CT Scanning and Digital Subtraction Angiography - Essay Example

Running Head: CT SCANNING AND DSA CT Scanning and DSA [The Writer’s Name] [The Name of the Institution] CT Scanning and DSA Introduction Multislice CT (MSCT) scanners permit scanning of a big capacity in a very diminutive time. This appears to be perfect for CT angiography (CTA) because the entire abdomen can be scanned while one held breathing by means of only a relatively little amount of contrast media. Multislice CT scanners are better than single slice CT scanners as regards to scanning period, effectual section thickness and dosage of contrast media. Volumetric data allow 3D visualization from any direction of view and, consequently, are better to projection methods such as conventional angiography. Computed tomography angiography has established dominance over conventional angiography as regards to detection of morphological changes and aneurysms of the abdominal capillary and its relevant branches. Multidetector row CT scanners are helpful in the uncovering of aortic aneurysms, stenoses, calcifications and occlusions. By means of axial and MPI projections attained the pre-eminent outcomes. Parallel results were also reported by Wilhelm et al. (Wilhelm et.al. 2000, 172). Their study was conducted to assess different reconstruction methods of helical CTA for the appraisal of haemodynamically related renal artery stenosis as compared to DSA. The most excellent results were acquired with axial projections chased by multiplanar reconstructions. In addition, it is revealed that the 3D-VR mode is also capable to convey dependable information about morphological and pathological modifications in the abdominal arteries. Still prior reports illustrate that helical CT is a dependable method for recognition of occlusion and aneurysms. The major dilemma with CTA seems to be the specific diagnosis of the level of stenosis. It was observed that an overestimation of the level of stenosis in 18% for axial projections was observed, while Rieker et al demonstrated an underestimation in 25% of cases. As regards to this perspective, the cause for the overestimation was that narrow windows were chosen in order not to miss any stenosis. Fewer most favourable bolus timing might, hence, direct to an overestimation of stenosis. The less fine bolus timing may also be the rationale for the low sensitivity of 3D-VR in discovery of stenoses and occlusions. The purpose was to afford an anatomical image of vessels by trouncing all other intra-abdominal configurations. This means location clip planes accurately and opting demonstrative parameters that contest with the contrasted vessels. A feebly contrasted vessel will not be demonstrated. Consequently, the extent of stenosis of minute vessels like mesenteric and renal arteries was chiefly overestimated. An overlying calcified plaque might masquerade the genuine level of stenosis in the 3D-VR and, to some extent, also in the axial reconstruction manner, as reported by Wilhelm et al. (Wilhelm et.al. 2000, 172). In distinction, the results confirm a low level sensitivity for 3D-VR and DSA in the detection of calcifications. In 3D-VR a small calcification might have been veiled by a well-contrasted vessel lumen as, generally, the objective was to show the lumen of the vessel and not the vessel wall itself. (Berg, 1998, 533-40) Digital subtraction angiography (DSA) Digital Subtraction Angiography (DSA) is the procedure to visualize blood vessels with contrast medium in a bony environment by subtracting the pre-contrast image (the mask) from the image with contrast medium. DSA, an electronic technique for imaging blood vessels, is useful in diagnosing arterial occlusion, including carotid artery stenosis and pulmonary artery thrombosis, and in detecting renal vascular disease. After contrast material is injected into an artery or vein, a radiographer produces fluoroscopic images. In traditional angiography, images of blood vessels are acquired on films by exposing the area of interest with time-controlled x-ray energy while injecting contrast medium into the blood vessels. The images thus obtained would also record other structure besides blood vessels as the x-ray beam passes through the body. In order to remove these distracting structures to see the vessels better, a mask images for subtraction is required. The mask image is simply an image of the same area without contrast administration. So, using manual darkroom technique, clear pictures of blood vessels are obtained by taking away the overlying background. In DSA, the images are acquired in digital format through the computer. With the help of the computer, all images would be recorded into the computer and subtracted automatically. As a result, we can have a near-instanteous film show of the blood vessels alone after x-ray. (Wikipedia Web Source) An adapted choice of the present parameters could develop the results for finding of calcifications in 3DVR. (Berg, 2002, 116-24; Prokop, 1999, 101-10) Digital subtraction angiography (DSA) shows restrictions in the exposure of calcifications since small calcifications might frequently only be diagnosed circuitously. (Key, 1987, 1083-88) Additional analysis must pursue to optimize contrast-enhanced CTA. Such enhancements have been observed and conveyed for magnetic resonance angiography (MRA) by Lee et al. (Lee, 2001, 754-66) In this connection, for CT examinations, numerous usual delay times have been accounted for (Bartolozzi, 1998, 679-84; Kirchner, 2000, 368-73) but no thorough optimization has been evaluated for CTA. While comparing diagnostic DSA with CTA, the physician all the time has to identify that if performing a DSA, there is constantly the likelihood of transcatheter therapy in the similar examination. The handling of lesions such as stenosis or pseudoaneurysm is viable at many centres. (Wicky, 1998, 828-33) In the cases like these, CTA means a larger weight of contrast media if it is essential to conduct a conventional DSA for therapy after CTA. Bartolozzi et al also observed the likelihood of substituting DSA by CTA in numerous clinical applications. He emphasizes the reality that 3DVR permits the depiction of anatomical formations with the same viewpoint of human vision, thus making it more convenient to comprehend anatomical associations and signposts between organs. (Bartolozzi, 1998, 679-84) Taking his view into account, results explain that for a radiologist, it is uncomplicated to understand the axial slices properly, while for the curing surgeon the 3D-VR is the most supportive part of the examination. However, in some cases, it is functional, yet for a skilled radiologist, to have a 3D-VR to locate an injury properly. Wicky et al. in this connection, report a traumatic injury at the convexity of the aortic arch. The tear was situated instantly distal to the source of the left subclavian artery and, as a result, on contrast-enhanced axial CT sections, five high-density areas revealed above the aortic arch. In this case, 3D-VR was caring for making the exact diagnosis. Becker et al. maintain this view. (Becker, 1997, 1473-77) They experimented and declared an easier diagnosis of aortic arch stenosis by 3D-VR techniques as regard to axial reconstruction. CTA in axial, MPI and 3D-VR reconstructions is a dependable instrument for the diagnosis of occlusions, aneurysms, arteriosclerosis and stenoses of the aorta and its branches. Numerous injections of contrast medium are necessary, If compared with angiography, CTA may be carried out subsequent a single bolus injection of contrast agent. Furthermore, 3D-VR may be submitted in any projection. (Mollet, 2004, 2265-60) Computer Tomography (CT) Scanning The first of the in-vivo imaging techniques, computer tomography (CT) scanning, came on stream in the early 1970s. The common feature of these procedures is that researchers can produce images of the structure or functional activity of the brains of living people. Computerised tomography (CT, but also known as computerised axial tomography, or CAT) provides structural images. To generate brain scans, low levels of X radiation are passed through an individual's head at a series of different angles (through 180°). A computer analyses each 'image' and generates what is, effectively, a compound X-ray. It can produce a 'slice-by-slice' picture of the entire brain, or other parts of the nervous system such as the spinal cord if required. A drawback of CT scanning is that the contrast between more and less dense tissue is not particularly good, although it can be improved by the administration of a dye (injected into the bloodstream just before the scan is taken). CT scans cannot measure functional activity but they have provided valuable information about structural changes seen in the brains of some people with dementia, and about the effects and location of brain damage in general. CTA and DSA Becker et al. embrace a parallel view when relating spiral CTA and 3D-VR in aortic coarctation. (Becker, 1997, 1473-77) Neri et al. explained the benefits of using a CTA dataset in the assessment of a pseudoaneurysm of the abdominal aorta with virtual angioscopy. (Neri, 1999, 1227-30) In the printed emergency case, the CTA assessment was capable to illustrate the graft and the pseudoaneurysm and the side of dehiscence. (Hartnell, 2001, 35-46) On the other hand, Three-dimensional contrast MRA has developed speedily over recent years. It is at the present a highly exact method of diagnosing vascular abnormalities of the peripheral and abdominal vessels. (Prince, 1995, 785-92) The three-dimensional sequence permits large data sets to be collected. This assists to beat the problems of venous augmentation and respiratory artefacts. (Niendorf, 1991, S-221-23) In comparison with CTA, the contrast medium implied, gadolinium has no nephrotoxic properties, and a lower osmolar weight than ionic contrast media. In this case, allergic reactions are atypical. Nowadays, MRA shows capable results. (Arlart, 1997, 257-63) Di Cesare et al. affirm a specificity and sensitivity of 100% in the finding of aneurysms of the abdominal aorta. (Di Cesare, 2000, 126-32) A number of reports demonstrate that there is little or no benefit in using conventional DSA or CTA if it is compared with contrast-enhanced MRA. (Joarder, 1994, 155-64) Presently, in many cases of aortic aneurysms MRA appears to be the method of choice if an elective examination is done. (Joarder, 1994, 155-64) Limitations of MRA appear to be the accurate diagnoses of stenosis and occlusion of aortic branches. While, Arlart et al. report a sensitivity of 87.5% and a specificity of 97.5% for the revealing of iliac stenosis. (Arlart, 1997, 257-63) Overestimating stenosis based on high display centre and narrow window level are no setback in MRA. CT scans provide reasonably well-defined images of brain structure. Intravenous Digital Subtraction Angiography (IV-DSA) Intravenous digital subtraction angiography (IV-DSA) is a form of angiography which was first developed in the late 1970s. IV-DSA uses a computer technique which compares an x-ray image of a region of the body before and after radiopaque iodine based dye has been injected intravenously into the body. Tissues and blood vessels on the first image are digitally subtracted from the second image, leaving a clear picture of the artery which can then be studied independently and in isolation from the rest of the body. Some studies have indicated that IV-DSA is not suitable for patients with diabetes or renal insufficiency because the dye load is significantly higher than is used in arteriography. However, IV-DSA has been used successfully to study the vessels of the brain and heart and has helped detect carotid artery obstruction and to map patterns of cerebral blood flow. It also helps detect and diagnose lesions in the carotid arteries which are a potential cause of strokes. IV-DSA has also been useful in assessing patients prior to surgery and after coronary artery bypass surgery and some transplant operations. (Answers.com) Multislice CT Angiography During the past few years, extensive research has been invested in the development of non-invasive CAwith MSCT, resulting in a considerable number of publications on the diagnostic accuracy of this technique. In 1998, the first generation of multi-slice scanners was introduced, allowing the simultaneous acquisition of four slices, thereby enabling MSCT systems to visualise the coronary arteries. Reported sensitivities and specificities ranged from 66% to 99%, with weighted means of 80% and 94% respectively. (Franiel, 2006, 2-3) To obtain these results, however, more than 20% of the available segments were on average excluded, representing an important limitation of the technique at that stage.( Marano, 2004, 353-62) More recently, results of the newer generation of 16- slice systems have become available. With these systems, sections as thin as 0.5 mm and a temporal resolution of 105–250 ms can be obtained. As a result, a considerable improvement in assessability (approximately 96%) as well as sensitivity (approximately 88%) has been observed, with no loss in specificity, as shown in Fig. 1. Further refinement is anticipated following the recent introduction of 64-slice scanners, although currently no studies are available regarding the diagnostic accuracy of these systems. (Nieman, 2002, 2051-54) Examples of 64-slice MSCT-CA in patients with normal and abnormal coronary arteries are provided in Figs. 2 and 3, respectively. Fig. 1. Bar graph showing the diagnostic accuracy of 4- and 16-slice MSCT for the evaluation of significant coronary artery stenoses. Assessable = the average percentage of coronary segments that were of sufficient image quality to include in the analyses concerning diagnostic accuracy. Fig. 2. An example of normal coronary arteries obtained with 64-slice MSCT. An intra-myocardial course of the LAD can be observed (b and h, arrows). LAD left anterior descending coronary artery, D1 first diagonal branch, LCx left circumflex coronary artery, MO marginal branch, RCA right coronary artery. Fig. 3. An example of a patient with a total occlusion of the LAD, obtained with 64-slice MSCT. The LAD is occluded (a, b, d, e, arrows), whereas the first diagonal branch and the LCx are diffusely diseased. Conventional coronary angiography showed comparable findings (c, f). LAD left anterior descending coronary artery, D1 first diagonal branch, LCx left circumflex coronary artery. Another application of MSCT that is currently under investigation is the assessment of coronary stents, which are difficult to image with MSCT. Their metal content leads to high-density artefacts, and subsequent obscuring of a considerable part of the stent lumen. In many studies addressing the diagnostic accuracy of MSCT, therefore, stented segments are still excluded from analysis. However, substantial progress has been achieved with the increased image quality of the newer generation of MSCT scanners. With four-slice systems, the stent lumen was virtually invisible, whereas with 16-slice systems improved visualisation has been reported, in particular in stents with either a large diameter or thinner struts (Ropers, 2003, 119-24) . With the recently introduced 64-slice systems as well as the previously discussed dedicated filters, an even higher percentage of stents will be eligible for assessment of patency. The beginning of multislice CT in the department of clinical radiology composes a gigantic leap that notably broadens the extent of vascular CT imaging. (Ropers, 2001, 792-95) The advancements over conventional spiral CT have been considered quantitative, chiefly in conditions of increased image acquirement speed which provides unparalleled degree coverage and spatial motion. Furthermore, noteworthy technical and diverse innovations, such as cardiac scanning potentials, have brought about a qualitative transfer towards applications that were thought to be away from the scope of CT imaging. (Sato, 2003, 401-5) This mode multislice CT offers a affluence of innovative chances for rapidly and exactly diagnosing supposed vascular disease in all organ systems; yet, as we shift towards faster and faster image attainment techniques, we are also in front of new challenges that necessitate progress of novel strategies in order to take full improvement of the amplified capabilities of multislice CT in its existing shape and prospective generations of CT scanners.( Willmann, 2004, 568-77) The diagnosis includes the detection of the onset of a dementing process and, under some circumstances, contributing a more precise diagnosis, such as Alzheimer's disease or multi-infarct dementia. Although physical techniques such as computed tomography (CT) scans have an increasingly important part to play in diagnosis, no amount of scanning can specify exactly what a patient is capable of achieving as far as social or cognitive performance is concerned. (Nieman, 2003, 749-56) Formal psychometric testing may also contribute to differential diagnosis by eliminating alternative explanations for poor cognitive performance, such as depression or concurrent psychiatric disturbance. Conclusion MSCT has been demonstrated to allow non-invasive coronary angiography. It is important to keep in mind that this technique visualises atherosclerosis and not ischaemia. Therefore, the technique cannot be compared directly with the currently available imaging modalities for assessment of coronary artery disease, such as nuclear myocardial perfusion imaging and stress echocardiography. Rather, these techniques visualise the consequences of atherosclerosis and indicate whether ischaemia is present or not. The precise role of these imaging modalities (MSCT for assessment of atherosclerosis and myocardial perfusion imaging or stress echocardiography for assessment of ischaemia) is to be established. A potential scenario could be to use these techniques mainly in patients with an intermediate likelihood of coronary artery disease in a sequential manner. MSCT could first be applied to rule out coronary artery disease; if such disease is present, myocardial perfusion imaging could then be used to clarify the consequences of the atherosclerosis: ischaemia or not. Several studies have compared CT findings to the gold standard of angiography. (Fabian, 2006, 666-77; , Cleverley, 2005, 281-86; Parker, 2006, 1267-72) Angiography has been shown to result in a low, but clinically relevant, rate of false-positive and false-negative (3%) results. (Malhotra, 2006, 1042-48) Studies that base clinical outcome only on patients’ hospital stay, without knowledge of subsequent events post discharge, may miss clinically significant aortic injury. One of the study groups (Wicky, 2006, 828-33) strove to definitively prove the 100% negative predictive value of helical CT in the detection of traumatic aortic injury by using thoroughly documented clinical follow-up data. Researchers performed a detailed follow-up of the 278 members of our study group, using provincial medical databases, and we found no evidence of missed thoracic aortic or proximal great vessel injury during the follow-up period. On the basis of the natural history of untreated traumatic aortic injuries, as described by Parmley et al, less than 2% of patients with thoracic aortic injury owing to blunt thoracic trauma are expected to survive beyond 4 months. Therefore, this study’s minimum clinical follow-up of 183 days (6 months) and median follow-up of 615 days (20.5 months) should be adequate to evaluate for life-threatening missed injury. No injury to the proximal branch vessels of the thoracic aorta was missed during the study period. Because of the infrequency of injury to the great vessels in the setting of blunt trauma (1% to 2%), the number of patients in the study group may not be high enough to establish the negative predictive value of CT for excluding such an injury. (Fishman, 2005, 1071-1027) Previous authors have noted that, if high clinical suspicion of in nominate artery injury exists, angiography should be considered.( Mirvis, 2004, 922-30; Karmy, 2007, 782-87) It is important to note that this opinion did not evaluate for minimal aortic injury. As described by Malhotra et al, a minimal aortic injury is an intimal flap measuring less than 1 cm in length with minimal or no periaortic hematoma. (Malhotra, 2006, 1042-48) Researchers (Dyer, 2005, 195-202; Scaglione, 2005, 2444,48) believe that helical CT did not miss clinically significant injury. We cannot exclude the possibility of unrecognized small intimal tears that did not result in clinical effect. 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