BASICS IN COMPUTED TOMOGRAPHY

INTRODUCTION Designed by Godfrey N. Hounsfield to overcome the visual representation challenges in radiography and conventional tomography by collimating the X-ray beam and transmitting it only through small cross-sections of the body

G.N.HOUNSFIELD ALLAN M. CORMACK In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology & Medicine with Allan MacLeod Cormack, Physics Professor who developed solutions to mathematical problems involved in CT

Important events YEAR EVENTS 1969 G.N. Hounsfield developed first clinically useful CT head scanner 1971 First clinically useful CT head scanner was installed at Atkinson-Morley Hospital (England) 1972 First paper on CT presented to British Institute of Radiology by Hounsfield and Dr. Ambrose 1974 Dr. Ledley introduced the whole body CT scanner (ACTA scanner) 1979 G.N. Hounsfield shared the Nobel Prize with Allan MacLeod Cormack

C.T. scan Computed tomography scan machines uses Xray CT combines X radiation and radiation detectors coupled with a computer to create cross sectional image of any part of the body Think like looking into a loaf of bread by cutting it into thin slices and then viewing the slices individually

BASIC PRINCIPLE The internal structure of an object can be reconstructed from multiple projections of the object CT scanning is a systematic collection and representation of projection data

Comparison of CT with Conventional Radiography Conventional radiography suffers from the collapsing of 3D structures onto a 2D image CT gives accurate diagnostic information about the distribution of structures inside the body

Comparison of CT with Conventional Radiography Imagine you are standing in front of a wall, holding a pineapple against your chest with your right hand and a banana out to your side with your left hand. Your friend is looking only at the wall, not at you. If there's a lamp in front of you, your friend will see the outline of you holding the banana, but not the pineapple -- the shadow of your torso blocks the pineapple. If the lamp is to your left, your friend will see the outline of the pineapple, but not the banana. In order to know that you are holding a pineapple and a banana, your friend would have to see your shadow in both positions and form a complete mental image. This is the basic idea of computer aided tomography In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles

Comparison of CT with Conventional Radiography

Comparison of CT with Conventional Tomography Limitations of Conventional tomography Image blurring Problems with Film/screen combination

Image Formation and CT Numbers Shades of gray that make up a CT image are determined by the density of a structure and the amount of x-ray energy that passes through it. This phenomenon is referred to as the attenuation of the x-ray The degree of beam attenuation on a CT image is quantified and expressed in Hounsfield units (HUs), which are also referred to as CT numbers air : –1000 HU water: 0HU cortical bone: +3000 HU MATTER LINEAR ATTENUATION COEFFICIENT ( µ ) FAT 0.194 WATER 0.222 CSF 0.227 PLASMA 0.230 RED BLOOD CELLS 0.247

Terminology Hounsfield Unit (HU)- mean attenuation of x-rays by different tissues Data Acquisition- method by which the patient is scanned to obtain enough data for image reconstruction Beam Geometry- the size, shape, and motion of the x-ray beam and its path Ray- part of the beam that falls on the detector. Scan plane- region where the x-ray tube and detectors rotate

GENERATIONS Data gathering techniques have developed in stages termed Generations relate the configuration of the x-ray tube to the detectors Scan time reduction is the predominant reason for introducing new configurations

FIRST GENERATION Narrow pencil beam Single detector per slice Translate –Rotate movements of Tube- detector combination Scan time 5min Designed only for evaluation of brain, axis of rotation passed through the centre of the patient’s head Over night image reconstruction

1st Generation CT Scanner Head kept enclosed in a water bath Two side-by-side detectors A reference detector

SECOND GENERATION Narrow fan beam(30-100) Linear detector array(30) Translate-Rotate movements of Tube-Detector combination Fewer linear movements are needed as there are more detectors to gather the data Between linear movements, the gantry rotated 30o Only 6 times the linear movements got repeated Scan time~20secs

THIRD GENERATION Rotate(tube)-Rotate(detectors) Translatory motion is completely eliminated Pulsed wide fan beam(500) Arc of detectors(600-900) Detectors are perfectly aligned with the X-Ray tube Both Xenon and scintillation crystal detectors can be used Scan time< 5secs

FOURTH GENERATION Continuous wide fan beam(500-550) Ring of detectors(> 2000) Rotate(tube)-Fixed(detector) X-ray tube rotates in a circle inside the detector ring When the tube is at predescribed angles, the exposed detectors are read Scan time< 2 secs

IIIrd Vs IVth Gen. CT scanners

CT Data Acquisition Components

DATA ACQUISITION The scanning process begins with data acquisition. Data Acquisition refers to a method by which the patient is systematically scanned by the X ray tube and detectors to collect enough information for image reconstruction. A basic data acquisition scheme consists of X ray tube Filters Collimators Detectors

CT Gantry

CT gantry internal components 1.X-ray tube & collimator 2.Detector assembly 3.Tube controller 4.High freq. generator 5.Onboard computer 6.Stationary computer

CT Patient Couch

X-RAY TUBE Rotating anode type More heat loading and heat dissipation capabilities Small focal spot size (0.6mm) to improve spatial resolution

FILTERS Compensation filter is being used To absorb low energy x rays To reduce patient dose To provide a more uniform beam

COLLIMATORS To decrease scatter radiation To reduce patient dose To improve image quality Collimator width determines the slice thickness

DETECTORS Scintillation crystal detector The detectors gather information by measuring the x-ray transmission through the patient Two types: Scintillation crystal detector (Cadmium tungstate+ Si Photodiode) Can be used in third and fourth generation scanners Xenon gas ionisation chamber Can be used in third generation scanners only

Detector Cross-talk Detector cross talk occurs when a photon strikes a detector, is partially absorbed and then enters the adjacent detector and is detected again Crosstalk produces two weak and signals coming from two different detectors Crosstalk is bad because it decreases resolution Crosstalk is minimized by using a crystal that is highly efficient in absorbing X-rays (high stopping power)

Gas filled detector’s efficiency Gas filled detectors are less efficient than solid state detectors. The problem can be partially overcome by the following 3 ways. By using Xenon (z=54), the heaviest of the inert gases By compressing the Xenon 8 to 11 atmospheres to increase its density By using a long chamber to increase the number of atoms along the path of the beam.

Disadvantage of Xenon gas detector Efficiency ~60% This low efficiency is caused by two factors Low density of the absorbing material Absorption of X-rays by the front window, which is needed to contain the high pressure gas

OTHER SCAN CONFIGURATIONS Interest in faster scan times evolves from a desire to image moving structures such as the wall of the heart and contrast material in blood vessel and heart chambers and to overcome motion artifacts due to cardiac rhythm and patient breathing Electron beam computed tomography Dynamic Spatial Reconstructor(DSR)

ELECTRON BEAM COMPUTED TOMOGRAPHY Electron gun Large Arcs of tungsten targets Detector ring 17 slices per second

What is the radiation dose with EBT? EBT scanning is usually 1/5th to 1/10th the radiation exposure as Spiral CT scanning.

Radiation dose from EBT scans compared to other sources of radiation EBT Coronary Calcium Scan 50 to 70 mrem Non-invasive Coronary Angiogram 80 to 120 mrem EBT Low-dose Lung Scan 100 to 150 mrem EBT Abdominal/Pelvis Scan 100 to 300 mrem Background Sunshine Radiation in 1 year 300 mrem Cross country airplane trip 2 mrem Standard Chest X-ray 10 mrem Standard Abdominal X-ray 48 mrem Standard Spine X-ray series 300 mrem Standard Coronary Angiogram 500 to 1000 mrem Standard G.I. X-ray series 600 mrem Spiral CT Whole body scan 600 to 1000 mrem Recommended safety limits for radiation exposure is <5,000 mrem per year.

Electron beam computed tomography (EBCT) Electron beam computed tomography scanning is a new test that can be used to detect calcium buildup in the lining of arteries. It may be used as a screening tool to detect hardening of the arteries in people who are at high risk of developing atheroscerosis. Electron beam CT scanning is much faster than standard CT scanning. Electron beam CT scanning can produce an image in a fraction of a second and can take an accurate picture of an artery even while the heart is beating Standard CT scanning is not fast enough to take pictures of a pumping heart. In standard CT, several pictures, or "slices," of the heart are taken from different angles. These pictures are then analyzed using a computer to create a three-dimensional view of the heart.

Electron beam computed tomography (EBCT) Results Fat and calcium buildup may be seen in the arteries during an electron beam CT scan. Aggressive treatment for atherosclerosis may be appropriate. If electron beam CT scanning does not show the presence of calcium buildup in the arteries, then the chances of having CAD are low. A high calcium score on an electron beam CT scan indicates a greater risk of having cardiovascular problems within the next 2 to 5 years, especially when a person also has multiple risk factors for developing coronary artery disease. However, the scan does have a fairly high rate of false +ve results.

DYNAMIC SPATIAL RECONSTRUCTOR is an experimental apparatus 28 X-ray tubes X-ray tubes are aligned with 28 light amplifiers and TV cameras that are placed behind a single curved fluorescent screen The gantry rotates about the patient at a rate of 50 RPM Data for an image acquired in about 16 ms. Reconstruct 250 C.S. images from each scan data

DSR

Disadvantages of DSR High Cost Mechanical motion is not eliminated

Dynamic Spatial Reconstructor The Dynamic Spatial Reconstructor (DSR) is a high-temporal resolution, three-dimensional (3-D) X-ray scanning device based on computed tomography (CT) principles designed for investigation of quantitative studies of cardiovascular structure and function Initial results show that 3-D dynamic images can be obtained from patients with minimal invasiveness and that these images may provide useful diagnostic information

Image Quality in CT Image quality is the visibility of diagnostically important structures in the CT image. The factors that affect CT image quality are Quantum mottle (noise) Resolution : Spatial and contrast Patient exposure The factors are all interrelated

Quantum mottle (Noise) Quantum mottle is the statistical fluctuations of X-photons absorbed by the detector The only way to decrease noise is to increase the number of photons absorbed by the detector The way to increase the number of photons absorbed is to increase x-ray dose to the patient Mottle becomes more visible as the accuracy of the reconstruction improves

RADIATION DOSE Even distribution of radiation dose to the tissues as exposures are from almost all angles No overlapping of scan fields takes place Exposure factors used are higher to improve spatial and contrast resolutions and to reduce noise

CT ARTIFACTS Artifacts are distortions or errors in the image that are unrelated to the object scanned Most common artifacts in CT are Motion artifacts Streak artifacts Beam hardening artifacts Partial volume averaging artifacts Ring artifacts

EFFECTS OF ARTIFACTS DETERIORATE IMAGE QUALITY SUBJECT INFORMATION IS LOST PATHOLOGICAL DETAILS ARE LOST

MOTION ARTIFACTS Cause : Patient movement Appearance: Blurred / streaks / ghost images Rectification: reduction in scan time Clear and concise instruction to the patient proper patient immobilization if needed,administration of sedatives/antiperistaltic drugs

Motion artifact

STREAK ARTIFACTS Cause: Presence and movements of objects of very high density(contrast media, metallic implants,surgical clips) Appearance: Streaks Remove the offending object if possible.

DENTURES PRODUCING STREAK ARTIFACT SURGICAL CLIP IN HEART PRODUCING STREAK ARTIFACT

OUT OF FIELD ARTIFACT CAUSE:- Scan FOV not covering the entire anatomy APPEARANCE:- Shading/streaks REMEDY:- Ensure that scan field of view is larger than the object to be scanned

RING ARTIFACTS CAUSE : Detector failure or miscalibration of a detector APPEARANCE:- Ring Rectification : regular quality assurance checks

RING APPEARANCE