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IELTS Writing Made Easy
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Flag for inappropriate content. Download now. Related titles. Carousel Previous Carousel Next. Jump to Page. Search inside document. CT images are formed from a matrix of voxels each with an assigned number representing the X-ray attenuation coefficient of a corresponding volume within the patient b. Filtered back projection is a process in which the number assigned to each voxel from each projection is adjusted by a function of the measured amounts in the neighbouring pencil beams c.
Each voxel is measured independently by a pencil beam falling on a particular detector. CT has a higher spatial resolution than conventional plain film radiography ce.
CT has a better contrast resolution than conventional plain film radiography Image formation in CT: a. The detectors sample a continuously produced X-ray beam approximately times per rotation b.
Each voxel in a particular image slice is represented by a single detector in the array c. The log of the ratio of the unattenuated beam to that detected is equal to the sum of linear attenuation coefficients of each voxel the beam passes through d. CT requires higher doses compared with conventional radiography since the beam suffers more attenuation for the same projection e. Scout views survey radiographs are produced line by line at a fixed projection angle e. Image reconstruction in CT: a.
Utilizes iterative processes b. With an infinite number of samples an object could be perfectly reproduced c. With back projection each object in the scanner field contributes to every voxel in that image slice d. Filtered back projection convolves the image profile fe. The effect of quantum noise in an image is reduced by narrowing the viewing window b.
Spatial resolution in the z-direction is unaffected by pitch c. ACT image is most commonly calculated on a x matrix d. Scanned projection radiographs are performed to allow planning of the CT sequence e. Ranges of Hounsfield unit HU and window settings: a. The CT number or HU for air is 0 b. The normal HU range for fat is to c. The normal HU range for muscle is 40 to 60 d. Soft tissue window settings might include a window width of and window centre of 30 HU e. Lung window settings might be: window width and window centre HU 6.
Regarding CT images: a. CT angiography can utilize maximum intensity projection MIP to enhance image quality b. A structure with an average CT number of is likely to be made up predominantly of muscle c. Filtered back projection is not required in image reconstruction when using multi-slice detectors CT gantry and scanner generations: a.
Modern multi-slice CT scanners are examples of 3rd generation scanners b. Multi-slice scanners with an excess of detector rows are unlikely to be produced d. The tube and detector array rotate at approximately 60 revolutions per second in modern CT scanners e.
CT X-ray tubes: a, The CT tube is mounted with its anode-cathode axis perpendicular to the axis of rotation of the scanner to minimize the heel effect b. CT tubes typically use a focal spot size of approximately 0.
The anode target material used in CT is rhodium because of its higher energy characteristic radiation e. CT anodes may require heat capacities of 4 MJ or more and typically require active cooling mechanisms 9. Collimation and filtration in CT: a. Filtration in CT is typically 2. Bow tie filters are used to minimize variance in beam hardening caused by the elliptical shape of the patient c.
To produce thin 0. Post-patient collimation is essential in multi-slice CT to reduce scatter e. The width of the collimation at the level of the axis is typically about 50 cm CT detector technology: a. Xenon filled ionization chambers are used in most modern multi-slice CT scanners b. Solid-state detectors are formed from a scintillant such as cadmium tungstate and a silicon photodiode c. Should have negligible afterglow d. Separation of detectors to prevent light crossover increases the detection efficiency of the array e.
Solid-state detectors can be produced to a width of approximately 0. A pitch of 1 indicates a contiguous data-set b. Settings of kV and mA might be used c. Are unchanged when scanning adults and children d.
Matrix size can be reduced to x in CT fluoroscopy to enable realtime reconstruction of images e. Regarding spiral and multi-slice CT: True spiral CT was not possible before the advent of slip ring technology b.
Means that several parallel beams are used in data acquisition cc. The number of slices a scanner is capable of producing per gantry revolution is determined by the number of detector rows d. Slice width is determined by collimation e. Beam divergence in the z-axis is a potential problem Noise in CT: a. Noise limits CT as it reduces contrast and limits the spatial resolution of small low contrast objects b. Quantum noise is the most significant type in CT c.
SNR may be improved by increasing the mA or the scan time per rotation d. SNR may be improved by decreasing the slice width on a multi-slice scanner e. Isotropic voxels describe any cuboidal-shaped voxel b. Axial CT images acquired using most modern scanners can be reformatted to coronal and sagittal images without losing any data quality c. MPR techniques can allow oblique sections to obtain true coronal and sagittal images even if the patient was rotated in the scanner d. The isotropic voxel can be used to create a three-dimensional 3D data map c.
Photon starvation occurs in obese patients b. Beam hardening artefact means CT numbers in the core of the patient tend to be higher than they should be c. A ring artefact can occur if a single detector is faulty d. Partial voluming can potentially make very small highly dense objects appear larger e.
Cone beam artefact does not occur for objects situated close to the centre of rotation of a multislice scanner Image artefacts: a. Photon starvation can be seen as a dark ring starved of photons by a detector fault b.
Slice misregistration can be minimized with spiral scanning techniques Motion artefacts may occur even on rapid spiral CT techniques. Streak artefacts rarely cause any significant degradation of the image in modern scanners due to metal correction algorithms e. Dose in CT: a. Multi-slice always requires more dose than single slice b.
Pitch is inversely proportional to dose in CT c. In spiral CT, thinner slices lead to an increased dose for the same scanned volume e. Automated modulation of the KV compensates for patient size so that variation in dose to the detector is small and therefore quantum mottle remains similar for all patients Regarding CT dose: a.
CTDI is routinely measured using a thermoluminescent detector b.
IELTS and TOEFL made easy by GEORGE JOHN
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Writing Academic English Pdf Writing clear paragraphs; Writing clearly, concisely and precisely; Signposting; Paraphrasing, summarising and quoting; Editing and proof-reading your work; Assignment types. However, all School programmes are English-language-based, and all students are expected to have a standard of written English sufficient to demonstrate their academic competence. No part of this publication may be reproduced in any format including photocopying or storing it in any medium by electronic means without prior written permission of the publishers or a licence permitting restricted copying issued by.
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The repetition of questions enables the students to solve them in an excellent. This test has been based upon the conduction. The GRE test is divided into three main sections. The Analytical Writing section is always presented first.
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