Oxford Textbook of Neuroimaging Edited by Massimo Filippi Neuroimaging Research Unit, and Department of Neurology, Institute of Experimental Neurology, Division of Neuroscience, San Raffaele Scientific Institute, Vita-Salute San Raffaele University, Milan, Italy Series Editor Christopher Kennard 1 1 Great Clarendon Street, Oxford, OX2 6DP, United Kingdom Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries © Oxford University Press 2015 The moral rights of the authors have been asserted First Edition published in 2015 Impression: 1 All rights reserved. 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CHAPTER 1 Computed tomography Qinghua Hou, Elizabeth Tong, Cong Gao, and Max Wintermark Introduction and limit the ionizing radiation harm. Unlike conventional X-ray imaging, CT images are not developed directly from the attenu- Since its first introduction in 1971, computed tomography (CT) has ation of projection X-rays. Instead, slices of specific areas of the evolved rapidly. With the evolution from fan beam to cone beam, body are imaged and divided into a matrix of voxels, the intensity and from axial to helical mode, the increase in the number of detec- of which is calculated from multiple projections from an X-ray tors (from a single detector array to 256/320 detector arrays), and source to a panel of detectors, both rotating in the CT gantry. the advent of dual energy technology, CT now offers very short The attenuation value of an individual voxel depends upon its scan time (even for extensive anatomical coverage), improved spa- contents and can be quantitatively scored in Hu (Hu; called after tial and temporal resolution, and reduced patient dose, both in the inventor of CT), which ranges from –1000 Hu to +1000 Hu, terms of radiation and contrast agent. CT is the imaging modal- with water in the middle (Hu = 0), and air (–1000 Hu) and bone ity of choice in the emergency setting. It is fast, its requirements (+1000 Hu) at the extremities of the scale. Modern CT scan- are low, and it is available 24 hours a day, 7 days a week. Modern ners can reconstruct axial, sagittal, coronal, oblique, 3D, or even CT scanners offer volumetric acquisitions, which improve the 4-dimensional (4D) images. capability to evaluate intracranial and spinal structures in recon- structed three-dimensional (3D) views. With the development of post-progressing software, CT has expanded its applications Single-slice versus multislice computed beyond the evaluation of structural anatomy to include functional tomography scanners analyses such as perfusion-CT (PCT) to assess cerebral blood sup- In very early single-slice CT scanners, a single row of detectors ply (Table 1.1). were arrayed in a line opposite to the X-ray tube inside the CT gan- Despite these technological advances, the basic imaging principle try. These scanners used a narrow beam of X-rays (pencil width), of CT has not been changed and relies on X-ray physics. The radia- which was collimated and developed very little scatter when it tion dose concerns associated with CT remains, although it has passed through the human body. With such scanners, one gantry been alleviated by a number of dose-reduction techniques imple- rotation could only sample one slice. mented by the imaging manufacturers. Nowadays, the radiation In multislice CT scanners, detectors are arrayed linearly in multi- dose of a non-contrast head computed tomography (NCT) ranges ple rows, so that the machine can cover multiple slices in one rota- from 1.7 to 2.5 mSv [1,2], lower than the background radiation for tion (up to 320 slices with the most advanced CT technology). Also, a person living in Boston for a year (approximately 3 mSv) [3] . The a fan beam X-ray is used. The benefits of this geometry are not only estimated lifetime risk of death from cancer that is attributable to limited to obtaining a larger coverage with one single rotation, but a single NCT is 0.01–0.02% [4]. With the advent of dual-energy can also shorten the scanning time. With multislice CT scanners, CT, the radiation dose associated with CT may be reduced further the spatial and temporal resolution, and the contrast resolution are when contrast-enhanced examination is applied [5]. also greatly improved. The latest generation of CT scanners (e.g. 320-dector row CT) Computed tomography technology uses a cone beam of X-rays with a flat panel detector for captur- ing the images. Cone-beam CT scanners can produce 3D images CT is an imaging technique based on X-rays, a form of elec- with a single rotation of the gantry [6] , and also time-resolved tromagnetic radiation with a wavelength range of 0.01–10 nm. CT-angiogram (4D CTA) showing the dynamic progression of the X-ray beams can traverse the human body and be used to create contrast through arteries and veins [7]. an image (they can penetrate, but do no harm), while at the same time the photon energy it carries makes it an ionizing radia- tion, which can cause harm to living tissue (in proportion to the Image reconstruction algorithms absorption). CT imaging uses a specific range of wavelengths of For a considerable time, filtered back-projection has dominated X-rays (wavelength below 0.2–0.1 nm) to obtain good penetration the field of CT image reconstruction. Filtered back-projection runs 4 SectiOn 1 fundamentals of neuroimaging techniques Table 1.1 Comparison of CT, MRI, and single photon emission tomography (SPECT)/positron emission tomography (PET) imaging CT MR SPECT/PET Spatial resolution 0.5 mm 1 mm 4–6 mm Scan time seconds 20–40 minutes 10–15 minutes [22] Availability 24/7 24/7 Business hours Contrast Iodinated-based Gadolinium-based Radionuclide tracer Vascular imaging CTA and CT-venogram MRA, contrast-enhanced (CE)-MRA, MRV Not applicable Functional imaging Perfusion and blood–brain Perfusion and diffusion, 2O consumption, and Blood supply and metabolic rates barrier permeability evaluation metabolism spectrum analysis, connectivity evaluation, blood–brain barrier imaging. analysis, etc. Artefacts Streak artefact (Fig. 1.1), Geometric distortion caused by susceptibility Photon scatter and attenuation beam-hardening. artefacts, chemical-shift artefacts, Gibbs’ artefacts Major limitations Ionizing radiation, low sensitive Longest scan time, metal implant and pace maker Ionizing radiation, limited availability in to posterior fossa lesions contra-indication the emergency setting the projection images back through the image to obtain a rough Computed tomography imaging modalities approximation of the original object (back-projection), and uses a high-pass filter to eliminate the image blurring caused by such NCT ‘back-projection’. Routine NCT is performed in the axial plane, parallel to the orbit- More recently, iterative reconstruction algorithms have been omeatal line, with 2.5-mm thick slices. For cervical spine imaging, developed, originally to reduce the noise of images. These algo- 0.625-mm thick slices are obtained, while 1.5-mm thick slices are rithms consist of a series of sequential reconstructions and the standard for imaging the thoracic and lumbar spine. corrections—forward- and back-projection reconstruction steps, NCT is used for the initial evaluation of many intracranial and and comparison of the projection data with the real measured raw spinal lesions, especially in the acute setting. NCT is the first-line data and correction of deviation in projection between each step. examination for acute traumatic brain injury (TBI) and stroke, The correction is based on the statistical counting of the detected since it is especially suited for detecting ischaemia, haemorrhage, photons in the reconstruction process. The iterative process can be and skull fracture. performed in the raw data domain, in the image domain alone, or NCT is less sensitive than magnetic resonance imaging (MRI) in both [8,9]. for the assessment of posterior fossa and brainstem lesions, due to Iterative reconstruction can reduce the radiation dose associated hard-beam artefacts (Fig.