Multislice Spiral CT Scan

The incidence of ischemic heart disease is increasing in Thailand, especially in the high risk group; diabetes mellitus, hypertension, high cholesterol levels, smoking, older age, and family history of ischemic heart disease. Ischemic Heart Disease (IHD), otherwise known as Coronary Artery Disease, is a condition that affects the supply of blood to the heart. The blood vessels are narrowed or blocked due to the deposition of cholesterol plaques on their walls. This reduces the supply of oxygen and nutrients to the heart musculature, which is essential for proper functioning of the heart. This may eventually result in a portion of the heart being suddenly deprived of its blood supply leading to the death of that area of heart tissue, resulting in a heart attack. As the heart is the pump that supplies oxygenated blood to the various vital organs, any defect in the heart immediately affects the supply of oxygen to the vital organs like the brain, kidneys etc. This leads to the death of tissue within these organs and their eventual failure or death.

Symptoms and Signs Quite often, the first sign of Ischemic Heart Disease may be the severe chest pain of Myocardial Infarction, which may be fatal. However, the warning symptoms occur in a large number of patients in the form of Angina Pectoris. The main symptom of Angina is pain over the central chest that may sometimes radiate down the left arm, to the jaw or to the back. The characteristic feature of the pain is that it is usually aggravated by exercise and relieved by rest, though variant forms may occur. The pain is also relieved by the use of nitrates kept under the tongue. The pain is usually described as a sense of compression or tightness in the middle of the chest, and may last for five to twenty minutes. The pain may be accompanied by sweating. The presence of anginal episodes is virtually diagnostic of IHD.

Investigations and Diagnosis1

Electrocardiography (ECG) may be normal in several patients at rest between attacks of Angina. However, during the episodes of pain there may be depression of the ST segment and a T wave inversion in several leads, indicating Ischaemia. In cases of Infarction (heart attack), there will be ST segment elevation in the ECG, which may gradually evolve. An Echocardiogram may help in showing any functional abnormalities in the various cardiac chambers and in assessing the pumping efficiency of the heart. An exercise testing (Treadmill Test) is often indicated in patients who have symptoms but have normal ECG patterns. Myocardial perfusion scanning with radioactive thallium may also be helpful in the diagnosis. Coronary Angiogram provides accurate information about the actual site and extent of the stenosis (narrowing) and helps in deciding the method of therapy required.

Bangkok Hospital has begun using a sophisticated new imaging tool called the multislice CT scanner, which speeds diagnosis and treatment of patients. The multislice CT scanner provides clearer pictures with more detail and in a lot less time than it takes for conventional spiral CT. Many terms for this new CT technique are used: "Multidetector helical/spiral CT", "Multi-row detector helical/spiral CT", "Multislice helical/spiral CT". So far there is no generally accepted term for the modality. The term "Multislice helical/spiral CT" seems to describe the major accomplishment of the technique (acquisition of more than one image per gantry rotation) most properly.

The multislice CT scanner is similar in appearance to the conventional spiral CT. The difference is in the number of images or slices the scanner generates per second. As the X-ray source and detectors move around the patient, the newer multislice CT captures up to four slices simultaneously in 0.5 seconds compared to spiral CT which captures one slice per second. Using the multislice CT, patients do not have to hold their breath for as long, and the technician uses less contrast material. In the next few months, the multislice CT scanner will be even faster with eight rows of detectors available, instead of four. Even scanners with 16 and 32 rows are on the horizon. The greater number of rows allows the scanner to acquire more images per second.

Philips 16 Slice Infinite Detector Technology (IDT) CT Scanner is the only a multi-slice scanner that is fast enough for dedicated cardiac imaging. The Mx8000 IDT delivers 420msec rotation times coupled with beat-to-beat variable delay-algorithm that automatically determines the cardiac phase with the least motion so the clinician doesn't have to. It is capable of acquiring up to 16 submillimeter slices simultaneously - 38 slices per second at a slice thickness of 0.75mm - for fast, high quality image resolution of distal coronary vasculature.

Performance enhancing features of the Philips MX 8000 IDT include: Tach? Technology - A complete, high-speed, multichannel data acquisition system that transfers analog to digital data from the detector array at a revolutionary 1 gigabit-per-second rate via a single 8mm x 8mm computer chip. It is the only mechanism available that is fast enough to allow a large area multislice CT to collect high resolution dynamic focal spot data. The result - low dose and high quality imaging. This technology makes the MX8000 IDT ready for many multislice configurations. Multi-cycle 3D Cone Beam Reconstruction Algorithm is a fast, accurate and dose-efficient algorithm that enables true three-dimensional data acquisition and reconstruction. This will avoid and/or correct for artifacts present in reconstruction by reducing pixel to noise ratio resulting in superb image quality. It makes the MX8000 IDT the world's first true volume CT scanner, acquiring submillimeter, subsecond isotropic detail with every acquisition.

Beat-to-Beat Variable Delay-Algorithm is a gating protocol that allows for consistent motionless images of the heart. Excellent image quality is achieved in patients with high heart rates (up to 115 BPM) without the need for administering Beta Blockers. This variable "delay algorithm" identifies with each heartbeat the cardiac phase with the least motion and automatically reconstructs gated images of that phase based on heart-cycle information acquired during the scan, resulting in artifact-free imaging. The system automatically adjusts to reconstruct images at the point of least motion.

Indications for Multislice Spiral CT scan in cardiac patients :

1. to evaluate the stenotic coronary artery, both interiorly and exteriorly
2. for calcium scoring in the heart chamber and coronary vessels
3. three-dimension anatomical views and two-dimension multiplanar views of the heart to detect abnormal heart muscles and valves
4. software to calculate parameters indicating heart function, such as stroke volume, ejection fraction, cardiac output, right ventricular and left ventricular function
5. patient do not have to hold breath too long, since the examination time is less than 10 sections
6. special features to examine cardiac patient with tachycardia, heart rate more than 120 beats/min without the use of beta-blocker

Multislice Spiral CT scan can also be used to diagnose cerebral vessel abnormalities, such as hemorrhage, thrombosis and brain edema. It shows the pictures in both 2-dimension, and 3-dimension view. Besides this, intraabdominal organs, intrathoracic structures, bone and joint, and vascular system can be applied to this technique as well.

Clinical indication for Multislice Spiral CT scan

1. suggestive symptoms of ischemic heart disease, such as anginal chest pain, dyspnea on exertion
2. inconclusive exercise stress test
3. preoperative coronary evaluation in patients proceeding major surgery
4. postoperative coronary evaluation in patients after coronary bypass graft or coronary stent intervention
5. early detection of coronary atherosclerosis in high-risk group, such as heavy smoker, diabetes, hypertension, hyperlipidemia, family history of ischemic heart disease

Before a procedure

1. drink at least 2 liters of water a day, 2-3 days before a procedure
2. no fasting if calcium scoring is performed, but 6-hour fasting when coronary angiogram is examined
3. take regular medications until the day of procedure
4. arrive at the appointment time, direct phone number is preferred for your convenience

During a procedure

1. history taking, drug allergy, allergic history to contrast media or seafood, and informed consent
2. taking off metallic ornaments and change dressing
3. contrast media is injected at antecubital vein in the arm
4. examination time approximately 20-25 minutes depending on details, no hospital admission
5. blood pressure and oxygen level are monitored closely, inform your doctor if abnormal detected
6. examination results are interpreted by cardiologist and radiologist within 2-4 days, in case of urgency preliminary report is available in 4 hours

After a procedure

Drink at least 2 liters of water a day for 2-3 days after the examination, in case contrast media is injected

Patients not suitable for this procedure

1. allergic to contrast media
2. chronic renal failure
3. history of congestive heart failure or pulmonary edema