# Understanding Your CT Coronary Angiography: A Patient's Guide to the Cleerly Standard Protocol

This article explains the standard protocol for CT coronary angiography (CCTA) when used with Cleerly, a software tool that analyzes heart scan images for plaque buildup. The protocol follows the 2016 SCCT guidelines and covers who can safely have the scan, how to prepare, exact scanner settings, contrast dye amounts, and which image files Cleerly can analyze. Key results include specific technical requirements such as a scan field of 18 cm or smaller, slice thickness of 1.0 mm or less, and a target contrast density of 400–450 Hounsfield Units in the left main coronary artery. Patients with cardiac stents and pacemakers are eligible for Cleerly analysis, while patients who have had bypass surgery are not.

# Understanding Your CT Coronary Angiography: A Patient's Guide to the Cleerly Standard Protocol

## Table of Contents

- Key Points
- What Is This Protocol and Why Does It Matter?
- Who Should Not Have This Test?
- How to Prepare for Your Scan
- Keeping Radiation Exposure as Low as Possible
- How the Scan Is Performed: Key Technical Requirements
- Contrast Dye: Volumes and Injection Rates
- How the Images Are Reconstructed
- Uploading Your Images to Cleerly
- Technical Tips for the Best Image Quality
- Which CT Series Cannot Be Analyzed
- Limitations of This Protocol
- What Patients Should Remember
- Frequently Asked Questions
- Source Information

## Key Points

- The Cleerly protocol requires a CT scan field of 18 cm or smaller, slice thickness of 1.0 mm or less, and specific contrast timing.
- Patients with cardiac stents or pacemakers are eligible for Cleerly analysis, but patients who had bypass surgery (CABG) are not.
- Before the scan, avoid caffeine for 12 hours, solid foods for 4 hours, and erectile dysfunction medication for 72 hours (24 hours for sildenafil).
- During the scan, beta blockers and nitroglycerin are standard to slow heart rate and widen coronary arteries for clearer images.
- Cleerly can only analyze contrast-enhanced, thin axial CCTA series, not calcium score exams, chest CTA, or 3D reformats.

## What Is This Protocol and Why Does It Matter?

Coronary computed tomographic angiography (CCTA) is a specialized CT scan that takes detailed pictures of the blood vessels that supply your heart muscle. These vessels, called coronary arteries, can develop plaque — fatty deposits that narrow the arteries and restrict blood flow. This narrowing is called stenosis, and it is a leading cause of heart attacks.

The Cleerly Standard Recommended Guidelines describe exactly how a CCTA should be performed so that the images are suitable for Cleerly's software analysis. Cleerly Labs is a web-based software application. Trained medical professionals use it as an interactive tool to view and analyze cardiac CT data, determining the presence and extent of coronary plaques (atherosclerosis) and stenosis in patients who underwent CCTA for evaluation of coronary artery disease (CAD) or suspected CAD.

In plain terms: the scan must be done in a very specific way so the software can read it accurately. The protocol aligns with the 2016 SCCT Guidelines for the performance and acquisition of coronary computed tomographic angiography, published by the Society of Cardiovascular Computed Tomography. Detailed scanning protocols can also be obtained from your CT machine's manufacturer through their clinical applications department.

These recommendations have been found to provide optimal results for plaque characterization, meaning the images are detailed enough for Cleerly to identify and measure plaque accurately. This matters to you because better images can lead to more accurate results and better treatment decisions.

## Who Should Not Have This Test?

Before a CCTA exam, a physician or provider reviews your medical history to check for any contraindications — conditions or circumstances that make the test unsafe or inappropriate. Here are the reasons you might not be able to have this scan:

-
**Severe allergic reaction to iodinated contrast:** The contrast dye used during the scan contains iodine. If you have a known history of a severe or anaphylactic (life-threatening) reaction to it, this test is not safe for you. This also applies to medications used to control heart rate and to vasodilators (drugs that widen blood vessels), including beta blockers and nitroglycerin.
-
**Kidney problems (renal impairment):** Your kidneys filter the contrast dye out of your body. If your kidneys are not working well, the dye can cause further damage.
-
**Pregnancy:** The radiation from a CT scan can harm a developing baby.
-
**Clinical instability:** If you are medically unstable — for example, in the middle of an acute myocardial infarction (heart attack), experiencing decompensated heart failure (when the heart cannot pump enough blood), or have severe hypotension (dangerously low blood pressure) — the scan should not be performed.
-
**Inability to cooperate:** The scan requires you to hold your breath and stay still. If you cannot follow these instructions, the images will be blurry and unusable.

Beyond the standard SCCT contraindications, there is one important additional rule for Cleerly. Patients who have undergone a Coronary Artery Bypass Graft (CABG) — a surgery that creates new routes around blocked arteries — should **not** be referred for a Cleerly Analysis. However, patients with cardiac stents (small mesh tubes placed in arteries) and pacemakers are eligible and **may** be referred to Cleerly for analysis.

## How to Prepare for Your Scan

Proper preparation helps ensure the best possible images. Your healthcare team will give you specific instructions, but here is what the standard protocol requires:

-
**Stop caffeine and stimulants 12 hours before your appointment.** This includes coffee, tea, energy drinks, and certain medications that stimulate the heart.
-
**Stop erectile dysfunction medication on a specific schedule:** Discontinue use within 72 hours before the exam (but only 24 hours for sildenafil, commonly known as Viagra). Also avoid taking these medications within 24 hours following the exam. This is important because these drugs can interact with nitroglycerin, which is often given during the scan.
-
**Avoid solid foods for 4 hours before the scan.** You may usually continue to take your routine medications under the direction of your physician.
-
**Stay well-hydrated.** Your care team will emphasize the importance of drinking fluids before the exam. Good hydration helps your kidneys clear the contrast dye afterward.
-
**Know what to expect.** The technologist should explain the breathing instructions, the IV contrast administration, and how long the exam will take. Clear expectations about what happens before and during the scan are critical. They ease anxiety and help keep your heart rate (HR) better managed — a lower, steady heart rate produces clearer images.

You should also receive a thorough explanation of why beta blockers and nitroglycerin are used. Beta blockers slow your heart rate so the CT scanner can capture crisp images of your arteries. Nitroglycerin widens (dilates) your coronary arteries so they are easier to see. Neither medication is optional; both are essential for a high-quality study.

## Keeping Radiation Exposure as Low as Possible

Radiation exposure is a legitimate concern for any CT scan. This protocol includes several specific strategies to keep your dose as low as possible without sacrificing image quality:

-
**Automatic Exposure Control (AEC):** The machine's milliampere (mA) setting — which controls how much X-ray current is used — is typically determined automatically by AEC based on your body size. If AEC is not available, the mA should be reduced to the lowest point that does not reduce image quality.
-
**Prospective scanning:** Whenever possible, the technologist should use prospective gating or other dose reduction techniques such as high-pitch spiral scanning. These approaches expose you to less radiation than older methods. A retrospective technique (which delivers more radiation) may be required only in cases of high or variable heart rates. This option depends on the scanner's capabilities.
-
**Minimized scan length (Z-axis):** The scan range should cover only the heart's anatomy — nothing more. Every extra centimeter of scanning adds radiation without adding useful information.
-
**Iso-centering:** The patient should be centered in the scanner so that the heart is at the machine's center, rather than moving the scan field to the heart. This positioning improves image quality and allows lower doses.
-
**Half-scan reconstruction:** This technique, which uses data from a half rotation of the scanner, should be used by default for all coronary CTA examinations.

Different CT vendors and scanners have different dose reduction options. Your facility's team consults the vendor's clinical applications department to ensure the best dose reduction techniques are used without negatively affecting image quality and diagnostic performance.

## How the Scan Is Performed: Key Technical Requirements

For Cleerly to analyze your scan, the images must meet specific technical standards. Here are the key requirements:

-
**Field of view (FOV) of 18 cm or less:** This is the diameter of the area being scanned. It must be small enough to include all heart anatomy and coronary vessels but not so large that it wastes data.
-
**Slice thickness of ≤ 1.0 mm with a 0.5 mm increment or less:** The scanner takes "slices" (cross-sectional images) through your heart. Thinner slices mean finer detail. The slice increment (the overlap between slices) should be about half the selected slice thickness.
-
**Patient identification data:** Your name, date of birth, and ordering provider information are required for all exams sent to Cleerly. This data may be auto-populated or manually entered on the scanner at the time of the exam.
-
**Scanner requirements:** The scan must be performed on a 64+ slice CT scanner with a rotation time of ≤ 350 milliseconds (ms). Faster rotation helps freeze the motion of your beating heart.
-
**Tube voltage (kVp) based on body size:** The strength of the X-ray beam is adjusted to your body mass index (BMI):

- 100 kVp for patients with a BMI of 30 or less
- 120 kVp for patients with a BMI greater than 30

If you are having a serial CCTA (a follow-up scan to compare with a previous one), the same kVp as your baseline exam should be used so the images can be compared accurately.
-
**Medication protocols:** Your facility follows its own beta-blocker and nitroglycerin protocol guidelines.
-
**ECG gating:** This technique times the scan to your heartbeat. Correct placement of radiolucent leads and patches (leads that do not block X-rays) is essential.
-
**IV access:** An 18-gauge (18G) IV in the RAC (right antecubital, the inner elbow area) is preferable for contrast injection. A 20G may be used if necessary.
-
**Dual contrast injector:** The facility uses a dual-head injector, and biphasic or triphasic injections are recommended for optimal contrast enhancement.

## Contrast Dye: Volumes and Injection Rates

The contrast dye (iodinated contrast) makes your coronary arteries visible on the scan. The amount and speed of injection depend on three factors: the iodine concentration of the dye, your body habitus (body size), and the kVp being used. The table below shows the standard protocol:

**At 100 kVp (for BMI ≤ 30):**

- Iodine concentration 320 mg/mL: 65–70 mL at 5.0–5.5 mL/s, followed by a 40 mL saline chaser
- Iodine concentration 350 mg/mL: 55–60 mL at 4.0–4.5 mL/s, followed by a 40 mL saline chaser
- Iodine concentration 370 mg/mL: 55–60 mL at 4.0–4.5 mL/s, followed by a 40 mL saline chaser

**At 120 kVp (for BMI > 30):**

- Iodine concentration 320 mg/mL: 78–85 mL at 6.0–6.5 mL/s for standard habitus; 90–97 mL at 7.0–7.5 mL/s for large habitus; followed by a 40 mL saline chaser
- Iodine concentration 350 mg/mL: 65–70 mL at 5.0–5.5 mL/s for standard habitus; 78–85 mL at 6.0–6.5 mL/s for large habitus; followed by a 40 mL saline chaser
- Iodine concentration 370 mg/mL: 65–70 mL at 5.0–5.5 mL/s for standard habitus; 78–85 mL at 6.0–6.5 mL/s for large habitus; followed by a 40 mL saline chaser

The saline chaser is a saltwater solution pushed through the IV after the contrast. It flushes the dye through your veins and into your heart, ensuring that the full dose reaches the coronary arteries and that no dye is wasted in arm veins.

Facilities use a formula to calculate the precise amount of contrast for each patient: **(scan delay time + acquisition time) × flow rate = contrast volume**. The scan delay time is how long after injection the scan starts, and the acquisition time is how long the scan takes. Multiplying the sum by the flow rate (mL/s) gives the exact volume of contrast needed.

The goal is to achieve a specific brightness of the coronary arteries on the scan. Cleerly's software prefers a Hounsfield Unit (HU) range of between 400 and 450 HU in the left main coronary artery, the main artery that supplies the left side of the heart. Hounsfield Units are the standard measure of tissue density on a CT scan. Following the injection parameters above and using the formula helps the technologist deliver the precise amount of contrast to hit this target.

## How the Images Are Reconstructed

After the scan is complete, the raw data must be reconstructed into images. This is a crucial step, and the protocol requires immediate review:

-
**Review while the patient is still on the scanner table:** Image reconstructions should be reviewed immediately after the scan is completed, while you are still on the table. This confirms that the quality of data acquisition is sufficient. If the images are inadequate, the scan can be repeated right away.
-
**Limit the reconstruction field of view:** The reconstructed field of view should be limited to the cardiac structures only.
-
**Iterative reconstruction (IR):** This post-processing algorithm should be applied on all CCTA exams. IR independently reduces image noise (the grainy speckle that degrades image quality) while preserving resolution. The result is a cleaner image, often at a lower radiation dose.
-
**Phase reconstruction in milliseconds:** When reconstructions of phases in percentages contain artifacts (blurring or streaking from heart motion), reconstructing in ms or -ms (milliseconds relative to the heartbeat) may improve the image quality outcome.

**Cleerly-specific reconstruction requirements:**

Successfully acquired images have a slice thickness of ≤ 1.0 mm with a slice increment at half the selected slice thickness. The specific phases (points in the cardiac cycle at which images are reconstructed) depend on how the scan was performed:

-
**If the CCTA was acquired prospectively** (a lower-dose technique), submit thin-set axial phases, for example 60%–80% of the R-R interval (the time between heartbeats on the ECG), in 5% intervals.
-
**If the CCTA was acquired retrospectively** (used for higher or irregular heart rates), upload the following thin-set axial phases: 30%, 35%, 40%, 50%, 60%, 70%, 75%, and 80%.
-
**Also upload your automatic phase selection (Best Phase)**, which the scanner identifies as the clearest point in the cardiac cycle.

**Important note for Canon scanners:** If your facility uses a Canon scanner, the Enhanced CT (multi-frame DICOM) option must be disabled per manufacturer instructions. Your Canon Service Representative can provide guidance. Cleerly software is currently unable to analyze any CCTA with this function enabled. Additionally, all cardiac series should be reconstructed from the Multi View tab, and axial thin datasets should be created at ≤ 0.5 mm × 0.5 mm increment or less. Reconstructions created on the volume dataset are currently unable to be analyzed by Cleerly software.

## Uploading Your Images to Cleerly

Once the images are reconstructed, they are sent electronically to Cleerly Labs for analysis. The process involves two key steps:

1.
**Network all acquired thin-section axial phases to Cleerly Labs.** The actual phases uploaded vary by each site's specific radiologist requirements.
1.
**Allow all phases to be reconstructed completely before uploading the exam.** Uploading partial or incomplete reconstructions can cause errors.

This networking process is handled by your facility's imaging team. No action is needed from you.

## Technical Tips for the Best Image Quality

Your comfort during the scan directly affects image quality. The protocol recommends the use of patient comfort tools, including arm rests, knee cushions, warm blankets, and dimmed lighting. A relaxed patient has a lower, steadier heart rate, which produces sharper images.

Beyond comfort, the protocol includes several technical tips for the imaging team:

-
**Axial images** should be reconstructed with a slice thickness of ≤ 1.0 mm × 0.5 mm increment or less, to include all native coronary arteries.
-
**Slice increment** of 50% of the slice thickness should be used.
-
**Reconstruction kernel:** A semi-sharp reconstruction kernel should be used for most patients. For cases that require maximum spatial resolution, a sharp kernel may be used to reduce blooming (a bright artifact that makes calcium deposits appear larger than they are) and increase edge definition. For obese patients, a soft or smooth kernel may be used to reduce image noise.
-
**Number of phases:** A sufficient number of phases should be reconstructed to find the optimal phase of the cardiac cycle (the R-R interval) with the least amount of coronary motion.
-
**ECG editing:** If available, ECG editing should be used to correct errors or artifacts that occurred during acquisition, and to designate ectopic beats (extra or skipped heartbeats) for exclusion or special handling during data reconstruction.

## Which CT Series Cannot Be Analyzed

Not every CT image can be processed by Cleerly. The software has strict requirements. Cleerly analyzes only **contrast-enhanced, thin axial CCTA reconstructed series**. The following types of images **cannot** be analyzed:

- CTA Chest (a CT of the entire chest rather than just the heart)
- Exams with bypass grafts present (CABG patients are not eligible for Cleerly analysis)
- Calcium score exams (a separate type of heart CT that measures calcium without contrast)
- 2D and 3D image reformats (reformatted views such as curved planar reconstructions or volume-rendered images)
- Dose reports (documents that record radiation dose)
- Wide field of view lung reformats (reconstructions that include the lungs)
- Bolus tracking data (the test injection used to time the scan)
- SmartPrep data (similar monitoring data used for scan timing)

If your scan includes any of these, your facility will need to ensure the correct contrast-enhanced thin axial CCTA series is selected for upload.

## Limitations of This Protocol

This document is a technical protocol, not a research study with patient outcomes. Several limitations are important to understand:

-
**It is a guideline, not a guarantee:** The protocol describes the optimal way to acquire images for Cleerly analysis. Following it does not guarantee that every scan will be analyzable in every case. Patient factors such as very high heart rates, arrhythmias, or inability to hold breath can still degrade image quality.
-
**It does not compare outcomes:** This document does not report data on how Cleerly-assisted analysis affects patient outcomes such as heart attack rates, survival, or quality of life. The clinical value of Cleerly analysis must be evaluated through separate research.
-
**It is not a diagnostic guideline:** The protocol covers image acquisition only. Decisions about diagnosis and treatment are made by your physician, not by the software or this protocol.
-
**It relies on specific equipment:** Some recommendations (such as the Canon-specific instructions, prospective gating, and AEC) depend on the capabilities of the scanner at your facility. Not every facility may have all the options described.
-
**Reference to a single guideline:** The protocol cites the 2016 SCCT Guidelines as its primary reference. Guidelines in this field are periodically updated, and more recent guidance may exist.

The original document is a copyrighted technical reference from Cleerly, Inc. It was not published in a peer-reviewed medical journal.

## What Patients Should Remember

If your doctor has referred you for a CCTA with Cleerly analysis, here is what you should know and do:

1.
**Tell your doctor about all your medical conditions**, especially kidney problems, allergies (particularly to contrast dye or iodine), and any history of reactions to medications.
1.
**Tell your doctor about every medication you take**, including erectile dysfunction medications, because some drugs must be paused before the scan.
1.
**Follow preparation instructions exactly:** Avoid caffeine for 12 hours before the exam, avoid solid food for 4 hours before, and drink plenty of fluids unless your doctor tells you otherwise.
1.
**Ask questions about what to expect.** Knowing the breathing instructions, the IV placement, and the exam duration will reduce your anxiety and help you cooperate during the scan.
1.
**Do not be alarmed if you receive beta blockers or nitroglycerin.** These medications are a standard part of the protocol. They slow your heart rate and widen your arteries to improve image quality.
1.
**If you have stents or a pacemaker, you can still have this test.** If you have had bypass surgery (CABG), Cleerly analysis is not available for your scan, and your doctor will use other methods to evaluate your arteries.
1.
**Stay still and follow breathing instructions during the scan.** The technologist may review images immediately while you are still on the table. If any images are unclear, they may need to repeat part of the scan.

Understanding the technical details of your heart scan can feel overwhelming. But this protocol exists for one reason: to make sure the images of your heart are as clear and accurate as possible. That accuracy translates into a more reliable assessment of your coronary plaque and narrowing, which helps your doctor make better-informed decisions about your heart health.

## Frequently Asked Questions

### Can I have a CT coronary angiogram with Cleerly analysis if I have stents or a pacemaker?

Yes. Patients with cardiac stents and pacemakers are eligible for Cleerly analysis. However, if you have had coronary artery bypass graft (CABG) surgery, Cleerly analysis is not available for your scan. Your doctor will use other methods to evaluate your arteries if you have had bypass surgery.

### What should I do to prepare for the CT coronary angiogram with Cleerly?

Stop caffeine and stimulants 12 hours before. Avoid solid foods for 4 hours. Stop erectile dysfunction medication 72 hours before, except sildenafil (Viagra) 24 hours before. Stay well-hydrated. Tell your doctor about all medications and medical conditions, especially kidney problems and allergies to contrast dye.

### Why do I need to take beta blockers and nitroglycerin before and during the scan?

Beta blockers slow your heart rate so the CT scanner can capture crisp images of your arteries. Nitroglycerin widens your coronary arteries so they are easier to see. Both medications are essential for a high-quality study. They are a standard part of the protocol, so do not be alarmed if you receive them.

### What is the target brightness for the coronary arteries in the Cleerly analysis?

The goal is to achieve a brightness of 400 to 450 Hounsfield Units (HU) in the left main coronary artery. Hounsfield Units measure tissue density on a CT scan. The imaging team uses specific contrast dye amounts and injection rates to hit this target, which helps Cleerly analyze plaque accurately.

### Could the scan be repeated if the images are not good enough?

Yes. After the scan, images are reviewed immediately while you are still on the scanner table. If the images are inadequate, the scan can be repeated right away. Your ability to stay still and follow breathing instructions helps prevent the need for a repeat scan.

### What do the numbers '64+ slice' and 'rotation time of 350 ms' mean for my scan?

The scan must be done on a CT scanner with at least 64 detector rows, which allows thinner slices and faster scanning. A rotation time of 350 milliseconds or less means the scanner rotates quickly around you, helping freeze motion from your beating heart. These technical requirements ensure images are sharp enough for Cleerly to analyze plaque accurately.

### I have coronary stents and a pacemaker — should I get a second opinion before a Cleerly CT coronary angiogram?

Patients with cardiac stents and pacemakers are eligible for Cleerly analysis of a CT coronary angiogram. However, the scan cannot be performed if you have had bypass surgery (CABG). Your doctor must also rule out contraindications such as severe allergic reaction to iodinated contrast, kidney problems, pregnancy, clinical instability, or inability to hold still. If any of these concerns apply, or if you are unsure whether the scan is safe or appropriate for you, a second opinion can help clarify your options. Diagnostic Detectives Network provides independent expert second opinions.

## Source Information

**Original article title:** CT Coronary Angiography Cleerly Standard Recommended guidelines

**Publisher:** Cleerly, Inc. — cleerlyhealth.com

**Reference cited in the original:** Abbara S, Blanke P, Maroules CD, et al. SCCT guidelines for the performance and acquisition of coronary computed tomographic angiography: A report of the Society of Cardiovascular Computed Tomography Guidelines Committee: Endorsed by the North American Society for Cardiovascular Imaging (NASCI). *J Cardiovasc Comput Tomogr.* 2016 Nov-Dec;10(6):435-449.

**Contact for technical support (for imaging facilities):** support@cleerlyhealth.com

This patient-friendly article is based on peer-reviewed research and technical guidelines. It is provided for educational purposes and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.


---
Publisher: Diagnostic Detectives Network (https://diagnosticdetectives.com) — independent multi-expert medical second opinions, worldwide, private-pay.
Author byline: Anton Titov, MD, PhD. Contact: https://diagnosticdetectives.com/pages/contact
Canonical page: https://diagnosticdetectives.com/products/understanding-your-ct-coronary-angiography-a-patients-guide-to-the-cleerly-standard-protocol