Imaging
Table 1: Choosing between the Different Contrast-based Imaging Options
GBCA-enhanced MRI Preferred over Radiocontrast-enhanced CT Scan CNS/spine
Acoustic neuroma AIDS-related brain lesion Aneurysm Brain tumor (primary or metastases) CNS infection, abscess, meningitis Cavernous sinus Cranial nerve lesion Meningeal disease Multiple sclerosis Pituitary lesion Posterior fossa and brainstem lesions Seizure evaluation Spinal cord tumor Discitis/vertebral osteomyelitis Metastasis to epidural/intraspinal/or vertebral bone regions
Body
Breast cancer staging Prostate cancer staging Liver mass
Musculoskeletal Soft tissue tumor Bone tumor Joint infection Osteomyelitis
Radiocontrast-enhanced CT Scan Preferred Over GBCA-enhanced MRI Head and neck
Proptosis Squamous cell cancer staging Vocal cord paralysis
Body
Adrenal gland lesion Aortic pathology Gastrointestinal tract disease Pancreatic mass Pulmonary embolus Pulmonary mass-hilar disease Renal mass Splenic lesion
CNS = central nervous system; CT = computed tomography; GBCA= gadolinium-based contrast agent; MRI = magnetic resonance imaging.
In this article, we review some of the key concepts regarding diagnostic imaging in patients with underlying kidney disease and present our suggestions for a rational approach to choosing a modality.
Contrast-enhanced Imaging: Does Benefit Outweigh Risk?
When choosing an appropriate imaging test for patients with kidney disease, if gaining information from imaging is critical, one can proceed with either contrast-enhanced MRI or CT scan despite the associated risks. To make this a knowledgeable decision, a general understanding of the pathologies that necessitate contrast-enhanced imaging is required. This will allow the clinician to explain to the patient the potential benefits that justify the associated risks. An extensive review of every organ system and associated pathology is beyond the scope of this article. However, the generalizations presented in Table 1 provide a reasonable guide for choosing between the different contrast-based imaging options.
Under normal conditions, GBCAs do not cross the blood–brain barrier 132
(BBB) due to the hydrophilic character of the associated chelating agent. In addition to providing detailed images of the central nervous system (CNS) vasculature, GBCAs also enhance lesions that disrupt the BBB, such as intracranial tumors, infections, and meningeal disease.2
Outside
of the CNS, GBCAs are useful for enhancing various tumors and certain inflammatory lesions, due to their accumulation in areas with abnormal vascularity. These agents are helpful for diagnosing and staging solid tumors, detecting osteomyelitis, and highlighting vital organ disease (of the liver, lung, spleen, intestines, etc).
In general, radiocontrast-enhanced CT scanning is used to highlight vascular structures. Additionally, this technique allows differentiation of soft tissues within the intestinal tract and maintains increased sensitivity for detecting focal pathology within some abdominal organs. Table 1 provides a more comprehensive list of pathologies that dictate imaging with either contrast-enhanced MRI or CT scan.
Gadolinium-based Contrast Agents: Chelate Differences
As mentioned previously, GBCAs have been associated with the development of NSF in patients with kidney disease. To fully grasp the pathogenesis of this disease, it is helpful to have a basic understanding of the chemical properties of gadolinium. Gadolinium has paramagnetic properties that disturb the relaxation of water protons. This process results in a shortened relaxation time that increases signal intensity, a characteristic very useful for imaging. Gadolinium belongs to the lanthanide group of metallic elements on the periodic table and exists in the +3 oxidation state when in aqueous solution. Free ionic gadolinium (Gd3+) competes with Ca2+ primarily because these cations share similar ionic radius sizes. Due to its higher binding affinity than Ca2+, Gd3+ readily reduces neuromuscular transmission by blocking Ca2+ channels in muscle and nerve tissue cells, and alters the kinetics of Ca2+-dependent catalysts when bound to these enzymes.3
This makes ionic gadolinium extremely
toxic in biological systems. To overcome this, an organic ligand is used as a chelating agent that renders the complex biochemically inert.
The specific characteristics of the various ligands have implications regarding the potential for GBCAs to cause NSF. There are two general biochemical categories of ligands—linear and macrocyclic. Ligands can be further subclassified, based on charge, into ionic and non-ionic. These properties ultimately determine the tightness with which the ligand binds Gd3+ and subsequently how stable the complex remains.3 In general, tighter binding is preferred, as biological media such as human blood have additional metals that compete with Gd3+ for organic ligand binding. These competitors include zinc, copper, and iron. In addition, endogenous anions such as phosphate, carbonate, and citrate can bind Gd3+, and therefore will compete with the ligand for Gd3+ binding. These various binding interactions may ultimately lead to transmetallation, a process that occurs when the Gd3+–ligand complex dissociates and the ligand binds an endogenous metal and, conversely, Gd3+ binds an endogenous anion.4
Macrocyclic-based
ligands form more stable complexes with Gd3+ and are less likely to undergo transmetallation than linear-based ligands.3
Another component that determines the potential for transmetallation is the length of time that the GBCA remains within the biological system.
US NEPHROLOGY
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