Which imaging modalities often requires the use of intravenous iodinated contrast media for studies of the abdomen?

Contrast media

Intravenous contrast media can be used to provide direct visualization of perfused tissues. Perfused tissue shows up as hyperdense areas in contrast to non-perfused tissues, which remain the same density. This technique is valuable if the clinician wishes to know if an imaged area is perfused or not, for example to differentiate a fluid-filled cystic mass from a soft-tissue neoplasm with vascularization extending throughout the mass (Fig. 19.2). After performing the initial (native) CT scan, the same sequence is repeated following administration of contrast medium. Essentially the same contrast media can be used in CT imaging as in conventional radiography and the risks and precautions are the same. For intravenous administration of contrast media for CT studies, tri-iodinated, water-soluble compounds are used. They are administered as an intravenous bolus at a dose of 370 mg iodine/kg body weight. They are distributed by the vascular system and later excreted by the urinary system. Perivascular leakage should be avoided, as most ionic preparations are locally irritant. Renal vascular effects and renal damage induced by contrast media are well studied in humans and dogs (Porter 1993). Maintaining the patient's hydration status can minimize these adverse renal effects. Although extremely rare, severe side effects such as cardiac arrest and anaphylaxis have been described following administration of tri-iodinated, water-soluble compounds. Contrast agents with low osmolarity may have fewer adverse effects. Additionally, in equine imaging, cost is the main limitation in the use of intravenous contrast media.

CT Tips

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CT covers the primary tumor within seconds with multidetector CT scanners.

Iodinated intravenous contrast medium can interferewith iodine uptake by thyroid cells and may delay postoperative adjuvant radioactive iodine therapy of differentiated thyroid cancer. A CT scan without contrast agent provides adequate information in this scenario, but subtle findings (e.g., small recurrent nodule in thyroid bed or small lymph node) may be missed.

Contrast-enhanced CT can be used for assessment of local extent of disease if treatment with radioactive iodine is not anticipated. Aggressive differentiated carcinoma and poorly differentiated or anaplastic thyroid cancers do not concentrate radioactive iodine, and CT with contrast is an acceptable modality in these cancers.

CT is useful for early detection of extracapsular nodal spread that is generally not seen on MRI. Focal metastasis/necrosis within normal-sized lymph nodes can be detected on contrast-enhanced CT earlier than on MRI.

The Head

MRI is the gold standard modality to image the brain in humans and animals. The use of intravenous contrast media is indicated when imaging the equine brain. The contrast material used with MRI is a gadolinium derivate. Doses of 0.02 mmol/kg in adult horses and 0.1 mmol/kg in foals of gadolinium-diethylenetriamine penta-acetic acid (gadolinium-DTPA) have been reported for imaging the brain.10,50 However, an optimal dose of intravenous gadolinium contrast in horses has not been established. T1 images are used for evaluating contrast enhancement. Many congenital, toxic, inflammatory or infectious (including equine protozoal encephalopathy), and neoplastic processes affecting the brain can be identified with MRI.10,51-53

Renal Adverse Reactions and Prevention

The term contrast media nephrotoxicity (CMN) refers to an increase in serum creatinine concentration by more than 25% or 0.5 mg/dL within 3 days of receiving IV contrast media in the absence of another cause.59 The underlying mechanism of injury is unclear, although it is thought that contrast can reduce renal perfusion and is toxic to tubular cells. CMN occurs more frequently but not exclusively in children with preexisting renal damage; additional risk factors include dehydration, cardiac failure, and the subclinical renal insufficiency seen in cyanotic children. It has been suggested that infants and children who receive more than 5 mL/kg of nonionic contrast agent are at increased risk for CMN.60 Contrast for angiography does not increase the risk of postbypass acute kidney injury61 and most CMN appears to resolve without intervention. However, a recent study of 233 heterogeneous children receiving contrast for CT scanning found an association between CMN and unfavorable outcome, suggesting the process may not be as benign as previously hoped.62 Many interventions have been given prophylactically to prevent CMN; preliminary studies with N-acetylcysteine (NAC) have been promising but no interventions have been more effective than normal saline hydration.63 Currently, the only modifiable risk factors are hydration status and dose of contrast used. When possible, potentially nephrotoxic drugs should be stopped at least 24 hours before the procedure. Gadolinium-based contrast materials are considered nonnephrotoxic in the normal MRI dose of up to 0.3 mmol/kg. However, there is some evidence that the increased doses required for cardiac angiography may confer adverse renal effects.64 Most radiologic contrast media have significant osmotic diuretic effects. During procedures requiring large volumes and/or repeated doses of contrast media, a significant diuresis may occur, causing concealed fluid losses and potential hypovolemia. Additional IV fluid may be required to avoid dehydration. Occasionally, bladder distention and retention can also occur, which should be considered in a child with unexplained postoperative distress.

Computed Tomography

Chest CT is currently the best way to visualize the pleural space.133 Chest CT has its greatest utility in distinguishing parenchymal and pleural abnormalities.135 With current protocols involving the rapid injection of intravenous contrast medium, the unaerated perfused lung parenchyma will enhance, whereas the pleural fluid will not.136 The use of CT to discriminate transudates from exudates by their attenuation (Hounsfield units) has not been found to be clinically useful due to a great deal of overlap.137

Chest CT is quite useful in distinguishing a parenchymal lung abscess located near the chest wall from an empyema with an air-fluid level. The most distinctive features are the margins of the abnormality. With empyema, the cavity walls are of uniform thickness both internally and externally (see eFigs. 80-2, 80-5) and the adjacent lung is usually compressed. The angle of contact with the chest wall may be obtuse (see eFig. 33-7B). In addition, most empyemas have a lenticular shape and demonstrate the “split pleura” sign (Fig. 79-5A see also eFigs. 80-2, 80-5).138 With lung abscess, the walls of the cavity are not of uniform thickness and the adjacent lung is not compressed (see eFigs. 33-4, 33-13B and C, 33-21D and EeFigs. 33-4eFigs. 33-13eFigs. 33-21). The angle of contact with the chest wall may be acute (Fig. 79-5B).

In diffuse pleural disease, chest CT is useful in distinguishing malignant from benign causes. Features associated with malignancy include circumferential pleural thickening, pleural nodules (see eFig. 53-5), parietal thickening greater than 1 cm, and mediastinal pleural involvement (see eFig. 53-4 and Video 79-1

CT pulmonary angiography (CTPA) has become a first-line imaging test for the evaluation of PE140 (see Chapters 18 and Chapter 57). The evaluation of a patient with a pleural effusion for PE can begin with a Doppler ultrasound of the lower extremities. If the ultrasound identifies thrombus, the patient can then be treated for thromboembolic disease. If it is negative, the patient may still have a PE. In the past the standard approach was to proceed to lung ventilation-perfusion scanning. However, this has largely been replaced by CTPA. CTPA is highly sensitive and specific for pulmonary emboli in the proximal, segmental pulmonary arteries.140 In contrast to ventilation-perfusion lung scanning, CTPA can establish alternate diagnoses as well. Where CTPA is not available, lung scanning can be used, perhaps after efforts to improve accuracy by withdrawing as much pleural liquid as possible. A diagnostic scan (normal or high probability) can then be used either to exclude the diagnosis or initiate anticoagulation. If nondiagnostic (e.g., of low or intermediate probability), the scan should be followed by pulmonary angiography.

Technique

Imaging protocols always include a precontrast image series to identify presence of mineralization in the renal parenchyma or collecting system, because mineralization cannot be differentiated from the hyperattenuating contrast medium. In most patients, intravenous contrast medium has to be administered (400 to 800 mg I/kg body weight) to outline renal parenchymal lesions and filling defects in the collecting system. Contraindications to using iodinated contrast media are the same as described in the Excretory Urography section. The renal parenchyma is enhanced immediately after contrast-medium administration. Optimal ureteral opacification occurs 3 minutes post injection.37 However, image series may have to be repeated several times to visualize the entire course of both ureters because of ureteral peristalsis. Ureteral opacification persists for about 1 hour.37 Positioning the patient in sternal recumbency with the bony pelvis elevated is helpful in outlining the ureterovesical junctions, because the contrast medium will accumulate in the bladder apex away from the neck, facilitating the distinction between the contrast-medium–filled ureters and the bladder neck (Fig. 41.7). Filling the bladder with carbon dioxide increases the contrast between the bladder and ureters further; however, the balloon of the Foley catheter may distort anatomy and impair visibility of the ureteral openings. Dual-phase renal angiography can be performed to assess the arterial and venous blood supply of the kidneys separately. This is used mainly for presurgical assessment of feline kidney donors.40,41

Pretreatment Evaluation

The presence of a Pancoast syndrome is not always associated with NSCLC. Other diseases, including lymphoma, tuberculosis, and primary chest wall tumors, may be associated with an apical mass and chest wall involvement. Transthoracic needle biopsy should be performed to establish a diagnosis before treatment.

Pancoast tumors are, by definition, classified as stage IIB or higher and require an extent-of-disease evaluation before treatment is initiated, including contrast-enhanced CT of the chest and upper abdomen, whole-body PET, and MRI of the brain (Fig. 30.4A–B). Because Pancoast tumors with mediastinal node metastases (N2 or N3 disease) have a poor prognosis, mediastinal staging via either endobronchial ultrasound or mediastinoscopy should be considered.

MRI with use of intravenous contrast medium is the modality of choice for evaluating structures of the thoracic inlet, including the brachial plexus, subclavian vessels, spine, and neural foramina and is crucial for preoperative planning.36 The extent of nerve root involvement must be assessed. Resection of the T1 nerve root usually does not cause motor function deficit, but resection of the C8 nerve root or lower trunk of the brachial plexus leads to loss of hand and arm function. A careful neurologic examination is informative and supplements MRI findings.37 Pain extending along the ulnar aspect of the forearm and hand is consistent with T1 involvement. Weakness of the intrinsic muscles of the hand indicates involvement of the C8 nerve root or lower trunk of the brachial plexus. Resection of a Pancoast tumor should be planned jointly with a spine neurosurgeon to allow optimal patient selection and the best chance of complete resection.

Patients must be evaluated to determine whether they can tolerate combined-modality therapy. Performance status, renal function, and neurologic function must be adequate in order for the patient to receive platinum-based chemotherapy. Pulmonary function tests and, when necessary, cardiac stress tests are done to evaluate the ability of the patient to tolerate pulmonary resection.

Technique and Protocols

CT imaging of the pancreas relies on the differential IV contrast enhancement between tumor tissue and normal pancreatic parenchyma. The use of IV contrast agent is mandatory for accurate diagnosis, and timing of the contrast injection and accurate delivery of the appropriate volume requires use of a dedicated CT power injector. As with liver imaging, several methods are available to determine the time of maximal arterial enhancement for each patient. These include fixed timing (subject to suboptimal imaging due to variation in cardiac function and hydration state), timing bolus (Kalra et al, 2004), and commercially available solutions in which scanning may be triggered automatically when a vascular structure reaches a predefined attenuation.

Initial scan protocols for dedicated pancreas evaluation/CT angiography recommended four phases, but more modern practice uses three acquisitions, omitting early arterial imaging because pancreatic parenchyma and peripancreatic arterial anatomy are both well evaluated in the late arterial phase (Fletcher et al, 2003). The precontrast scan permits evaluation of pancreatic calcifications and allows localization of the gland and pertinent arteries in the z-axis for subsequent acquisition of contrast-enhanced phases. IV contrast medium is injected at a high flow rate of 4 to 6 mL/seconds. The late arterial or pancreatic parenchymal phase, acquired roughly 30 to 40 seconds after initiating contrast injection, is designed to maximize differences in contrast enhancement between pancreatic neoplasms and adjacent normal pancreatic tissue and is also useful in evaluating hypervascular liver metastases seen in patients with pancreatic endocrine neoplasms. Last, the routine portal venous phase acquired at approximately 70 to 90 seconds from the start of IV contrast injection provides the best evaluation of hepatic metastases from pancreatic ductal adenocarcinoma (Bashir & Gupta, 2012; Cantisani et al, 2003; Takeshita et al, 2002) and in some cases offers the best contrast to identify the primary pancreatic lesions themselves.

The dedicated pancreas protocol uses 750 to 1000 mL of oral water as a negative contrast agent administered before the examination, to aid distinction of enhanced vessels from the gastrointestinal tract (Lawler & Fishman, 2002; Soriano et al, 2004; Takeshita et al, 2002) and to facilitate identification of tumor invasion into adjacent bowel. Avoiding positive oral contrast also facilitates analysis on the 3D workstation. Images are typically reviewed at 2.5-mm collimation in all planes.

Routine evaluation for the follow-up of pancreatic cancer in a postoperative patient or for evaluation of response to medical therapy uses 500 to 1000 mL of oral water-soluble iodinated contrast solution or equivalent 2% barium sulfate suspension administered 45 minutes before scanning. This examination type requires an injection rate of only 2.5 mL/sec, so a small 20-gauge catheter may be used. Only the routine portal venous phase is acquired. Five-millimeter axial slices are typically reviewed, although thinner images may always be requested by the radiologist provided the raw image data are still extant.

Investigations

Dubin-Johnson Syndrome

In 1954, Dubin and Johnson373 and Sprinz and Nelson374 described a syndrome with chronic nonhemolytic jaundice characterized by accumulation of conjugated bilirubin in serum and grossly pigmented, but otherwise histologically normal, livers.