The contrast inserted can be done so with one of the two techniques. These include either a test bolus or automatic bolus triggering technique. Consideration of the amount of bolus must take into account the speed and the duration of the procedure. For example in eight and 16 detector row CTs, 80-120mL contrast medium must be injected at 3-5mL/sec in order to provide constant contrast for the 20 to 40 second scanning. Iodine and saline chasing are the two contrasts that are injected.
With iodine, the concentration is 1g/sec and with saline chasing the iodinated contrast is immediately followed by 50 ml of saline. The later technique has shown better results due to better contrast medium bolus utility, high and consistent vascular enhancement and prevention of streak artifacts, which causes faulty evaluation of the right coronary artery. (Schoepf et al, 2004) IMAGING OF CARDIAC CT: The accumulation of the thousands of image that are attained during the procedure and identifying the ones that are of relevance is the next important and challenging task that a radiologist comes across with.
Currently there are four methods that are used to display and evaluate the relevant data of the CT scan. (Schoepf et al, 2004) The maximum intensity projection method is ideally used for the visualization of the coronary arterial tree. This method is able to provide information through a more intuitive format, and is able to classify the information in to relevant sections, making it an easier job for the radiologist. The multi-planer reformation method utilized the isotropic nature of the CT scans and arranges them into arbitrary imaging planes.
Curved multi-planer reconstructions aid in following the course of the coronary arterial tree. (Schoepf et al, 2004) Three dimensional display or volume rendering creates three dimensional images from the two dimensional data obtained through the CT. The final advancement and method of CT evaluation is through the help of soft wares that allow isolation and reconstruction of the various images into coherent data, and thereby increase ease of data assimilation. (Schoepf et al, 2004)
Two modes of cardiac imaging are currently carried out. The prospective ECG gating works by R-wave detection and subsequent triggering which causes creation of axial images. The retrospective tagging protocol takes spiral acquisition with simultaneous ECG. The introduction of the electron beam computed tomography opened new doors in the dynamic image capturing capacity of the CT. this method made it possible to capture images of the beating heart with out losing any of its details.
With the consequent introduction of the MSCT technology, the methods are fast becoming a widespread diagnostic modality in general public screening programs. (Morin et al, 2003) As mentioned, the prospective EBCT scanners work by exposing the patient to the radiation at an appointed time in the cardiac cycle. This is in contrast to the MSCT technique, where prospective as well as retrospective gating can be used for imaging purposes. The prospective gating technique is preferred due to the reduced radiation exposure when compared to the retrospective procedure.
(Morin et al, 2003) Image noise is another important issue to be resolved in each patient, for otherwise the results of the images obtained will be of suboptimum quality. The photons emitted by the X-ray tube are directly proportional to the voltage applied to the X-ray beam, and in patients of large sizes or scans which have narrow widths, the voltage must be increased. This results in increased radiation exposure for the patients. In the case of EBCT, the predetermined scanner settings do not allow variations above or below the range, which is possible with the MCST.
This gives the MSCT scanner the advantage, for a radiologist can easily adjust the voltage values according to the individual patient. (Morin et al, 2003) PATIENT PROCEDURES FOR VARIOUS CARDIAC CT SCANS: CT angiography procedures in a patient require strict adherence to protocol and good preparation of the patient for the procedure. Since CT imaging is improved at heart beats less than 65 beats per minute, many patients are given medication to reduce the heart rate. However, there are other approaches to reducing the heart beat as well.
For example, holding of breath can help reduce the heart beats in a patient with heart rates over 70 beats per minute. In practice, the most commonly used agent is the beta blocker drugs such as metoprolol and alongside short acting nitroglycerine is given to enhance the visualization of the coronary vessels. The use of nitroglycerine, however, is still subjected to clinical trials in such cases and therefore, is not used as widely as the beta blockers. Beta blockers can be used orally or intravenously for the procedure.
The concurrent use of ECG tracing is a good method to correlate the cardiac monitoring with the evidence obtained from the CT. both of these procedures is therefore carried out simultaneously. In such cases, the patient is positioned in a supine position and the ECG leads are placed according to the method. Since angiography CT requires the use of the contrast medium, this is inserted in the body via the 18 gauge needle, with a flow rate of 5 ml/s. (Hoffman et al, 2006)