Reaction Severity
Symptoms
Recommendation
Mild
Limited urticaria, itchy throat, nasal congestion, sneezing, conjunctivitis
Premedication not required
Moderate
Diffuse urticarial/erythema, facial edema or throat tightness without dyspnea
Premedication, consider alternate test
Severe
Diffuse or facial edema with dyspnea, laryngeal edema with dyspnea or hypoxia, erythema with hypotension, bronchospasm/wheezing, anaphylaxis
Premedication, consider alternate test
Recurrent allergic-like reaction in 10 % of premedicated patients.
No increased risk with history of specific allergy (i.e. Shell fish).
Need for test should be confirmed, alternative test considered.
Premedication recommended if history of moderate or severe contrast reaction requiring treatment
Premedication strategies: 50 mg prednisone or 32 mg methylprednisolone by mouth, 13, 7 and 1 hours prior to contrast injection and diphenhydramine 50 mg by mouth, intramuscular or intravenous 1 hour prior to injection.
In urgent situations, methylprednisolone sodium succinate 40 mg IV every 4 hours until contrast injection and diphenhydramine 50 mg IV 1 hour before injection can be used. Optimal premedication effect is achieved 4–6 hours after steroid administration but effects develop as early as 1 hours after administration.
Moderate/severe renal dysfunction
Risk of contrast-induced nephropathy rises with increased Creatinine (Cr) or decreasing glomerular filtration rate (GFR).
Higher risk in diabetics, elderly, hypotension, dehydrated patients, patients on non-steroidal anti-inflammatory drugs and low body mass index
No universally accepted guidelines by Cr or GFR
Chronic stable renal dysfunction:
Cr 1.5–2.0 or GFR 40–60 consider prehydration
Cr >2.0 or GFR <40 strongly consider alternate test
Acute kidney injury: no absolute guideline, strongly consider alternate test, prehydration and nephrology consultation
Suggest renal function testing prior to CT with contrast:
Age >60
History of renal disease, including:
Dialysis
Kidney transplant
Single kidney
Renal cancer
Renal surgery
History of hypertension requiring medical therapy
History of diabetes mellitus
Metformin or metformin-containing drug combinations
Scan Protocol
Contrast Phases
Non-contrast
Calcium score calculation.
Often helpful to differentiate calcification, and high-density surgical material from contrast on subsequent phases.
Contrast Timing
Contrast injected into a peripheral vein returns to the right heart, circulates through the lungs, the left heart, and into the systemic arteries including aorta and coronaries. In healthy subjects this takes approximately 12–18 seconds; in patients with systolic dysfunction or valvular disease this can take up to 35 seconds or more.
Techniques for scanning with adequate contrast opacification include:
1.
Set delay, usually 18–20 s
2.
Bolus-tracking, opacification is monitored in the aorta and scanning is triggered as soon as a threshold is met
3.
Test bolus, a separate, small contrast injection is given before scanning and opacification of the aorta is monitored to measure the time of peak opacification.
Test bolus is the most reliable but results in a small increase in scan time, contrast volume and radiation dose.
Gating
Gating is the process of synchronizing image acquisition or reconstruction with the cardiac cycle. Cardiac gating requires monitoring of the cardiac cycle using ECG leads. Scanners are programmed to sense the R-wave in the ECG tracing. Image acquisition can then be triggered at any desired point of the cardiac cycle, typically end systole or end diastole to minimize motion, or throughout the whole cycle to facilitate cine imaging.
Prospective Triggering
Images are only taken during specific, predetermined segments of the cardiac cycle. Patient selection: Slow, steady heart rate, ability to hold breath up to 15–20 s. Benefits: Less radiation. Challenges: Fewer phases of cardiac cycle available for interpretation. Longer scan time and breath-hold.
Retrospective Gating
Images are taken throughout the entire cardiac cycle. Patient selection: Higher heart rate with more variability, poor breath-holding/shortness-of-breath. Benefits: Shorter scan time/breath hold. More phases of cardiac cycle available for interpretation. Challenges: Higher radiation dose.
High-Pitch
Fast, spiral acquisition of the heart. Patient selection: Slow heart rate, poor breath-holding. Benefits: Low radiation dose, very fast acquisition. Reliable high quality images of all cardiac structures except for coronaries. Challenges: single phase of cardiac cycle available for interpretation, may be corrupted by motion artifact.
Angiographic
Images of the heart are acquired during peak arterial enhancement allowing detailed evaluation of the arterial structures.
Perfusion
Perfusion information is available on a typical angiographic phase. Contrast begins to reach the capillary bed 2–4 s after the coronary arteries. A myocardial perfusion deficit on angiographic phase scan is considered a resting perfusion defect; the culprit coronary lesion is considered hemodynamically significant to the degree that blood flow is limited even at rest [7]. Perfusion can be compared between stress and rest using two separate angiographic acquisitions, one with administration of adenosine or regadenoson.
Delayed
Additional images are acquired, without additional contrast administration, after the angiographic phase following a time delay. A delay of ~2 min is adequate for contrast to accumulate in areas with very slow blood flow, useful for:
Differentiation of late filling versus thrombosed false lumen in dissection
Differentiation of late filling versus intracavitary thrombus i.e. Left atrial appendage or left ventricular aneurysm
Extravasation of contrast from slow bleeding vessels
Identification of slow-filling subtle graft endoleaks
Enhancement of vessel wall in vasculitis
Further delay of ~7 min is adequate for contrast to accumulate in the myocardial interstitium, useful for:
Infarcted myocardium where the interstitial space is expanded by the presence of fibrotic scar tissue
Acute/subacute myocardial damage (infarct or myocarditis) where capillary permeability is abnormally increased.
Additional Considerations
Many features are specific to various manufacturers resulting in varying capabilities depending on the brand and model of scanner available. The following is a brief description of some of these features:
Slices
64-slice is the minimum recommendation for cardiac CT. Depending on the manufacturer, this corresponds to detector width ranging from 19.2 to 40 mm. The detector width equals the length of the patient scanned simultaneously. More slices/wider detector means a faster scan, less artifact, and shorter breath-holding for the patient [8]. At the time of this writing, scanners are available with 320 slices on a single source and 192 dual-source, covering 16 and 22 cm in a single CT rotation.
Dual source
Scanners with 2 x-ray sources allow extremely fast acquisition. The scanner collects twice the amount of information compared to single source, improving temporal resolution 2-fold. Coronary evaluation becomes feasible at higher heart rates.
Dual Energy
Reduces artifacts from metal such as coronary stents and can improve myocardial perfusion evaluation.
Indications for Cardiac CT
Indications for coronary artery calcium scoring, and cardiac CT angiography are listed below. For evaluation of coronary artery disease, pretest risk assessment is a major component of determining appropriate use of CT.
Pretest Risk Assessment
Low, intermediate or high risk is based on the Thrombolysis in Myocardial Infarction (TIMI) risk score for symptomatic patients and Absolute Risk Assessment Scores such as Adult Treatment Panel III (ATP III) or Framingham Risk Score in asymptomatic patients [3].
TIMI Risk Score
One point for each of the following:
Age ≥65 years
≥3 risk factors for coronary artery disease (hypertension, diabetes mellitus, family history, lipids, smoking)
Prior coronary stenosis ≥50 %
ST – segment deviation on electrocardiogram at presentation
≥2 anginal events in the prior 24 hours
Aspirin use in prior 7 days
Elevated serum cardiac markers
For various TIMI risk scores, the risk of all-cause mortality, new or recurrent myocardial infarction, or severe recurrent ischemia requiring urgent revascularization at 14 days following presentation is as follows [9]:
TIMI risk score < div class='tao-gold-member'>
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