Practical tips and tricks for achieving a better image quality during coronary CT angiography
Contributed by Dr. Vishnu Ravila
Counseling the patient about the procedure of the scan
- Please remember that the patient might have probably come for a scan for the first time in his life and will be having multiple queries about the procedure
- Please be patient in explaining the procedure and the process of the scan to the patient because understanding the importance of every step will make following instructions easier
- Explain to the patient the importance of heart rate control before proceeding with the scan.
- Explain to the patient about the technique of the breath-hold which is in mid inspiration, and request them to practice a uniform method of breath-hold that can be used while the patient is in the gantry
- Also, explain to them about the phenomena like the generalized experience of feeling heat/ warmth while the contrast is entering the body so that the patient will not panic during the study
- Most often, the quality of the scan is proportionate to the amount of time spent preparing the patient
- When a patient comes for a coronary CT angiography measure the height and weight of the patient and note it down
- Ask for the time of the last meal and make sure that there is at least 3-4 hours of gap before giving the contrast
- Measure the heart rate using a pulse oximeter or manually.
- If the heart rate is more than 65 bpm consider giving a stat dose of 15 mg of Ivabradine (The actual target heart rate might depend on the equipment which you are going to use. Some scanners can get excellent images even if the heart rate is more than 70 bpm but for most of the scanners out there, a heart rate of 65 bpm is the sweet spot)
- After giving the dose of Ivabradine one can check for the decrease in the heart rate. (If patient is fasting, check after one hour as peak of action is reached then.) If the heart rate does not come down to the range of 65 bpm then please consider using an IV beta-blocker (like IV Metaprolol) under supervision to decrease the heart rate. Please do not wait for more than hour or more hoping that the stat dose of Ivabradine will further decrease the heart rate
- (If the baseline heart rate is very high, like >90BPM, we can ask the patient to take heart rate lowering medication 2 days before the scan so that the heart rate can be more stable during the time of the scan. We use a 10mg Ivabradine as a single dose per day for 2 days before the scanning)
- While the patient is waiting for the heart rate to come down you can take the help of a Nurse or a Phlebotomist to secure 18 gauge cannula in the antecubital fossa in a large caliber vein
- Once the target heart rate is achieved, place the patient in the gantry with heart at the iso center.
- Position the ECG leads as recommended by the scanner manufacturer
- Now will be a good time to check the patency of the IV line connecting it to a pressure injector and giving a short bolus of saline
- Before proceeding to perform a calcium score, 2-3 puffs of nitroglycerin spray can be administered. which will usually take 5 minutes to achieve maximal coronary vasodilatation. Once the calcium scoring scan is done check for any artifacts due to improper breath-hold by the patient. This is best done by examining the sternum in the sagittal view or domes of the diaphragm and the lung parenchyma in the Lung Window
- If the breath-hold is not satisfactory please talk to the patient again and reiterate that proper breath-holding is very crucial for the contrast scan. We can try to motivate patients with positive talk and encouraging them
- If the patient is still apprehensive and not able to follow instructions you can ask the attendee of the patient to hold the nose and mouth of the patient after giving them lead aprons
- Once you are satisfied with the quality of the breath-hold please proceed further for the contrast study
- If the patient weight is less than 80 kg you can attempt to do a scan using a low Kv technique, with 100 kV
- If the patient weight is more than 80 kg you can use an iodinated contrast with a higher concentration of iodine to achieve good opacification of coronary arteries
- These small changes will drastically improve the opacification of coronary arteries which will make the various post-processing tasks very easy
- If you have a high-end CT scanner and the patient has a very stable heart rate with less than 65 bpm you can attempt to do a prospective triggering for the scan. Please bear in mind that any slight variation of the heart rate while acquiring a prospective triggering scan can potentially ruin the study
- If there is a variation of the heart rate around 65 bpm please choose a retrospective gated scan.
- Kvp value of 110, dose modulation with mAs and appropriate FOV will reduce radiation dose.
- A dual phasic injection of the contrast equivalent to the patient body weight in ml at the rate of 5 mL/sec with initial contrast followed by half the quantity of normal saline chaser should be administered
For example, if the patient weight is 90 kg give 90 mL of iodinated contrast at 5 mL/sec followed by 45 mL of a normal saline chase
- Once the scan is done check for the adequacy of Image Quality as done after the calcium scoring in the sagittal view for the sharp sternal margins and diaphragm and the lung parenchyma in the lung window for any motion blur
Make sure that you get a best diastolic and a best systolic reconstruction from the retrospectively gated study.
Additionally, you can also make a multiphasic reconstruction at 10% R-R intervals for example from 20%-80% of the cardiac phases if there has been a larger variation of the heart rate during the study
Transfer the data sets to the CT work station for further post-processing
Post-processing must include at least 2 views showing an entire course of the vessel using curved planar reformat in case of normal caliber vessels
In case of diseased segments, make additional rotated views of curved planar images to give a better view of the plaque and the corresponding IVUS view or luminal cross-sectional views
VRT images with coronary tree should be done.