Computer Tomography Scan Angiography for Carotid Artery Stenosis - Essay Example

Carotid Artery Stenosis Introduction The carotid arteries are the primary arteries which carry fresh, oxygenated blood to the brain and hence it is vital that they function properly. Any anatomical aberration or pathological lesion can severely affect the blood supply to the brain and result in serious consequences. Carotid Artery Stenosis is a condition which develops gradually and its detection at an early stage is vital for undertaking appropriate surgical or medical intervention. Extracranial carotid disease accounts for nearly one half of all cases of patients dying due to cerebrovascular accident (CVA) culminating in stroke (Nadalo & Walters. 2009). The annual mortality figure for patients affected by CVA is 150,000 worldwide and almost 600,000 others suffer morbidity with symptoms ranging from aphasia and blindness to paralysis (Nadalo & Walters. 2009). The authors therefore describe the goals of carotid imaging to be early detection, clinical staging, surgical road mapping, and post therapeutic monitoring. Risk factors for carotid artery Stenosis are identical to those of coronary artery disease and include smoking, sedentary life styles, intake of high fat/cholesterol diet, obesity, uncontrolled hypertension and diabetes. Elderly people above the age of 60 years are more prone to the development of this condition. In cases where carotid artery Stenosis is severe, the ipsilateral cerebral hemisphere might be underperfused, especially when adequate collateral circulation is deficient. This can lead to further complications like focal thrombosis in areas of atherosclerosis (Nadalo & Walters, 2009). Anatomy The twin carotid arteries supplying the head and neck regions, each further divide into the external and the internal carotid arteries (Gray’s Anatomy). The external carotid artery supplies blood to the neck region and exterior regions of the face and head while the internal carotid nourishes the parts within the cranial and orbital cavities (Gray’s Anatomy). The External Carotid is a bit smaller in the child as compared to the internal carotid artery, but in the adult the size is almost equal. It is also more superficial in location at its origin which is located opposite to the upper border of the thyroid cartilage. The branches of the External Carotid artery are divided into four sets. The Anterior Branch which further differentiates into Superior Thyroid, Lingual & External maxillary branches; The Posterior Branch differentiating into Occipital and Posterior Auricular; The Ascending Branch differentiating into Ascending and Pharyngeal and the Terminal Branch sub dividing into the Superficial Temporal and Internal Maxillary branches. The Internal Carotid is much more tortuous in its course and supplies the anterior parts of the brain, eye and its appendages, forehead and the nose. It may have either one or two flexures near the base of the skull and its course follows a distinctive S shape along the Sphenoid bone of the skull. The Internal Carotid has been provided different names depending upon its course and regional relations. The different regions have been named the cervical, petrous, cavernous and the cerebral regions of the Internal Carotid Artery. It is positioned more towards the middle line of the neck. The cervical portion does not give off any branches but the rest three do. The petrous region gives off Caroticotympanic, Artery of the Pterygoid canal, Cavernous, Hypohyseal and theSsemilunar branches; the cavernous region gives off the Anterior Meningeal, Ophthalmic Anterior Cerebral and Middle Cerebral branches and the cerebral region of the Internal Carotid gives off Posterior Communicating and Choroidal branches. The wall of the large arteries such as carotid artery consists of three layers – intima, the innermost layer lined by endothelial cells on the luminal side and internal elastic lamina on the outside, the middle layer containing smooth muscle cells and the outer adventitia made up of connective tissue, fibroblasts and smooth muscle cells (Abayomi. 2004). Case Clinical History The patient was a 59 year old white male with history of hypertension, diabetes and CABG in 1994 (Goldman. 2004). He was an ex- smoker and a pacemaker was implanted in 1997 as he was suffering from bradycardia. In 1999, he developed hoarseness of voice and was diagnosed as positive for laryngeal squamous cell carcinoma. After 2 months of radiation therapy there was improvement in his condition and he did not experience any deterioration until 2003. However, subsequently he complained of dysphonia, diplopia and dizziness. Further tests revealed hypothyroidism and he was accordingly treated with levothyroxin. However, his condition did not improve and he complained of visual disturbance as well as continued hoarse voice. Neurological examination led to the suggestion of radiological studies which included craniocervical computerized tomographic angiogram and intracranial MRI. CTA revealed an aberrant origin of the left vertebral artery from the aortic arch, origin Stenosis ranging from moderate to severe, occlusion of the right internal carotid artery, 60% Stenosis of the left internal carotid artery and severe basilar artery Stenosis. MRI revealed a chronic infarct of the right occipital lobe. This led to the suggestion of a 4-vessel cerebral arteriogram study was suggested and carried out. Right internal carotid artery occlusion was confirmed and so was the left internal carotid artery Stenosis along with severe long basilar artery Stenosis. The condition was considered too risky for any surgical intervention and only pharmacological approach was tried. The patient survived for just six more months. Pathology Any interruption, however brief, to the blood flow towards the brain can result in a transient ischemic attack (TIA) (Nadalo & Walters. 2009). The interruption can be due either to atherosclerosis of the carotid arteries supplying the brain or due to embolus formation, which can temporarily affect the blood flow. Circulating thrombi and hemorrhage in the brain can also result in interruption of blood flow to the brain. Depending upon the region of the brain which suffers the resultant ischemia due to reduced tissue perfusion, symptoms can vary from visual, behavioral, movement, speech and thought process aberrations to frank stroke. During TIA, permanent damage is unlikely and the symptoms may appear for a brief duration of 8-14 minutes with recovery within an hour, although in some cases they may persist for as long as 24 hours. During such episodes, permanent damage to the brain is unlikely as compensatory circulation might improve the situation. Proper diagnosis for the cause of TIA therefore assumes primary importance because if it is due to some hemorrhage rather than blood flow blockade, the traditional treatment of blood thinning agents and anticoagulants used for cardiac artery Stenosis might jeopardize the situation altogether. If TIA is diagnosed for the first time in an elderly patient, chance of frank stroke is 5% in the first month following TIA and 20-25% of the patients are expected to experience a full stroke within the subsequent two years (Nadalo & Walters. 2009). If symptoms of TIA persist for a longer duration than 24 hours, the patient is treated with a regimen recommended for a full stroke incident. Severe unilateral Stenosis of the affected carotid artery can lead to reduced perfusion of the affected cerebral hemisphere which heightens the risk for the development of thrombus within the anterior and the middle cerebral arteries and within other peripheral arterial circulatory branches within the cranium. In case of pre existing atherosclerosis, focal thrombosis can occur. Coexistent cardiovascular diseases, chronic lung disease and attempts to lower systemic hypertension artificially can exacerbate the condition in TIA. Patient Preparation Proper medical history of the patient is essential before selecting the diagnostic procedure to be adopted. Presentation of symptoms in the clinical setting, family history and risk factors should be considered before undertaking diagnostic procedures. Duplex carotid sonography should be used for initial evaluation which can be followed by Computerized Tomographic Angiography (CTA) or magnetic resonance imaging (MRI) scans of the carotid artery suspected for stenosis. Patients with renal disease may not tolerate the high doses of iodinated contrast agents and MRA (Magnetic Resonance Angiography) may not be suitable for patients using implanted pacemakers or on whom ferromagnetic cerebral aneurysm clips have been used. Sedation is also a negative factor as it is required for MRA studies and might be contraindicated in certain patients. A patient undergoing a prospective CT scan needs to undertake a few precautions one day prior to the scheduled scan and needs to be educated about the whole procedure. Any allergies and preexisting conditions such as renal malfunction, diabetes and cardiovascular disorders need to be taken into consideration. Regular laboratory procedures such as blood glucose and urea nitrogen (BUN) levels as well as creatinine clearance tests need to be performed to save from possible complications. Food and water needs to be withdrawn 2-4 hours prior to the actual procedure though the patient may be allowed to sip a little amount of water. A day prior to and on the scan day, the patient should not have indulged in taking of any vasoactive drugs such as sildenafil or nitroglycerine. Intravenous contrast medium needs to be injected before the scan preferably into the antecubital vein and the patient should be psychologically prepared for this. Nervous patients may need a pre scan sedative in order to assist them in staying still during the scanning procedure. In case of carotid artery Stenosis the clinical decision for surgery depends upon the grade/degree of Stenosis as determined by diagnostic studies (Prehn et al. 2008). Although CTA is considered as the gold standard for optimum diagnosis, surgeons prefer less invasive techniques due to safety and economic considerations. Scanning Protocol of CTA The use of the 16-slice multidetector computerized tomographic (MDCT) scanner in conjunction with spiral CT is increasingly becoming a popular mode for the diagnosis of carotid artery Stenosis (Goldman. 2004) as it involves a rapid procedure lasting just 10 minutes on the average and also eliminates the precautions for claustrophobia and presence of metallic implants in patients which are an issue with MRI. The procedure also demonstrates high levels of sensitivity as well as specificity when used for imaging multivascular beds. The 16-detector CT scanners can elucidate resolution of arteries as small as 2 mm in diameter. Scanners with 32, 40 and more detectors are also under development which will further enhance the details elucidation capacity of the scans. Moreover, the amount of iodine-based contrast to be injected for such scans is as less as 100-150 ml and can be safely used in even allergic patients after administration of appropriate pre medications for allergy. Peri-procedural hydration, injection of iso – osmolar contrast and N-acetylcysteine to decrease the incidence of azotemia can be safely employed in patients with renal insufficiency. For the actual scan, the patient is either positioned in supine or ‘modified swimmer’s’ positions in order to eliminate artifacts and maintain stationary position during the scan (Hallett & Flieschmann. 2006). Active participation of the patient is obtained by issuing instructions to maintain stillness during the scan. A standard protocol involves setting the imaging protocol at 0.6 (0.75) mm collimation and a pitch of 0.9 (1.0) (Prehn et al. 2008) employing the Siemens Sensation CTA scanner. Radiation exposure parameters are set at 120 (120) kVp and 270 (200) eff.mAs. this gives a CT dose index (CTDIvol)of 20.66 (15.60) mGy. The field of View (FOV) of 140 and matrix size (512x512) results in a voxel size of 0.27mm x 027mm x 1.0mm. the non-ionic contrast medium is used at a dose rate of 100 ml administered intra venously at a rate of 4.0 ml per second is sufficient for the scan. The scan is usually started by making use of a bolus triggering software with a threshold of 70HU over the baseline. Commercial medical software can then be used for making 3D reconstructions which aid in the mapping the sites of Stenosis. Diameter and cross sectional areas can be measured and the volume of the lesion can be calculated by employing appropriate statistics software. MDCT can be performed either using the single bolus or double bolus injection of the contrast material (Sukuma et al. 2003). In the single bolus method, both head and the neck can be scanned in one session by continuous injection of the contrast material at a flow rate of 3ml per second until a total dose of 100 ml is utilized. When the double bolus method is employed, the head is scanned first after delivery of 70 ml of the contrast material at a flow rate of 3ml/second and the neck is scanned after a booster injection of the remaining dose of the contrast material is injected at a flow rate of 2ml/second, utilizing 30 ml of the remaining contrast material. Both methods yield excellent 3D reconstruction in carotid artery angiography. The above protocol has been effectively used on a Toshiba Aquilion MDCT scanner using the following scanning parameters (Sukuma et al. 2003): 1. Voltage Current: 135kV/250 mA each for the head and neck scans. 2. Slice Thickness: 0.5 mm for head and 2 mm for neck. 3. Table Speed: 3mm/sec for head and 12mm/sec for neck. 4. Reconstruction Pitch: 0.5 mm for head and 1.0 mm for neck. 5. Helical Pitch: 3, in both regions. 6. Scan Range: 80.5-92.5 mm for the head region and 114.0 -180.5 mm for neck region. 7. Scan Time: 26-33 seconds for the head region and 11-15 seconds for the neck region. The above methods yield excellent images which can be reconstructed to give good three dimensional images. Contrast Protocol for the Examination The volume and bolus injection speed during MDCT have been standardized up to a certain extent but may vary due to individual patient characteristics such as cardiovascular condition, renal efficiency, body weight/size and physiological parameters such as cardiac output and central blood volume (Hallett & Flieschmann. 2006). There is an inverse relationship between cardiac output and arterial opacification. Increase in cardiac output results in lower arterial opacification. The contrast medium injection rate and the injection duration need to be individualized according to the physical characteristics of the patient. The increase in injection flow rates and higher concentrations of iodine usually increase arterial enhancement. But the total concentration has to be kept within the maximum limits recommended in Europe (400mg/l) and the United States (370mg/ml), respectively. The total injection volume however is relatively unimportant in CTA as compared to scans in other organs such as liver (Hallett & Flieschmann. 2006). Short injections require higher injection rate while longer duration injections may have a cumulative effect due to recirculation of the contrast medium. Scan timing has an important bearing on the overall result and for slow acquisitions with table speed of 30mm/sec and less injection duration has to be synchronized with scan time (Hallett & Flieschmann. 2006). Contrast medium transit time (tCMT) is an important aspect affecting results and has to be adjusted in patients with poor circulation. However, in modern machines it can be an automated affair with bolus triggering software taking care of the procedure. Additional time delay is programmed according to individual requirements to allow the movement of the table until the beginning of data acquisition. Injection rate and injection duration are the two parameters which can be exploited to obtain better acquisitions. In a study conducted to evaluate single and double bolus injection of contrast medium of the head and neck region, the protocols used for single bolus was a flow rate of 3ml/sec for a total does of 100 ml of the contrast medium followed by a 20 second scan delay before scan, with a total examination time of 70 seconds (Sakuma et al. 2004). In the double bolus group contrast material was initially injected at a rate of 3ml/s until a dose of 70 ml of the contrast medium was consumed and only the head was scanned. A boost injection at a flow rate of 2ml/s to consume the rest 30 ml of contrast medium was given and the neck was scanned after a 15 sec scan delay. In both methods, foci of calcification, arteries and the carotid bifurcations were well visualized (Sakuma et al. 2004). Strategies to reduce radiation Dose The recommendation of CT scans for diagnosis has increased remarkably in the last decade. Radiation injury can result in predisposition to subsequent risk of cancer and other abnormalities, particularly in the young patients. Methods and strategies need to be developed in order to minimize radiation exposure and still obtain the diagnostic picture. Tube current setting is one aspect which can be kept at a low level (Donnelly et al. 2001). Current settings of 80-140 mA have been recommended as safe and it is possible to get a god scan at even as low as 12.5 mA in some cases. It has been calculated that a 50% reduction in tube current results in a 50% reduction in radiation dose. However, 5-20% reduction from the current standard has been accepted as possible (Donnelly et al. 2001). Another factor that can be altered is the pitch which if doubled, reduces the radiation dose by half. Increasing pitch from 1:1 to 1.5:1 decreases the radiation dose by 33% without affecting diagnostic quality (Donnelly et al. 2001). Modern equipment have integrated automatic exposure control devices and software to minimize radiation exposure and injury. Image Display/ Appearance and Analysis (2D/3D Reconstructions) The data obtained through MDCT scans is collected and subjected to inbuilt or external computer software which produce multiplaner reconstructions (MPR), maximum intensity projection (MIP), shaded surface display (SSD) and volume rendering (VR) 2D/3D reconstructions and carotid Stenosis is graded according to NASCET (North American Symptomatic Carotid Endarterectomy Trial)criteria (Saba et al. 2009). Quantification of degree of Stenosis is done by selecting a plane perpendicular to the lumen centerline. Diameter of the stenosed segment is compared with a more distal normal segment of the affected artery. This value is known as ratio percent. Plaque ulceration is measured by outpouching of contrast material adjacent to the carotid wall. Axial images are used to determine plaque type as well as density. Plaque with a density value of Read More