CT imaging procedures of the heart have improved at a rapid pace in the recent years. The efficacy of diagnosis has increased considerably and newer technologies are now underway to improve scanning and diagnostic procedures. CT is now looked as the diagnostic modality that may rid of surgical and invasive procedures for heart. Its ability to identify pathosis in the heart at early stage makes it an ideal method of screening at risk individuals.
CT IMAGING OF THE HEART
CT imaging has come a long way since its introduction in the field of medicine as a diagnostic modality. While imaging in the past was confined to radiographic assessments only, the advent of CT led to improvements in the diagnostic capabilities of the physicians. The first in the series of the CT scans were the conventional collimation scans, which although were a good addition to diagnostics; remained a small part in the diagnostic planning. It was later with the addition of the high resolution CT scanning that true possibilities of this diagnostic were realized.
High resolution CT for the first time enabled the physician to discriminate between parenchyma and various tissues of the organ, and thereby diagnose with accuracy various diseases. It was later on that the introduction of spiral and multi detector CT scanners further improved the diagnostic value of the CT scan. (Muller, 2002) This was many steps ahead of the conventional radiographic techniques in use. For radiographs were not able to differentiate body structures beyond hard tissues and soft tissues, and could not assess accurately the densities of the various organs.
CT imaging became one of the widely used modalities in the cases of brain and body scans. It especially became a method of choice for cancer patients, where detection and metastasis were known with increased accuracy. CT then found its way into the brain scans especially in patients suffering from stroke or brain occupying lesions. Later on CT became a good method to study the lungs and the thoracic region. However, the introduction into the cardiac CT was delayed considerably. There were many reasons why early CT systems were not used in cardiac imaging.
The first challenge was the small size of the coronary arteries that were aimed for imaging, along with the tortuous path as well as movement due to cardiac and respiratory motion. (White and Reed, 2004) The tortuous course of the arteries makes it very difficult to locate and much less trace them with accuracy. This technique virtually became useless due to this draw back for heart cases, as coronary artery blockages could not be viewed. The technique factors that made this procedure difficult at the earlier stages included the lack of speed and poor temporal resolution in the initial CT models.
The heart is especially difficult to view and scan due to the continuous motion of the heart. This makes viewing of the coronary arteries even more difficult due to their small size. The introduction of the first models of electron beam tomography was not successful due to its expensiveness and therefore was not employed in many hospitals for a long time. The multi-detector and the spiral CTs were the next in line in the evolution of the CT scans. This method was in many ways superior as it gave high details of the small coronary structures.
Indeed the diagnostic and detail value of the spiral CT has been compared to that of conventional angiography, making it a good diagnostic procedure for various heart conditions as well. The only problem that the physicians encountered with CT was the lack of resolution in the images. Again this was attributed to the constant movement of the heart, and the changes that took place due to breathing. These problems still cause difficulty in the imaging with the current models. With increased advancements in the diagnostics, it is hoped that this problem will also be taken care of.
(Muller, 2002) The concept behind the CT scan is the acquisition of multiple radiographs of an area under review. These radiographs are taken at different angles around the organ to be viewed, and therefore aims to collect as much radiographic images within one sitting. These multiple x-ray images are combined together to create multiple cross sectional views of the organ or area under study. These methods can either use a contrast or not, depending upon the type of procedure. The contrast used is generally iodine, but others are also used albeit occasionally.
Now with the recent advancements in the procedure, the use of multi-detector scanning is undertaken, which result in high resolution and quality images of the heart. (History of Innovations, 2006) These scans with the contrast are able to provide very high resolution images and two dimensional and three dimensional views, thereby revolutionizing the imaging process of the diagnostics. The CT scan has made it possible to fuse the visual as well as virtual demonstration of the organs on to a screen and thereby giving accurate assessment of the various body functions and organs.
The areas where these diagnostics are seeking improvement include improvements in speed, quality and dimensions of the display, and improved access to the data provided. Another area of development includes the reduction in the intensity of the X-ray beams to reduce adverse effects. Since the anatomical information of the individual patient is reproduced exactly, the treatment plan is more accurate and in lieu with the patient needs. In many respects, the current methods of CT and MRI can be termed as virtual dissection of the body, as it is able to provide visualization of the organs, possible only through anatomical dissection in the past.
(Robb, 2006) This technique has been found to be especially useful in emergency cases of cardiac pathologies, where rapid scanning can help save lives. CT scanning had introduced three new advantages to the radiographic imaging of the past. The image produced was digital, which made it possible to manipulate it, and magnify it considerably to understand the pathosis. The differences of tissue density were well understood which was not possible with conventional radiographs, and various cross sectional views help establish a 3 dimensional view which was limited to one dimension with radiographs.
These developments have taken years to take place. However now, thanks to these technologies, it is possible to image a beating heart, and blood flow in the heart, brain, lungs and the extremities. (Robb, 2006) The introduction of CT scan in the diagnostic modality of heart conditions is not an old concept but is a recent modality. Initially it was cardiac catheterization and angiography that dominated the heart diagnostics. These procedures although were very successful and accurate, still involved an element of invasive surgery in them. Patients were more anxious with these procedures.
With the introduction of multidimensional CTs, there is new hope for a less invasive diagnostic procedure. If contrast is not injected, the CT scan of the heart is completely non invasive in nature. CT imaging can diagnose the aneurysm formation in the aorta, and detect any possible diseases of the heart taking place, including congenital problems. With the electron beam CT, the diagnosis of calcium buildup with in the arteries can take place. The diagnosis is far more superior to other invasive procedures, as a three dimensional image is achieved, which helps to identify the exact sequence of events taking place in a patient.
While ECG and EKG were used to predict the changes taking place in the heart, this technology removes heart diagnosis from prediction to confirmation. Even the simpler of the CT scans can help create a very vivid image of the pathology taking place. The multi-slice CT is unique in that it is able to create a virtual 3 dimensional structure of the patient’s heart, enabling the physician to clearly see the problem areas. The procedure is not invasive, is less time consuming and is less expensive for the patient.
The surgical procedure is virtually eliminated in the cardiac diagnostics. It is here that the introduction of cardiac MRI must also be taken into consideration. MRI use in the conditions of heart began almost at the same time when CT scans were being used. MRI in many respects has preceded the use of CT scans in cardiac imaging. MRI works on the principals of radiofrequency and magnetic waves, which creates high quality pictures of motion. (History of Innovations, 2006)This immediately eliminates the need for any x-ray exposure.
The magnetic waves are able to identify early signs of diseases. The images can then be adjusted for static modes or for dynamic modes. MRI have been preferred for the imaging of the heart due to decreased x-ray exposure and increased area of visuography. Abnormalities in the cardiac movements are easily seen through this procedure, along with the patterns of blood flow in the heart and great vessels. Any areas that are receiving less blood supply in the heart can be also diagnosed, which can help in prevention of any disease.
Indications for the use of MRI in the heart include ischemic heart disease, thick wall of the myocardium, right ventricular abnormalities, pericardial disease, tumors and valvular diseases, and many more. (History of Innovations, 2006) INDICATIONS FOR THE USE OF CT SCAN OF HEART CT imaging for heart is currently being used for a variety of purposes. These can be classified into CT imaging for calcium scoring, cardiac anatomy assessment, coronary artery evaluation, functional cardiac assessment and cardiac perfusion.
With the wide availability of the procedure in many hospitals, the CT scan of the heart holds much potential. One of the most exciting applications of the CT heart is in the emergency departments, where CT scans can save considerable time and help in rapid diagnosis of the condition of the patient. CT heart is an ideal diagnostic for patients arriving with chest pain, for placing biventricular pacemaker leads and for percutaneous coronary interventions for people having chronic total vascular occlusions.
It is also very helpful in the new arrived cases of cardiomyopathy and in discrimination of ischemic from non ischemic causes of the heart pathology. The correct identification of plaques within the patients can also take place in short time through this procedure. The reason for the increase use and application of CT heart are because it is a safe and reliable procedure and is highly accurate with no invasive nature. It is an ideal tool for correctly identifying heavy calcifications in vessels that are 1. 5 mm or more in diameter. (Escolar et al, 2006)
With the ease of handling and the short duration of time that is needed for the procedure, many hospitals in the United States are switching to CT scanning of the heart. The costs of this intervention are far less than those for cardiac angiographies. With over 40% of the American population at risk of cardiac diseases, earlier intervention, diagnosis and screening can result in significant drop of mortality and morbidity rates. Among such interventions is the introduction of screening programs that aim to encourage people who are motivated and to increase awareness among the public.
CALCIUM SCORING OF THE HEART: The role of calcium in the prediction of cardiac diseases is very important. Patients with high blood calcium levels are more than ten times at risk of developing cardiac problems in the future. There are many CT techniques that can be used for cardiac calcium assessment. These include EBT and Agatston scores. The role of MDCT is still under trial investigations, and its efficacy is being tested. The new addition of two new measurements is calcium volume and calcium mass respectively. (White and Reed, 2004) A big area of debate in this procedure is the cost of the CT scan.
Many researches think that it is unnecessary to carry out coronary calcium scores, when preventive strategies such as aspirin use, and advice to adopt a healthy life style are present. The coronary artery calcium scores are now one of the best methods to detect and predict cardiovascular events in patients who may be asymptomatic at the time. The current CT scan modalities are extremely sensitive to detect any calcium deposition that may be taking place in the patient’s arteries. Studies have shown strong graded association between CAC scores and cardiac events.
This method has become popular as it is able to timely identify asymptomatic yet high risk patients. However, there is need to collect data and adjust it to different ages and sex of the people to come up with accurate predictions. Other complicating factors such as diabetes or history of cardiac diseases may be an important indicator and modifier of the cardiac event. This association is the reason why many hospitals are now employing CT scanning for screening purposes among individuals. (LaMonte et al, 2006) UNDERSTANDING THE ANATOMY OF THE HEART:
With more detail and resolution of the CT imaging, it is now possible to identify the true dimensions of the structure of the heart. Cardiac chamber size and the thickness of the walls of the heart can be accurately assessed. Apart from the right sided valves, most of the structures of the heart are easily deciphered. Tissues and areas that can be easily delineated include the mitral and aortic valves and their thicknesses, thrombi present at the apex of the left ventricle, left atrial appendage, cardiac tumors such as myxomas and metastasis, and congenital heart conditions.
The left atrium and pulmonary veins are now among the newer areas where cardiac CT is used in order to plan atrial fibrillation ablation therapy. (White and Reed, 2004) Understanding the anatomy of the heart can help in carrying out any surgical procedures that may be required by the patient, and may aid the surgeons in the correct placement and sequence of the surgical procedure. It can also help in deciding whether a diagnosis and intervention required will be of invasive or non invasive nature. (Escolar et al, 2006) FUNCTIONAL ASSESMENT OF THE CARDIAC TISSUE: The procedure is as effective an assessment tool as it is a diagnostic one.
With the evolution of scanners in the MDCT design, normal functions of the heart such as ejection fraction, stroke volume, wall motion, and wall thickness can be easily identified. (White and Reed, 2004) DELINEATING CARDIAC PERFUSION AND QUALITY: CT scans can be used very successfully to understand and delineate the cardiac perfusion rates in different areas of the heart. (White and Reed, 2004) The visualization of the coronary atherosclerotic plaque is another important diagnostic achievement of this procedure. MDCT is able to assess the presence, amount, and composition of the non calcified atherosclerotic plaques.
It can also assess the degree of remodeling that has taken place in the proximal segments. While this specification is up to 90% accurate in the case of mixed and calcified plaques, same cannot be said for completely uncalcified plaques, where accuracy drops to 60% to 85%. (Hoffman et al, 2006) CORONORY ARTERY STATUS MEASUREMENT: The identification of any disease process in the coronary artery is the main method to predict and prevent any cardiac mishaps. Different methods can be used to understand the topography and condition of the coronary arteries and their trees.
This is especially useful in patients with increased heart beats, where concomitant dose of beta blockers can enhance the visualization of the coronary artery anatomy. The introduction of sub millimeter techniques has widely improved this diagnostic procedure, and the use of different modalities such as curved planer, thin-slab and volumetric techniques largely enhance the final results of the diagnostics. These methods are especially useful to evaluate the status of the cardiac stents and bypass grafts in cardiac patients, and any pathological change that may be taking place in it.
It can be a very helpful tool after cardiac surgeries to assess the patency of the various arteries such as the coronary and the internal mammary artery. Similarly, any anomalies in the coronary tissue are also easily recognized. (White and Reed, 2004) Coronary artery stenosis can be very confidently predicted by the use of MDCTs especially the 64-slice MDCT variant. This method has been able to identify significant stenosis in smaller coronary artery segments as well as side branches. (Hoffman et al, 2006) CONTRAINDICATIONS FOR USE OF CT SCAN OF HEART
CT scan in the heart is a relatively safe procedure and can be carried out with no side effects in a majority of the patients. However, certain cases may not be the best candidates for the procedure. The most common of these are women who are pregnant, as the increased dose of radiation may be harmful for the child. In many of the cases, the use of iodine takes place as a contrast medium. Therefore, patient must be asked about history of allergy to shellfish, iodine, or any medications. Patients who are undergoing radiotherapy may not be ideal candidates for the procedure.
CT heart is not an ideal procedure for those who are above 60 years of age, or have any kidney problems. This is because the excretion of the contrast medium may be compromised in kidney patients. (History of Innovations, 2006) DIFFERENCE BETWEEN CONVENTIONAL AND CARDIAC CT SCANS: Cardiac CT scans are very different from CT scans of other systems and organs. This is due to the high resolution that is required. For this a 16-detector-row CT scanner is used. (Becker, 2003) The speed of the table is also reduced considerably to only 6mm/sec in order to gain more exposures and images.
The result is an increase in the number of radiographs which increases the resolution. The procedure therefore is of longer duration than other CT procedures. In many of the cases, concurrent ECG tracing is carried out so as to correlate the various cycles of cardiac contraction to the CT images. At the moment the detector rows available are in two forms, the 4-detector-rows and the 16-detector row. 16 detector rows are preferable due to the reduced scanning time, small increase in the dosage of radiation when compared to 4-detector-rows.
The image produced is clearer for 16 detector rows than with 4 detector rows, and even small structures such as the right coronary artery is better visualized with the 16 detector. (Becker, 2003) CONTRAST MEDIA IN THE CT OF HEART: The use of contrast media depends upon the type of procedure that is being carried out. For example there are two variations of the MDCT procedure. The first one, the calcium scoring CT is utilized without the use of contrasts and instead utilized the ECG gating. However, CT angiography requires the use of contrast media, so that multiple cardiac phases are reproduced easily.
(White and Reed, 2004) With the advent of faster CTs there is renewed emphasis on the use of contrasts for detection purposes. The technique for contrast is very specific and therefore requires it to be adequate, consistent and providing homogenous contrast in the entire coronary artery tree. Proper contrast is essential for two dimensional and three dimensional findings to take place. The trick is to introduce an amount that simultaneously delineates the structure and allows identification of abnormalities and masses without obscuring it.
This issue has been the biggest problem in the contrast CT scans for heart, and has resulted in false positive and false negative results in the past. The indications of this technique include CT coronary angiography techniques, contrast enhanced CT of coronary artery anomalies, by pass grafts and stents, and CAD detection. (Schoepf, 2004) 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) CT IMAGING WITH ECG: ECG is one of the popular methods for reconstruction of the images received from the CT scan of the heart. ECG as mentioned can work either prospectively or retrospectively for the reconstruction technique.
However, the methods are limited in cases where a pacemaker is present in the patient, as it is the pacemaker that directs the contraction of the heart. Other problems include the inability to record premature ventricular contraction. (United States Patent 6721386, 2004) Any improvement in the image reconstruction will require the adjustment and removal of the problems mentioned above. The new techniques utilize both ECG signaling and input as well as the input of mechanical motion signals. By correlating these two sets of variables, a correct reproduction of the various phases of the cardiac cycle can be established.
(United States Patent 6721386, 2004) In order to make this intervention successful, the apparatus required will comprise of three components. These include the imaging device, the ECG monitor and a cardiac motion sensor. Each instrument has a certain function to fulfill. The radiation imaging device will obtain the CT images of the patient. The ECG equipment will record the cardiac data of the patient and the motion sensor will detect cardiac data associated with mechanical motion of a cardiac region of the subject.
The computer and the software then synchronize and compare the information from the three sources and compiles to give a final result. This method is ideal for 3 D visualization of the heart, and in identifying the topography of the associated vasculature. Calcifications are easily diagnosed through this process, and therefore this system has applicability in cardiac scoring of the patients. This process is also good in procedures such as contrast angiographies, and is able to give detailed account of any abnormalities in the cardiac function due to the present of the ECG as well as the motion sensors.
(United States Patent 6721386, 2004) PROBLEMS AND POTENTIAL ERRORS IN EVALUATING CARDIAC CT RESULTS; There are many factors of the procedure that must be clear in the mind of the operator and the physician before evaluating the cardiac ct results and findings. The type of image evaluated is the first and foremost issue. Experts claim that the best image evaluations are carried out by the original axial images obtained, and therefore, must be given priority over other images and angles. This is especially so in the cases of coronary artery stenosis evaluation.
The multiplaner reformatted or MPR images should then be used as an aid to the axial images to reach diagnosis. The identification of any plaques in the lumen should then be confirmed using long axis views which would give a cross sectional image of the area. This improves the diagnosis of the findings considerably. The visualization of the stents may not be optimal in the CT angiographies. This is due to the high density of the stents, coupled with beam hardening artifacts from the stent struts. Understanding anatomy of the coronary veins is important to identify any variations and not to mistake them with any pathology.
Identification of artifacts such as motion, misalignment and slab artifacts is essential for proper identification of any cardiac pathology. Other artifacts may include blooming artifacts, beam hardening artifacts and respiratory artifacts. (Hoffman et al, 2006) These artifacts can be outlined as those that are pertaining to the patient, those that are procedure related and finally those that are reconstruction related. Among the patient related factors obesity is a main issue in CT scanning. Those patients who have weights above 80 kgs or those who have large breast may have artifacts in their imaging.
Since this procedure requires the patient to hold their breath for 8 to 10 seconds, failure to do so may result in faulty results. Motion relation artifacts can be eliminated by calming the patient and removing anxiety. The present of metal due to bypass surgery can also lead to streak artifacts in the results. (Telkar, 2007) Procedure related errors may be due to improper placement of the electrodes, improper or poor contact and poor planning of the procedure. The use of contrasts in the procedure can lead to two types of artifacts, known as the streak and the slab artifacts respectively.
(Telkar, 2007) The reconstruction related artifacts can be due to high or variable heart rates. There are many methods employed to remove or minimize these artifacts. Multisector reconstructions are a good way to minimize artifacts. ECG traces can be manipulated by placing the syncs in the correct position or inserting extra syncs. (Telkar, 2007) One of the biggest concerns in the use of CT cardiac is the high dose of radiation that the patient is subjected to. These doses have been recorded to be as high as 15mSv. For the reduction of these high doses, many strategies are employed.
These include X-Ray beam filtration, X-ray beam collimation, X-ray beam current modulation and adaptation for body habitus. The most common method however, is the dose modulation procedure where the X-ray tube output is changed during the various stages of the cardiac cycle. This dose is increased to 100% in diastole, while is minimized to 20% in systole. The effect is the reduction of the total radiation dose by almost 50%. (Fishman, 2006) One of the recent researches have put many others in concern about the false positive results of the CT scans.
A report in Science Daily has claimed that the new PET-CT scanners along with their soft wares are predicting as high as 40% false positive in patients and is indicating coronary disease in them. (Science Daily, 2007) According to the report the researcher K Lance Gould claims that there are many false negatives regarding coronary artery diseases due to this procedure. Similarly in 23 percent of the cases, the PET-CT scanner has shown high abnormalities in the structure of the heart. Many factors are considered to be the cause of this faulty detection.
The inability of the software to identify the changes due to breathing and heart rhythm can cause false positive results, which can be dangerous in an inexperienced physician’s hands. Gould claims that only a perfectly conducted technique results in accurate results. PET is a recent intervention in diagnostics of the heart, and has been predominantly used for brain, lungs and body imaging. Failure to appreciate the differences in the cardiac imaging is the reason for faulty results. However, Gould does admit that there are certain advantages of the technique.
Again, if done properly, the technique is highly accurate. It is also a fast way of obtaining information and has a short time for the procedure. It has been very much credited for good diagnosis of the heart. Until the doctor or the physician is able to identify the technological aberrations in the technique, Gould recommends not using it or consulting with more experienced personnel. (Science Daily, 2007) THE NEW DIRECTIONS IN CT IMAGING OF THE HEART: With improved hardware and software systems being introduced in the market, the evolutions i