Radiologic adjuncts

Radiologic adjuncts are a class of pharmaceutical agents used in conjunction with radiologic imaging procedures to optimize the diagnostic yield, safety, accuracy, or therapeutic application of those procedures. Unlike contrast agents or radiopharmaceuticals—which directly enhance imaging signals—radiologic adjuncts serve supportive roles. They modify physiological functions, reduce image artifacts, improve patient comfort, or enhance target specificity, making the imaging procedure more effective or interpretable.

They may include pharmacologic stress agents, sedatives, anticholinergics, diuretics, enzyme inhibitors, or vasodilators, administered prior to or during radiologic procedures. Their application spans numerous imaging modalities such as nuclear medicine, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and angiography.

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1. Definition and Scope

Radiologic adjuncts are pharmaceutical or biologic substances administered as ancillary components during diagnostic or interventional imaging. Their purpose is to:

• Enhance physiologic contrast or imaging accuracy

• Improve patient compliance or comfort

• Minimize imaging artifacts or interference

• Support theranostic procedures (combined therapy and diagnostics)

These agents are not radiopaque, nor are they radioactive. Instead, they act by modulating patient physiology or drug pharmacokinetics in the context of the imaging process.

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2. Classification of Radiologic Adjuncts

Radiologic adjuncts can be grouped based on their functional roles in imaging:

A. Pharmacologic Stress Agents

Used in cardiac nuclear imaging and MRI to simulate exercise.

• Adenosine – Coronary vasodilator for myocardial perfusion imaging

• Regadenoson – Selective A2A adenosine receptor agonist

• Dipyridamole – Inhibits adenosine uptake to cause vasodilation

• Dobutamine – β1 agonist to induce cardiac stress (for MRI or echocardiography)

B. Diuretics

Enhance visualization of renal and urinary systems by promoting tracer excretion.

• Furosemide – Used in renal scintigraphy (e.g., MAG3 scan) to assess drainage

C. Anticholinergics / Smooth Muscle Relaxants

Reduce peristalsis and motion artifacts in abdominal and pelvic imaging.

• Hyoscine butylbromide (Buscopan) – Used in MRI colonography, CT enterography

• Glucagon – Reduces bowel motility during MRI or barium procedures

D. Sedatives / Anxiolytics

Improve patient cooperation and reduce motion.

• Midazolam – Benzodiazepine for sedation in MRI or PET-CT

• Lorazepam – For claustrophobic patients during MRI

E. Gastrointestinal Motility Modulators

Enhance or suppress gastric emptying or intestinal transit.

• Metoclopramide – Increases gastric emptying, often used with SPECT studies

• Erythromycin – Prokinetic agent used in gastric emptying studies

F. Secretagogues and Enzyme Inhibitors

Stimulate or suppress organ secretions for better visualization.

• Cholecystokinin (sincalide) – Stimulates gallbladder contraction in HIDA scans

• Cimetidine / Ranitidine – H2 receptor blockers used in Meckel’s diverticulum imaging

• Pentagastrin – Stimulates gastric mucosa during radiolabeled gastrin studies

G. Vasodilators and Vasoconstrictors

Used to alter perfusion or tracer distribution.

• Nitroglycerin – Enhances coronary flow in cardiac imaging

• Phenylephrine – May reduce nasal congestion in sinus CT

H. Radiotracer Kinetics Modulators

Alters biodistribution or uptake of specific tracers.

• Potassium perchlorate – Blocks thyroidal uptake of pertechnetate

• Levothyroxine withdrawal / rhTSH – Enhances radioiodine uptake in thyroid imaging

I. Others

• Mannitol / PEG – Oral osmotic agents for bowel distension before CT enterography

• Water / Juice – Neutral oral contrast in MRCP or small bowel MRI

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3. Mechanisms of Action

Radiologic adjuncts influence patient physiology or tracer dynamics to aid imaging.

Agent Class	Mechanism	Imaging Purpose
Vasodilators	Coronary smooth muscle relaxation via A2A or β1 pathway	Myocardial perfusion (stress test simulation)
Diuretics	Inhibit Na⁺/K⁺/Cl⁻ reabsorption to increase urine flow	Renal scans to evaluate obstruction
Anticholinergics	Block muscarinic receptors to inhibit peristalsis	Reduce motion in abdominal CT or MRI
Sedatives	GABA-A agonists for CNS depression	Decrease anxiety, involuntary movement
GI Prokinetics	Dopamine antagonism or motilin agonism	Gastric emptying studies
Hormonal Agents	Stimulate bile or acid secretion	HIDA scan, Meckel's diverticulum detection

4. Clinical Applications and Imaging Contexts

Imaging Modality	Radiologic Adjunct Used	Clinical Indication
Cardiac SPECT/PET	Regadenoson, adenosine, dipyridamole	Pharmacologic stress testing
Renal Scintigraphy	Furosemide	Diuretic renogram for obstructive uropathy
MRI Abdomen/Pelvis	Buscopan, glucagon	Reducing bowel movement artifacts
Gastric Emptying Scan	Metoclopramide, erythromycin	Enhancing motility for accurate T½ estimation
HIDA Scan	CCK (sincalide), morphine	Gallbladder ejection fraction; biliary obstruction
PET/CT	Lorazepam, water	Minimize patient movement; GI contrast
Meckel’s Scan	Cimetidine, ranitidine	Inhibit acid secretion to retain Tc-99m pertechnetate
MRI Enterography	Mannitol, PEG	Small bowel distension
Thyroid Scans	rhTSH, perchlorate	Optimize radioactive iodine uptake

5. Dosing and Administration

Dosing of adjunct agents varies depending on agent class and patient condition:

• Regadenoson: 0.4 mg IV bolus (standard fixed dose for stress testing)

• Furosemide: 20–40 mg IV during MAG3 or DTPA renogram

• Hyoscine (Buscopan): 20 mg IV or IM before MRI/CT enterography

• Metoclopramide: 10 mg orally or IV prior to gastric emptying scintigraphy

• Sincalide (CCK): 0.02 mcg/kg IV over 3 minutes (HIDA)

• Midazolam: 1–2 mg IV for procedural sedation

Agents should be tailored to:

• Patient comorbidities (e.g., cardiac status, renal function)

• Pediatric vs adult dosing

• Imaging protocol timing

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6. Adverse Effects and Contraindications

Agent	Adverse Effects	Contraindications
Adenosine/Regadenoson	Chest pain, hypotension, AV block	Severe asthma/COPD, AV block, bradycardia
Dipyridamole	Headache, dizziness, flushing	Hypotension, unstable angina
Furosemide	Dehydration, electrolyte imbalance	Anuria, sulfa allergy
Hyoscine (Buscopan)	Dry mouth, tachycardia, urinary retention	Glaucoma, myasthenia gravis
Glucagon	Nausea, vomiting, hyperglycemia	Pheochromocytoma, insulinoma
Metoclopramide	Extrapyramidal symptoms, sedation	Parkinson's disease, GI obstruction
Midazolam	Respiratory depression, drowsiness	Narrow-angle glaucoma, severe hypotension
Sincalide (CCK)	Abdominal pain, nausea, hypotension	Hypersensitivity, bowel obstruction

Monitoring, especially in elderly and renally impaired patients, is essential during administration.

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7. Drug Interactions

Radiologic adjuncts may interact with:

Adjunct Agent	Interaction Risk
Adenosine	Antagonized by caffeine, theophylline
Dipyridamole	Potentiated by aspirin or anticoagulants
Metoclopramide	Interacts with antipsychotics (EPS risk)
Midazolam	Potentiated by opioids, CYP3A4 inhibitors
Glucagon	Potentiates anticoagulant effect of warfarin

Patients should be screened for recent use of interfering agents prior to imaging.

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8. Pharmacokinetics Overview

Agent	Onset (min)	Duration	Metabolism/Excretion
Regadenoson	<1	~10 min	Hepatic metabolism, renal excretion
Furosemide	5–10	2–4 hrs	Renal
Hyoscine	1–2	~30–60 min	Hepatic, renal
Midazolam	1–5	30–120 min	Hepatic (CYP3A4), renal
Metoclopramide	30 (oral)	1–2 hrs	Hepatic, renal
Glucagon	1–10	10–30 min	Hepatic, renal

9. Regulatory and Clinical Guidelines

Radiologic adjuncts are regulated under:

• FDA: Approved with imaging-specific indications

• EMA: Similar authorizations across Europe

• ACR and SNMMI: Provide practice guidelines for use

Considerations include:

• GMP compliance for injectable preparations

• Radiology department protocols based on organ system

• Informed consent, especially for agents with sedative or stress-inducing properties

Examples of approved adjuncts with imaging indications:

• Adenoscan (adenosine)

• Lexiscan (regadenoson)

• Bloxiverz (neostigmine, sometimes used post-imaging)

• Glucagen Diagnostic Kit (glucagon)

• Diuril (chlorothiazide; rarely used now in renography)

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10. Future Directions in Radiologic Adjunct Use

• Development of targeted GI motility suppressors for high-resolution abdominal imaging

• AI-assisted adjunct administration protocols to personalize agent selection

• Nanotechnology-based adjuncts (e.g., smart vesicles that respond to stimuli)

• Pharmacogenomic profiling to identify responders/non-responders to agents like regadenoson

• Use of biosensors to monitor physiologic response to adjuncts in real-time