastric or naso-duodenal \200\224tip locations are key), sternal wires (are these intact?), prosthetic valves, implantable cardi oversion devices (ICDs/pacers), vagus nerve stimulators, Swan \200\223Ganz catheters, etc. The diaphragm: The right hemidiaphragm is higher than the left by up to 3 cm in 95% of case s. The lateral costophrenic angles should be sharp and acute. They may be ill-defined in hy perinflation or with effusion. On a lat- eral view, the right hemidiaphragm passes through the heart to the anterior chest wall, whereas the left ends at the posterior heart border. Causes of raised hemidiaphragm include eventration of diaphragm, enlarged liver or spleen, phrenic nerve paralysis, or lobar collapse. Trachea: The trachea is central or maybe slight ly to the right. Trachea may be deviated toward the lesion (collapse), away from the lesion (tension pneumothorax) or deviated from patient rotation. Paratracheal line is com- monly <5 mm; its thickening and nodularity suggest paratracheal lymph- adenopathy/mass. Heart: Lo ok for the landmarks. The cardiothoracic ratio is the ratio of the heart width to the chest width. It should be <50% (essential to have a PA 691 movie as the heart is magnified on AP or supine films). Ratios larger than this suggest car diomegaly due to any number of causes. Mediastinum: Mediastinal widening may be due to aort ic aneurysm or dis- section, mediastinal fluid, enlarged lymph nodes, thymus, thyroid, or t u- mor. The hila: Composed of pulmonary arteries, pulmonary veins, lymph nodes, and airways . Left is higher than right ( 1 cm) 95% of the time. Look for change in density and/or roun ded configuration. This could be due to tu- mor, enlarged lymph nodes, hypoinflation, or ju st a rotated movie. The lungs \200\234Too black \200\235: \212 Pneumothorax: Absent vascular markings and visceral pleura/lung edge visible Bullous change : I.e., emphysema Chronic pulmonary embolus: Localized oligemia (regional lack of visual- i zation of pulmonary blood vessels) Hyperinflation in chronic obstructive pulmonary disease (COPD) Absent anterior chest wall structures: I.e., mastectomy or Poland \200\231s syndrome \200\234Too White \200\235: \212 \212 \212 \212 \212 Consolidation: Lobar or segmental opacity, plus air bronchogram if air- ways remain air-fil led but little volume change (unlike collapse) Collapse/atelectasis: Volume loss causes shi ft of the normal landmarks (hila, fissures, diaphragm, etc.) Pulmonary nodules ( \200\234co in \200\235 lesions <3 cm in diameter): These are often \200\234incidentalomas \200\235 an d need to be differentiated into either benign or malignant lesions based on their characte ristics \200\224but most importantly in comparison to older films. \200\234Ring \200\235 s hadows: Either airways with peribronchial \200\234cuffing \200\235 (pulmonary edema, bronc hiectasis), or cavitating lesions (e.g., abscess [bacterial, fun- gal]) or tumor Interstiti al pattern: Pulmonary interstitium is normally not seen on CXR. Abnormal interstitial patte rns maybe seen as linear opacities ( Kerley \200\231s B lines , interlobular lymphatics see n in low-pressure pulmonary edema or lymphangitic spread of tumor); discoid atelectasis, li near band of atelecta- sis; reticulonodular pattern , multiple intersecting irregular lines interspersed with small 1 \200\2232 mm nodules, seen in pulmonary fibrosis or interstitial pneumonitis. \212 \212 \212 \212 Acute conditions not to miss \212Pneumothorax is commonly associated with trauma or can be iatrogenic. It may occur spontaneously in emphysema. In tension pneumothorax, the lung may be severely collapsed with mediastinal shift to the opposite side, with scooping of the ip silateral hemidiaphragm. There may also be subcutaneous air/subcutaneous emphysema. This is a medical emergency and requires immediate decompression followed by placement of a chest tube. in a patient with chest pain and/or history of trauma Widened mediastinum may represe nt aortic aneurysm or dissection, traumatic transection, or mediastinal hematoma. Free air in the abdomen sents free intraperitoneal air and may suggest a bowel perforation. \212 692 \212 seen under the right hemidiaphragm; this repre- Don \200\231t \212 After identifying one example of Succumb to \200\234satisfaction of search \200\235: patho logy on the image, don \200\231t give up your search until you have thor- oughly examined t he whole movie, including all the corners; there may be two or more important findings! y g o o d a R Figure 18.1 Posteroanterior chest radiograph of a female showing basic anatomical structure s l i Figure 18.2 Lateral chest radiograph of a female showing basic anatomical structures 693 Figure 18.3 3D coronal CT of the chest demonstrating heart and vascular structures Figure 18.4 3D coronal CT of the chest demonstrating heart and vascular structures 694 y g o o d a R Figure 18.5 3D sagittal CT of the chest demonstrating midline structures of the chest and v ertebral column l i Figure 18.6 3D sagittal CT of the chest demonstrating structures on the left side of the ch est 695 Figure 18.7 3D coronal CT of the chest demonstrating bronchial and pulmonary vascular tree \212 : Apices, aortopulmonary (AP) angle, retro- Forget to recheck danger areas cardiac region, and paratracheal area to the radiologist and to check the radi- Fail to give an adequate history ologist \200\231s final interpretation . Basic image interpretation can be easy, but subtle abnormalities require the trained eye of a radiologist. \212 The abdominal movie See Figures 18.8 \200\22318.13 . Before you start \212 : A true abdominal series of radiographs Order the best possible exam includes three films: 1. Upright PA chest (to look for free ai r under the diaphragms) 2. Upright abdomen after at least 5 min of upright positioning to a llow air to percolate superiorly and appear 3. Supine abdomen \212Double-check the name, date and time on the movie : Make sure the patient \200\231s na me matches and that you are looking at the appropriate time point. Radiographs (plain films ) only identify the interfaces between tissues of differ- ent densities: Interfaces made by adjacent tissues that are of the same den- sity cannot be seen. For example, you cannot se e the interface between two adjacent loops of fluid-filled small bowel. What you can identi fy are abnormal contours and mass effects created by the underlying pathology (e.g., loops of small bowel displaced by a large pelvic mass). Look thoroughly and systematically: you l ook at all abdominal films; review by organ system. Compare to prior films whenever pos- Co mpare new versus old findings: sible, as age and progression of findings can be crucial to diagnosis and treatment. Although abdominal radiographs can offer valu- Don \200\231t forge t the patient: able clues to diagnosis, it is possible for a very sick patient to have a no rmal-appearing abdominal series of radiographs. \212 \212 Develop a repeatable method by which 696 \212 \212 y g o o d a R Figure 18 .8 Abnormal gas patterns on plain abdominal radiography l i (A) (B) (C) (A) Cecal volvulus: Erect abdominal radiograph demonstrating a grossly dilated loop of bowe l (cecum) projected in the upper abdomen ( white arrow ). Note the lack of gas in the rest of the large bowel and dilated small bowel loops ( black arrow ). (B) Small bowel obstructi on: Multiple loops of distended small bowel. Note the valvulae conniventes stretch- ing acr oss the bowel circumferentially ( arrow ). (C) Large bowel obstruc- tion: Plain radiograph showing distended large bowel loops. Note grossly dilated cecum ( white arrow ). From Musson RE, et al. Postgrad Med J. 2011;87:274 \200\223287. Adapted with permission fro m PMJ, The Fellowship of Postgraduate Medicine . Observe the gas pattern and position See Figure 18.13 . Small bowel is recognized by its ce ntral position and valvulae conniventes (the mucosal folds) that transverse the full width of the bowel lumen. Large bowel is more peripheral and the haustrae, colonic folds, extend only partially across the lumen. Abnormal position may point to pathology (e.g., centrally displaced loops in ascites; bowel displaced to the left lower quadrant by splenomegaly). Lo ok for dilated bowel loops, defined as >3 cm diameter for the small bowel and as >6 cm diam eter for the large bowel. Also look for air \200\223fluid levels in the bowel on upright mo vie. \212 If the dilated loops are small bowel only, it is probably a small bowel obstruction. If the dilated loops are large bowel only, it may be a large bowel obstruc- tion or a cecal or si gmoid volvulus. A volvulus appears as a large bean-shaped loop of large bowel arising from pelvis. If the dilated loops are seen in the small and large bowel, it may be an adynamic i leus, a large bowel obstruction with an incompetent ileocecal valve, gastroenteritis, or ex cessive aerophagia. \212 \212 Localized peritoneal inflammation can cause a localized dilatation of bowel in response to inflammatory irritation (focal ileus). This may be seen on the plain movie as a \200\234se ntinel loop \200\235 of intraluminal gas and can provide a clue to the site of pathology. L ook for extraluminal/extraintestinal gas Look for gas in the peritoneal cavity, especially air under the hemidiaphragm, which can indicate a perforated viscus. Look in the portal vei n territory (peripheral liver) or biliary system (central liver and gallbladder), where air may be seen after passing a stone, after ERCP, or with gas-forming biliary infection or wi th bowel necrosis. Look at the GU system (enterovesical fistula, 697 pyelonephritis), in the peritoneum ( double wall sign: Visualization of the outer wall of b owel loops caused by gas outside the bowel loop and normal intraluminal gas), triangle sign (represents a triangular pocket of air between two loops of bowel and the abdominal wall), football sign (a large collection of air that seems to outline the entire abdominal cavity ; air surrounding the falciform ligament, may have the appearance of the laces of the footb all), the colonic wall (pneumatosis coli, infective colitis), or a subphrenic abscess. Some times, free air in the abdomen can be normal post surgery or after certain procedures, such as ERCP, laparoscopy, etc. Free air can be detected on the left lateral decubitus radiogra ph because free air collects around the inferior edge of the liver, which forms the least d ependent part of the abdomen in that position. Look for calcification in the abdomen or pel vis Calcification in an abdominal movie maybe normal or may indicate pathology. Normal stru ctures that calcify include costal cartilage, mesenteric lymph nodes, pelvic veins (phlebol ith), and prostate gland. Pathological calcifica- tion can be approached systematically: \212 Vascular: Arteries (atherosclerosis, the eggshell calcification of an aneurysm) \212 GU: Renal calcification or ureteric stones (usually jagged; look along the line of th e ureter), bladder (stone, tumor, tuberculosis [TB], schistoso- miasis), uterine fibroids, dermoid cyst of the ovary (which may contain teeth) \212 GI : Adrenal (TB), pancreatic (chronic pancreatitis), liver, or spleen granu- lomas; gallstones (only 10% radiopaque), appendicoliths, incision or injec- tion sites; may all ca lcify. Bones of spine Look for metastases, which may be osteolytic (low density, black), osteoblas tic (higher density, white), or mixed. Malignancies that have predominant osteoblastic lesi ons include prostate, carcinoid, gastrinoma, small cell lung cancer, and Hodgkin \200\231s lymphoma. Osteolytic lesions can indicate renal cell and thyroid cancer, melanoma, multiple myeloma, non-small cell lung cancer and non-Hodgkin \200\231s lymphoma. Mixed lesions are most commonly secondary to breast cancer, but osteosarcoma, Ewing \200\231s sarcoma, as wel l as others, can also present as mixed lesions. Paget \200\231s disease maybe identified as a \200\234picture frame vertebral body. \200\235 Other conditions visible on spine includ e osteomalacia, collapse, osteoarthritis, hyperparathyroidism ( \200\234rugger jersey \200 \235 spine), or ankylosing spondylitis ( \200\234bamboo spine \200\235). Bones of pelvis Lo ok at the hip joint for any signs of fracture. Advanced osteonecrosis may be seen as loss o f spherical margin and collapse of the femoral head. Osteoarthritis may present as joint-sp ace narrowing with degenerative changes such as osteophytes. Soft tissues These can be eval uated by the fatty rim (preperitoneal fat lines) surrounding the organs. The loss of the no rmal fat planes may suggest the presence of pathology; for example, the psoas lines may be obliterated in retroperitoneal processes such as inflammation, hemorrhage, or peritonitis. Note kidney size and shape (normally 2 \200\2233 vertebral bodies \200\231 length and paral lel to the psoas line). Iatrogenic or incidental objects You will be surprised to find that the abdominal cavity can be a hiding place for a wide array of objects, left both intentio nally (e.g., inferior vena cava [IVC] filters, stents, surgical pins, screws, clips, etc.) and unintentionally (surgical sponges, clamps, etc.) For intentionally placed objects, make sure that they are located where they 698 should be (e.g., an IVC filter should not be located in chest). Presence of such objects ca n reveal a lot about a patient \200\231s history. Acute conditions not to miss \212Pneumop eritoneum: T his usually means perforation unless recent y g o o d a R l i instrumentation. Portal venous air (peripheral liver) and/or pneumatosis coli (air in the w all of bowel, usually colon): This often means necrotic bowel. \212 Don \200\231t \212 \212 Forget to look for lower lung pathology on the upright abdominal movie. Forget to check the \200\234corners \200\235 of any plain films as these can hide subtle and often missed pat hology. Figure 18.9 Anterio-posterior abdominal radiograph of a female demonstrating structures in the abdomen and pelvis 699 Figure 18.10 Supine abdominal radiograph Figure 18.11 3D coronal CT of the abdomen \200\224I STOMACH LIVER SPLEEN VERTEBRAL BODY LEFT ADRENAL GLAND RIGHT ADRENAL GLAND LEFT KIDNEY RIGHT KIDNEY LEFT PSOAS MUSCLE RIGHT PSOAS MUSCLE LEFT ILIAC CREST RIGHT ILIAC CREST LEFT FEMUR (GREATER TROCHATER) RIGHT FEMUR (GREATER TROCHATER) 700 3D CORONAL ABDOMEN CT y g o o d a R Figure 18.12 3D coronal CT of the abdomen \200\224II l i Figure 18.13 3D coronal CT of the abdomen \200\224III 701 Modalities Ultrasonography \200\234Ultrasound \200\235 refers to the sound waves with freq uency greater than the upper limit of human hearing. In ultrasonography, a sound wave that passes through a tissue interface is reflected back to the US transducer, and these reflect ed waves are used to construct an image of an organ or structure. Keys to US image formatio n are attenuation (i.e., when the signal strength is progressively reduced as waves penetra te deeper into the tissues due to its conversion to heat) and reflection (i.e., when the US wave hits a tissue boundary and is partially or completely reflected back to the transduce r). These key properties help to create the US image. Beyond seeing organs and tissues, oth er key features can help in making a diagnosis using US. Two of these are acoustic shadowin g (Figure 18.14A), which is a band of markedly reduced echogenicity (black) behind a strong ly reflecting structure. Fibrous tissue, calcified structures, such as bones and stones, le ad to this kind of acoustic shadowing. US is also unable to penetrate air-containing struc- tures, which also leads to shadowing. In contrast, acoustic enhancement (Figure 18.14B) is a bright band (white) behind an anechoic structure that does not reflect any sound waves ( e.g., bladder or a cyst). This phenomenon can help us differentiate an anechoic simple cyst showing distal acoustic enhancement from a hypoechoic solid breast lesion. US is an extrem ely powerful tool that can be tailored to any number of diag- nostic and interventional use s. It does not use ionizing radiation to produce images; hence, it is safe even in early st ages of pregnancy. Furthermore, US is portable and can be used at the bedside of critically ill patients. There are essentially no contraindications to US. However, it may be of limi ted value in obese people because of poor tissue penetration. Bone, gas, and calcifica- tio ns reflect US waves and can interfere with the ability to image subjacent structures. Of al l the modalities, US is the most operator-dependent, and reliability of results may vary. U S is the imaging modality of choice in obstetrics and is often the first line of investigat ion for abdominal pathologies and pelvic lesions. US is often used to guide needle aspirati on, biopsies, sclerotherapies, and thermal ablations. Doppler US allows for the assessment of the direction and velocities of blood flow within blood vessels. It is based on the prin cipal of Doppler shift , which is defined as the change in the frequency of a sound wave wh en the source and observer are in relative motion. When incident sound waves encounter the red blood cells in the circulating blood, they undergo a frequency shift proportional to th e speed of red cell movement. This change in frequency helps determine the blood flow veloc ity. There are various modalities of Doppler US, such as color flow imaging, in which the v elocities are displayed on a color scale. The transducer detects the flowing blood and arbi trarily assigns a red and blue color depending on whether the flow is toward or away from t he transducer. It is most commonly used in assessment of regurgitant flow and intracardiac shunts. Duplex US combines the Doppler flow information and conventional imaging informatio n, sometimes called B-mode. It gives both structural and functional information and is used extensively. 702 y g o o d a R Figure 18.14 l i (A) US of gallbladder demonstrates a single echogenic focus (gallstone) with acoustic shado wing ( arrowheads ) (B) US of the kidney demonstrates a cyst with acoustic enhancement ( arrowheads ) 703 Computed tomography The advent of CT (also called CAT) has revolutionized medical imaging. The principal component of CT scanning is an x-ray tube that rotates around the patient and generates an x-ray beam. The opposed array of detectors record the radiation that passes t hrough the tissues and is used to construct the images. Since its introduction, there have been several gen- erations of scanners. In first-generation conventional \200\234step and shoot \200\235 CT, the patient is moved into a desired position and x-ray tube rotates arou nd the patient to acquire individual slices. Conventional CT scans are still used for some applications such as high-resolution CT (HRCT) of lungs. Conventional CTs have almost entir ely been replaced by spiral or helical CT scanners. In helical CT, the x-ray beam rotates c ontinuously around the patient in a spiral manner so that a volume of tissue is scanned rat her than individual slices. The large amount of raw three-dimensional (3-D) data are then r eformatted into axial, coronal, sagittal, or oblique planes. These datasets can also be use d for 3-D and maximum intensity projections (MIP) postprocessing. All modern CT scanners ar e designed as multidetector (MDCT) or multislice scanners, in which multiple active rows of detectors acquire information as tubes rotate around patients. Multidetector imaging allow s for faster scanning, thinner slices, higher imaging resolution, and bet- ter 3-D reconstr uction from the entire volume of a scanned body part. CT uses point attenuation for density measurements, which are expressed as Hounsfield units (HU). It can be used to differentiat e between tissue types (air, fat, muscle, blood, fluid, bone). In Hounsfield units, bone ha s attenuation of +1,000, water is 0, air is \200\2231,000, fat is <0, and the remaining ti ssues fall in between, depending on tissue composition (see Figure 18.15). Contrast use in CT CT can be performed with or without IV or oral contrast. Tissue structures can be viewed in arterial, venous, and delayed phases after the injection of IV contrast. IV contrast is used to differentiate enhancing vascular structures and lesions (e.g., infections or neopl asia) from nonenhancing structures (e.g., cysts, free fluid). The contrast used for CT is s imilar to that used in angiographic procedures; therefore, standard precautions and consent are recommended. Oral contrast (or water for 3-D reconstructions) should be given prior to abdominal or pelvic scanning to help delineate the bowel (the exception are protocols for renal stone studies and CT angiography). To fill as much of the bowel as possible, the gene ral rule is to give oral contrast early and often. CT angiography High-resolution, contrast -enhanced MDCT is able to produce 3-D reconstructed images of nearly every arterial territo ry. CT has already begun to replace much diagnostic angiography in both emergent and noneme rgent settings. All the advances in CT have created some difficulties, including increased cost and increased computer workload and data storage. Compared to conventional radiography (plain x-rays), radiation doses from CT scan- ning are considerably higher. The ever-incre asing use of CT scanning has Figure 18.15 Figure demonstrates the Hounsfield unit scale on a linear grayscale diagram. 704 increased the overall population radiation exposure. The long-term effects of this increase d exposure are still being examined. y g o o d a R l Magnetic resonance imaging MRI is a powerful imaging modality that does not use ionizing ra diation. Instead, it uses a powerful magnetic field to image protons within the soft tis- s ues of the body. When a patient is placed in the magnetic field (0.2 \200\2237 Tesla), the body \200\231s nuclei align themselves in the direction of the external magnetic field. A r adiofrequency (RF) pulse is then generated that excites the protons in the soft tissue, cau sing them to attain a higher energy level. When the RF pulse stops, the protons relax back into their equilibrium position, releasing the energy. This radiofrequency signal or echo e mitted by the protons while relaxing is detected and used to create an image. The time elap sed between a RF pulse and to the peak of the echo is called the echo time (TE). The elapse d time between successive RF pulses is called the repetition time (TR). The two types of re laxation times \200\224T1 or longitudinal relaxation and T2 or transverse relaxation \200 \224differ for various tissues and are the basis for varying contrasts seen on MRIs. The em ission and reception of the RF waves are carried out by the coils that are placed over the body structure that is being imaged. MR images are described in terms of signal intensity. Tissues that are whiter are hyperintense and those with low signals or black are hypointens e . A pulse sequence with a short TR and a short TE accentuates the T1 differences between tissues (T1-weighted). A T1-weighted image provides excellent anatomic detail and produces increased signal intensity over baseline when a contrast agent is given (showing more vascu larity). A sequence with a long TR and long TE are T2-weighted , which is good for visualiz ing pathology. A mixture of both reflects proton density weighting, which reflects the wate r content of the structure. Advantages and disadvantages MRI takes advantage of multiple pa rameters (T1, T2, proton density) and has many imaging protocols or sequences (spin echo, i nversion recovery, gradient echo), which provides an excellent soft-tissue contrast. Variou s sequences are available and can be tailored to ask very specific clinical questions. The exposure to magnetic fields of the strength used in clinical MRI is considered to be harmle ss. Because MR \200\231s safety in pregnant patients has yet to be determined, it should be used cautiously, especially in the first trimester. Although MRI is an extremely powerful and evolving technique, it is limited by a lack of portability and its relatively slow spee d of imaging. MRI machines can be noisy and claustrophobic, although newer machines with op en or wide bores are available. Contraindications to MRI The specific contraindications to MRI will vary from institution to institution. Absolute contraindications: \212 i Non-MRI compliant aneurysm clips or other surgical clips placed in the last 6 wks Neurostim ulator device Pacemaker or defibrillator (this may vary) Cochlear implant Metallic ocular f oreign body (suspect this in patients with a welding his- tory \200\224evaluate with orbit x-ray) Other metallic implanted devices such as insulin pumps Metallic shrapnel Obesity ove r the maximum table allowance (will vary, usually <350 lb). \212 \212 \212 \212 705 \212 \212 \212 Relative contraindications: \212 \212 Claustrophobia Unstable patients (only MR-compatible life support equipment can be used in the scanning area) Dependence on infusion pumps \212 MRI development is currently one of the most active areas of research within radiology, and its indications and uses continue to expand. Nuclear medicine (radionuclide scanning) Radioisotope scanning involves administration of a small amount of radioac- tive tracer that localizes to a specific organ or cellular recept or. The organ is subsequently imaged to study the disease process. The radioactive tracer c onsists of a ligand (nonradioactive) that is combined with a radionuclide. The commonly use d radionuclides include technetium-99m ( 99 Tcm), indium-111 ( 111 In), iodine-123 ( 123 I) , and gallium-67 ( 67 Ga). All these isotopes emit -rays, which are captured by a -ca mera and subsequently transformed into ana- log and/or digital information. Imaging perform ed by a rotating -camera is termed single photon emission computed tomography or SPECT. Anatomic modalities such as CT or MRI are considered anatomic imaging modalities that, in g eneral, provide exquisite anatomical detail. Radioisotope studies fill this gap by providin g physiological or functional information. Using CT and MRI in conjunction with radioisotop e imaging provides com- plementary information that is often of greater clinical relevance than either type of test individually. Because of this, most SPECT cameras have been combin ed with CT scanners to provide fusion images that display both function and anatomy simulta neously. Bone scan 99 Tcm-MDP (methylene diphosphonate) is the commonly used radiotracer; i t is retained in areas with increased osteoblastic activity. Common indications: Assessment of metastatic disease from a known primary, assessment of extent of Paget \200\231s diseas e, diagnosis of occult or stress fractures, Identification of osteoid osteoma, characteriza tion of metabolic bone disease. Note that bone scanning it typically negative in cases of m ultiple myeloma. Cardiac scan Myocardial perfusion imaging is used mainly to evaluate atypi cal chest pain, assess the extent of ischemic heart disease, and assess viable myocardium p rior to revascularization. Dipyridamole, adenosine, or dobutamine are used as stressors if arthritis or abnormal ECG (left bundle branch block, or on digoxin with changes to the ECG) preclude exercise, or in those in whom the exercise test is inconclusive. SPECT imaging is performed after injection of 201 TI (thallium-201), or 99 Tcm-MIBI (methoxy isobutyl isoni trile). Radionuclide ventriculography/ Multigated acquisition scan (MUGA) ECG gated imaging is performed after radiolabeling red blood cells with radiotracer and measuring the radioa ctivity. It provides objective and reproducible data on ventricular ejection fractions, reg ional wall motion abnormalities, and information on ventricular aneurysms ) 99 Tcm -MAA (macroaggregates of albumin) is commonly Lung scan (V used to assess perfusion (Q) and 133 Xe is used to assess ventilation (V) /Q . scanning relies on the physiological principle of reduction in segmental V perfusion with maintained normal ventilation in pulmonary embo lism. Matched reduction in perfusion and ventilation can be seen in parenchymal lung disord ers. Scans should be interpreted in conjunction with a current CXR, and reports are in the form of probabilities: High probability (>80% likelihood of pulmonary embolism [PE]), low p robability (<20% likelihood of PE), intermediate probability ( 20 \200\22380% likelihood of PE), or very low probability (<5% likelihood of PE). Note that CT pulmonary angiography is both sensitive and specific technique for the assessment of acute pulmonary embolism and i s more commonly used. Hepatobiliary scan (HIDA) 99 Tcm-mebrofenin is taken up by the hepato cytes, secreted into the bile, then disperses with the bile into the bile ducts, gallbladde r, and intestine. This test is used mainly to diagnose acute cholecystitis, for biliary lea ks, and for quantitative information on . /Q 706 gallbladder kinetics (i.e., assessment of chronic cholecystitis) when used in conjunction w ith an IV infusion of cholecystokinin (CCK). Thyroid scan 99 Tcm-pertechnetate or 123 I-Iod ide may be used for assessment of solitary thyroid nodule, assessment of congenital hypothy roidism and localization of ectopic thyroid tissue, diagnosis of autonomous functioning thy roid adenomas, and for the differentiation of Graves \200\231 from multinodular goiter. Rad ioactive iodine is also used to treat hyperthyroid patients with Graves \200\231 disease, t oxic adenoma, or multinodular goiter. Therapeutic sodium 131 I is given orally and is rapid ly taken up by the thyroid gland, where it emits high-energy rays that locally destroys the gland. It is also used as an adjuvant therapy after thyroidectomy (to destroy residual tumor or metastatic disease) in patients with well-differentiated thyroid cancer. Parathyro id scan 99 Tcm -MIBI is used to localize a parathyroid adenoma once a biochemical diagnosis of primary hyperparathyroidism is made. Adrenal scan Pheochromocytomas and other tumors of neuroectodermal origin can be localized using 123 I-MIBG (metaiodobenzylguanidine). Renal scan Radioisotope scanning and computer-assisted analysis of the images of the kidneys (ren ography) provide vital functional information. Standard renography with 99 Tcm-DTPA or 99 T cm-MAG3 (Mertiatide) provides data on split renal function and uptake and excretory pattern s of each kidney. Diuretic renography performed by injecting furosemide at the end of a sta ndard renogram helps to distinguish physiological dilatation of the renal pelvis from obstr uctive nephropathy. Captopril renography performed by repeating the standard renogram after administration of PO captopril (25 \200\22350 mg) helps to confirm renovascular disorder ( unilateral better than bilateral). 99 Tcm -DMSA (dimethyl succinic acid), a renal cortical imaging agent, is useful in assessing chronic pyelonephritis, renal scarring, cortical cyst s, and tumors. Inflammation/infection scan (tagged white blood cell scan) 111 In-oxine labe led leukocytes are typically used to evaluate osteomyelitis (in conjunction with either a p hase bone scan using 99 Tcm-MDP or a marrow scan using 99 Tcm-sulfur colloid). However, leu kocytes radiolabeled with 111 In-oxine or 99 Tcm-HMPAO can also be used for the evaluation of fever of unknown origin, localization of abscesses (5 \200\22310 d old), evaluation of c hronic inflammation, and assessment of mediastinal lymphomas, prosthetic infection (especia lly hip), and in patients with inflammatory bowel disease. Radiolabeled red blood cell scan s (tagged red blood cell scans) are useful in the localization of an unexplained active GI bleeding that is occurring at a rate of 0.1 \200\2230.5 mL/min. It is more sensitive than a ngiography in localizing a GI bleed. It can be a key to localizing bleeding \200\224and can help direct therapeutic angiography, endoscopy, or surgery. y g o o d a R l i Positron emission tomography Molecules labeled with positron-emitting radionuclides are inj ected and postannihilation event photon paths are analyzed by crystal detectors to produce 3-D images. 18F-FDG (fluorodeoxyglucose), an analog of glucose, is the agent most commonly used. Currently, positron emission tomography (PET) is used primarily for the evaluation of recurrent or met- astatic tumors, for assessing tumor response to chemo-/radiotherapy, and for assessing an unknown lesion \200\231s metabolic activity or neoplastic potential (e.g. , a newly appearing lung nodule). Studies are frequently performed in a hybrid PET/CT scann er for accurate physiologic/anatomic correlation. In the brain, 18F-FDG PET can also be use d to determine the site of epi- leptogenic foci when there is no anatomic lesion seen on MR I or CT. PET can also be used to diagnose dementia (even before symptoms start) and 707 to distinguish Alzheimer \200\231s from other dementias (e.g., symmetrical hypo- metabolism in parietal and temporal lobes not the frontal lobes, which are affected in Pick \200\231s dementia). PET development is currently one of the most active areas of research within ra diology, and its indications and uses continue to expand. Organ systems Gastrointestinal radiology Abdominal US Abdominal US has found its major use in suspected hepa- tobiliary disease. It is the choice of imaging for patients who present with acute right upper quadrant pain. It can detect >95% of gallstones and can characterize the gall bladder wall along with localized tenderness (Murphy \200\231s sign) in the case of acute cholecystitis. Note that right upper quadrant and abdominal US require that the pa tient remain NPO 4 \200\2238 h prior to the examination in order to allow the gallbladder t o expand and to minimize abdominal gas. Other clinical indications for hepatobiliary US inc lude evalu- ating obstructive jaundice (dilation in obstructive jaundice; although the caus e, i.e., stone vs. tumor, is less reliably seen), screening for hepatocellular carcinoma in cirrhotic patients (live size, texture [e.g., fatty or shrunken cir- rhotic liver and soli d or cystic masses within it]), evaluating patients before and after liver transplantation (patency and size of the portal vein, spon- taneous or surgically created portosystemic shu nts), and evaluating shunt patency with TIPS. Doppler of the portal and splanchnic veins is used to rule out thrombosis and assess direction of flow in the portal veins in cir- rhosi s, screen for vascular complications after orthotopic liver transplant, and check the paten cy of the stents. Abdominal CT CT is the modality of choice for the evaluation of nearly al l abdominal pathology in both solid and luminal organs. With few exceptions (such as renal stone protocols), abdominal CT should be performed with both oral and IV contrast. This pro vides the most contrast between abdominal structures, allows for evaluation of post-traumat ic injuries (e.g., active hemorrhage, parenchymal organ and hollow organ injuries), infecti ons/abscess, masses, adenopathy, and vascular disease throughout the abdomen. CT can be use d to assess for bowel disease such as obstruction, cancer, perforation/pneumoperitoneum, an d inflammatory/ infectious disease. Figure 18.16 shows CT scans of some acute abdomen patho logies seen commonly in the emergency room. CTA of the abdomen is used for the evaluation o f the aorta, renal arteries, celiac trunk, and superior and inferior mesenteric arteries, a nd can be used to assess for aneurysm, dissection, stenosis, and atherosclerotic disease. A bdominal MRI MRI of the abdomen can be used for the evaluation of masses or infection withi n any of the solid abdominal and, to a lesser degree, luminal organs. MRI is most often use d for the evaluation of lesions that are poorly seen on CT (e.g., metastatic disease to the liver), for the characterization of lesions that are indeterminate with other modalities ( e.g. \200\234complex \200\235 renal cysts or \200\234indeterminate \200\235 liver or adre nal lesions), and for patients who are unable to be evaluated with CT (e.g., allergy to iod inated contrast). MRI is often used to assess the biliary tree (MRCP) to evaluate pancreati c ductal anomalies (e.g., divisum) or for the diagnosis of biliary obstruction. MRA of the abdomen is used for the evaluation of the renal arteries, celiac trunk, and superior and in ferior mesenteric arteries, and of stenosis and atherosclerotic disease. MRA is also used t o assess for arterial dissection or aneurysm. GI fluoroscopy In all of these studies, radio paque contrast (either barium or water-soluble contrast, depending on perforation and aspir ation risks) 708 y g o o d a R Figure 18.16 l i (A) Axial CT with IV and oral contrast demonstrating thickened and dilated appendix ( circl e ) with mild periappendiceal fat stranding. (B) Coronal CT scan with oral and IV contrast demonstrates multiple diverticula in sigmoid colon. There is bowel wall thickening and moderate fat stranding consistent with diverticul itis ( circle ). Images courtesy of Dr. Karen Horton, MD, Johns Hopkins Hospital.