Computed Tomography (Part 1) RST 2024 PDF

Computed Tomography - (Part 1) RST 2024 - Rodnick Vassallo  Learning Outcomes Understand the concept of computed tomography Describe the technological features of a contemporary CT scanner Be able to describe data acquisition and reconstruction in computed tomograp...

Computed Tomography - (Part 1) RST 2024 - Rodnick Vassallo  Learning Outcomes Understand the concept of computed tomography Describe the technological features of a contemporary CT scanner Be able to describe data acquisition and reconstruction in computed tomography Be able to describe the concept of ‘volume’ or helical scanning  What is CT? An imaging procedure that uses x-ray equipment and computer-processed measurements taken from different angles to produce cross-sectional images Tomography - from the Greek word ‘tomos’ meaning ‘a slice’ Previously know as Computed Axial Tomography or CAT scan Invented by Sir Godfrey Hounsfield, a British electrical engineer First person to process x-ray images using a computer  History of CT First clinical CT scan done 1 st October 1971 at Atkinson Morley Hospital, Wimbledon It took 160 readings with each scan taking over 5 minutes Images from these scans took 2.5 hours to be processed on a large computer  Why CT? Limitations with projection radiography – Superimposition of structures – Differentiation of structures with low contrast resolution (subtle difference of shades of grey) CT Scanners Today  Overall CT Process Motorised x-ray tube that rotates in a gantry Fan shaped beam which ‘slices’ the anatomical area being scanned An array of digital detectors located opposite to the x-ray source pick up the attenuated beam Images are processed and viewed in multiple planes Equipment - CT Gantry Internal View https://youtu.be/4jEtTNKM68I?si=KCRIb3zNXMxfDd8P  CT Detectors What is the function of a CT detector? – Conversion of x-rays into an electrical signal Xenon vs Solid State Detector (SSD) Efficiency 60-70% Efficiency 98% Solid State Detector  CT Detector Characteristics Small in size for better spatial resolution Allows for multiple detectors to be combined together with the possibility of forming a detector array High detection efficiency and a high sensitivity to capture/process a wide dynamic range of x-ray intensities A fast response time with negligible afterglow (persistence of an image/signal following radiation) High stability to function under the high speeds of the rotating gantry CT X-Ray Tube  CT X-Ray Tube Longer continuous exposure times at higher KV and mA Larger and thicker anode to absorb and dissipate large amounts of heat Modified cathode assembly to produce a smaller focal spot (0.6mm) Glass envelope replaced with metal envelope and ceramic insulators (metal ceramic tube) – heat dissipation ‘Flying’ focal spot allowing for control of focus position on anode  Advantages of Metal Ceramic Tubes Higher tube loading Longer tube life Spiral groove bearing uses liquid metal alloy as lubricant (gallium)  Straton X-ray Tube New design allowing the entire tube body to rotate rather than just the anode The electron beam in the tube is shaped and controlled by magnetic deflection coil Allows all the bearings to be located outside the evacuated tube and enables the anode to be cooled more efficiently Low heat storage capacity Cooled down within 20 seconds  Slip Ring Technology What is a slip ring? – An electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure  Slip Ring Technology Just like bumping cars get electrical power by brushing against a conductive ceiling, a slip ring has grooves along which electrical contactor brushes slide Transmission of power/data is made possible through electrical connections made by stationary brushes pressing against rotating circular conductors Eliminates the need for any wired connections! CT Scanner Generations  CT Scanner Generations 5th Generation – Developed specifically for cardiac scanning – Known as electron beam scanners (EBCT) 6th Generation – Spiral/Helical CT scanners – Slip ring technology 7th Generation – Multi-detector array CT (MDCT) https://youtu.be/Ni4Hsi3GhXo?si=dPytG66CcIFgeKZZ  Image Acquisition – What Does CT Measure? Attenuation A measure of how easily a material can be penetrated by an x-ray beam. It quantifies how much the beam is "attenuated" (weakened) by the material it is passing through The density of the tissue passed by the x-ray beam can be measured from the calculation of the attenuation coefficient  Attenuation Coefficient The attenuation coefficient (µ) describes the fraction of a beam of x-rays that is absorbed or scattered per unit thickness of the absorber This value varies depending on the number of atoms in a cubic volume of material and the probability of a photon being scattered or absorbed from the nucleus or an electron of one of these atoms  Attenuation Coefficient It = Io e -µx Io µ Io = incident beam x It = transmitted beam µ = attenuation coefficient It x = thickness of body part imaged (e = mathematical constant 2.71828)  Image Reconstruction Unlike x-ray radiography, the detectors of the CT scanner do not produce an image. They measure the transmission of a thin beam of x-rays through a part of the body along a 360⁰ path – this the ray sum The image of the section of the object irradiated by the x-ray, is reconstructed from a large number of ray sums. It gathers together all the ray sums coming from the volumes of material through the detectors to produce an attenuation profile Attenuation profile  Back Projection How is the resulting image actually produced? – CT uses a reconstruction algorithm called back projection – Calculations are worked out in reverse A reconstruction algorithm used in CT to produce the typical cross-sectional images whereby attenuation calculations are worked out in reverse based on the angle they were originally acquired.  Back Projection Example Each sample acquired in a CT system is equal to the sum of the image values along a ray pointing to that sample For example, view 1 is found by adding all the voxels in each column View 3 is found by adding all the voxels in each row The other views, such as view 2, sum the pixels along rays that are at that angle  Back Projection Example A back projection is formed by ’smearing’ each view back through the image in the direction (at the angle) it was originally acquired. The final back projected image is then taken as the sum of all the back projected views This image looks similar to the real picture but is blurry - we smeared bright pixels across the entire image instead of putting them exactly where they belonged  Back Projection Object Image 2 projections 4 projections 8 projections 16 projections 32 projections  Back Projection Algorithm The back projection algorithm can be compared to a large game of 3D Sudoku https://www.youtube.com/watch?v=BmkdAqd5ReY (From 3:12)  Filtered Back Projection To fix the blurring problem created by standard back projection, we use filtered back projection Refers to altering the projection data before the back-projections The particular type of filter needed is a sharpening filter. This type of filter picks up sharp edges within the projection and subtracts out the extra smearing caused by back projection Filtered Back Projection  Image Filtering Image filtering can be further applied using specific algorithms depending on the sharpness or smoothness required – post processing  Image Filtering Increasing the sharpness filter too much will result in noise!  Iterative Reconstruction An image reconstruction algorithm used in CT that begins with an image assumption, and compares it to real time measured values while making constant adjustments until the two are in agreement Computer technology limited early scanners in their ability to perform iterative reconstruction. However, this image reconstruction algorithm is now widely used due to the improvement of computer processing power It provides the ability to overcome noise associated with filtered back projection thus improving image quality, reduce artifacts and decrease radiation dose  Iterative Reconstruction https://www.itnonline.com/article/basics-understanding-what-iterative-reconstruction  Image Matrix The image produced by the CT scanner consists of a square matrix of picture elements (pixel), each of which represents a voxel (volume element) of the tissue of the patient The typical CT image is composed of 512 rows, each of 512 pixels, i.e., a square matrix of 512 x 512 = 262,144 pixels (one for each voxel)  Displaying The CT Image The CT image is made up of a grey scale image of varying densities (levels of grey) Each voxel represents a small section of the body part imaged and has an associated number based on the x-ray attenuation of that section (Hounsfield Units)  Hounsfield Units (HU) A linear scale of grey scale values (densities) based on the measured attenuation coefficients Water and air are used as the reference and given a value of 0 and -1000 respectively  Hounsfield Units (HU) Tissue HU Bone 1000 to 3000 Liver 40 to 60 White Matter 46 Grey Matter 43 Blood 40 Muscle 10 to 40 Kidney 30 CSF 15 Water 0 Fat -50 to -100 Lung -500 Air -1000  CT Windowing Process in which the CT image greyscale component of an image is manipulated via the CT numbers This will change the appearance of the picture to highlight particular structures Window Width (WW) -the measure of the range of CT numbers that an image contains Window Level (WL) -also referred to as window centre, is the midpoint of the range of the CT numbers displayed  Window Width Small Window Width Large Window Width Short grey scale Long grey scale Small block of Large block of CT nos assigned CT nos assigned grey levels grey levels Small transition Large transition zone of white zone of white to black to black Level centred Used where near average large latitude is CT no of organ required of interest Used to simultaneously display tissues greatly differing in attenuation  WW & WL WW contrast WL brightness  Helical CT The x-ray tube is continuously rotating in the same direction within the gantry whilst the patient is continuously moving in one direction (along the z- axis)  Axial vs Helical CT Gantry stops and rotates to Also known as spiral/volume CT acquire data from single slice Gantry keeps rotating X-rays switched off continuously releasing x-ray Patient moves to next slice beams Rotates to acquire data from The table simultaneously moves next slice This results in a continuous spiral scanning pattern  Helical CT Requirements: – Slip ring technology – High power x-ray tubes – Interpolation algorithms Advantages: – Speed; no need to pause between scans for table movement – Longer scans; avoids respiratory mis-registration from different breaths as scan performed during one breath – Pitch can be used to reduce scan time and/or radiation dose and still cover the same volume – Overlapping slices allows better reconstruction and helps in showing smaller lesions; can be done at any position and interval – More effective use of contrast agent as faster scanning enables scanning during multiple phases in one contrast injection e.g. portal venous, angiographic, delayed  Pitch Speed of the table movement through the gantry defines helical pitch A measure of the overlap through scanning  Pitch A pitch number > 1 = table travels more than the width of the beam i.e. there are gaps A pitch number < 1 = table travels less than the width of the beam i.e. there is overlap A higher pitch: – Lower radiation dose – Quicker scan – Lower image quality (lower SNR) as fewer projections obtained A lower pitch: – Better z-axis resolution so better image quality – Higher patient dose  Image Reconstruction - Interpolation The volume scanned in a single rotation differs between axial scanning and helical scanning because of the table movement Interpolation: Estimation of a value at a certain position using known data from nearby points This results in the creation of ‘virtual’ slices  Interpolation Interpolation Algorithm: mathematical process required to reconstruct axial images from the spiral volume data set Modern helical CT averages (or 'interpolates') data from two projections 180 degrees apart. This interpolation is done to convert the helical path to transverse slices  Slice Thickness vs Slice Interval ‘Acquired’ slice thickness: – The thickness of each slice set in the scan parameters – Smallest thickness cannot be less than the smallest detector size (but can be more) ‘Reconstruction’ slice thickness: – Will depend on the acquired slice thickness – Determined in the reconstruction parameters  Slice Thickness vs Slice Interval Slice thickness determines the trade-off in image quality between spatial resolution and image noise Thinner slices = increase in spatial resolution but also increase in image noise Slice of bread analogy (the thicker the slice, the less detail is visible) https://youtu.be/lqA823kTlGY?t=4  Slice Thickness vs Slice Interval The distance between the centre of two adjacent slices Determines the number of images in a series Contiguous: Interval = Thickness – where one slice ends, the next one starts Non-Contiguous: Interval > Thickness – some areas of anatomy will be missed between two adjacent slices (creates less images in the series) Overlapped: Interval < Thickness – some areas of anatomy will be shown in two adjacent slices (creates more images in the series) Slice Thickness vs Slice Interval  Scanning Parameters vs Reconstruction Parameters  Scanning Parameters vs Reconstruction Parameters  Scanning Parameters vs Reconstruction Parameters  Revise – Key Concepts 1 Why was CT necessary? What is CT measuring? What is the function of a CT detector? What are the main characteristics of a CT detector? What are the features of a high-powered CT X-ray tube? What are the functions and benefits of slip ring technology? How are CT images reconstructed? What is back projection/filtered back projection? What is iterative reconstruction and why is it an improvement of filtered back projection?  Revise – Key Concepts 2 What are Hounsfield units and what is their significance? What is CT windowing and how are Window Width and Window Level applied? What are the advantages of helical CT and how does it compare to axial scanning? What is pitch and what impact does it have? What is interpolation and how is this used in helical scanning? What is the difference between acquired and reconstructed slice thickness? What is the implication of having thinner/thicker slices? What does ‘slice interval’ refer to and what slice interval options are available? What is the advantage of image reconstruction?  Further Reading http://www.wikiradiography.net/page/Slip+Rings https://www.dspguide.com/ch25/5.htm http://xrayphysics.com/index.html https://radiopaedia.org/articles/computed-tomography?lang=gb http://www.sprawls.org/resources/CTIMG/module.htm http://www.impactscan.org/slides/impactcourse/basic_principles_of_ct/index.html https://www.sciencedirect.com/science/article/abs/pii/S0720048X18303747 https://www.radtrain.com.au/post/copy-of-ct-slice-thickness-and-interval-explained  Questions? [email protected]