Computed Tomographic Imaging Part I: Basics of Computed Tomographic Imaging

Basics of Computed Tomographic Imaging CT Artifacts Basics of X-ray Basics of Computed Tomographic Imaging CT Artifacts Next generation CT techniques Contrast agents Small molecule Nanoparticulate Macromolecular avb

can cause serious damage to the macromolecules of life In the middle of the spectrum are heat waves (infrared), visible light and UV rays. The shorter UV rays can be damaging to life. X-rays (Röntgen rays) were discovered and artificially produced in the laboratory towards the end of the 19th century.

Basic Difference between Major Modalities CT: Synthesis of multiple X-ray images of a ‘slice’. MRI: Imaging protons excited by radio waves. Ultrasound: High - frequency ‘sound’ waves reflected from tissue junctions. CT MRI US All these methods illustrate structure of the body in some form of sectional view.

X Rays X-rays are a part of the natural electromagnetic spectrum. All electromagnetic waves travel at the same speed through vacuum – 300,000 km/sec. A wave has two attributes – wavelength and frequency. The product of the two equals the speed at which it travels. Waves with longer wavelength (lower frequency) have lower energy. The shorter the wavelength, greater the energy of the wave. At one end of the spectrum we have radio waves with wavelengths measured in metres, centimetres or millimetres (frequencies range from few kHz to tens of mHz).

Use of X-rays for Various Applications

X Ray Tube Principles Artificially X rays are produced by decelerating high-velocity electrons. X-ray tube has a source of electrons, a means of accelerating them to high velocities and something to stop them so that they lose their energy. The electron source is the cathode, heated by a filament. The anode has a positive voltage (thousands of volts) and attracts the electrons so that they reach a high velocity. The disc-like surface of the anode also stops the electrons. The X-rays produced go out through the window. Only a small fraction of the energy is in the form of X-rays, a lot is ‘wasted’ as heat. The anode is specially designed to withstand the heat and the ‘tube’ also has a cooling mechanism. Heater Cathode Window Anode Key Point : X-rays are produced by deceleration of high velocity electrons.

X Ray Imaging X-rays, after having passed through the body, are made to strike a photographic film, much like a black-and-white camera film. The film has a coating of halides (chlorides/bromides) of silver. The halides affected by X-rays are reduced to metallic silver after treatment with “developers”. The unaffected (“unexposed”) halides are washed out chemically and the film, rinsed with water, is dried. The finely particulate silver actually appears dark (rather than shiny!). Thus, areas of the film exposed by X-rays are dark, unexposed areas are transparent. X-ray films are viewed as “negative” films against an illuminated background. Fluoroscopy: X-ray images can also be viewed with a fluorescent screen like that of a monitor. In such an image exposed areas are bright, unexposed areas dark. It exposes the patient to much higher doses of X-radiation and is far more hazardous.

Understanding the Image As X-rays from the source pass through the body, they lose their energy. The loss of energy, called attenuation, depends on some tissue characteristics. Some tissues are “transparent” to X-rays, some are “translucent” (partially transparent) and some are “opaque” to X-rays. A totally opaque material will absorb all the X-rays, allowing none to pass through. A “transparent” tissue between the source and the film implies that more X-rays strike the film, affecting more silver halide, leading to a black image, an “opaque” tissue will block a lot of X-rays, less or no silver is affected and the image is white. Intermediate degrees of transparency give rise to shades of gray in the image. Key Points : X-rays are absorbed, or lose their energy to a variable extent as they pass through tissues of the body. The X-ray film is exposed to a correspondingly variable degree and shows light and dark areas.

Understanding the Image Attenuation The most important (but not exclusive) factor is the presence of ‘heavy’ elements in the tissues. The term ‘heavy’ refers to the atomic mass (as in the periodic table of elements), which does not necessarily correspond with the density or specific gravity. Most body tissues are carbon-, hydrogen-, oxygen- and nitrogen based. The atomic masses of these elements are 12, 1, 16 and 14 respectively. The common heavier elements are calcium (40) and iron (56). Bone has a great concentration of calcium. Muscle tissue has a fair degree of calcium abundance and blood, of iron. This does not make all bone or blood opaque to X-rays! The thickness of the tissue and the relative abundance of heavy elements also matters. Thus, a thick mass of muscle or blood may be more opaque than a thin plate of bone. X-ray image for studying ‘soft’ tissues uses less energetic X-rays or shorter exposure than one taken for studying bone. X-ray attenuation depends largely on the average atomic mass in a tissue, though thickness and density do have a role to play.

Attenuation Patterns The cross section of the knee in the lower part of the picture shows how X-rays may be attenuated. Note the two hollow bones (most long bones in the body are hollow). The large masses are the muscles, with blood vessels and nerves scattered among them. Most significantly, note that X-rays passing through the region labeled ‘A’ face a much larger thickness of bone compared to those passing through ‘B’. The muscles, though much thicker, still do not offer as much “opacity” as the bones, the skin and the softer tissues even less. The air outside the leg is virtually transparent X-ray source Film Muscle Skin A B Knee X-ray beam

Image Densities In a nut shell On films Bone – calcium – greater attenuation : white image Soft tissues – less attenuation – gray image Air – least attenuation, dark areas However … thickness also matters! In fluoroscopy the pattern is reversed.

Directional Terms While being subjected to X-ray imaging, a patient or a part of the patient’s body may be positioned differently with reference to the source and the film. Scenario 1: If the beam enters the front of the patient’s body and emerges from the back that is, the patient faces the source and the film is behind the patient – we describe the image as an anteroposterior (A-P) view. An image taken in the reverse manner (X-rays going from the back to the front, with the film in front) is a PA view. Most chest X-ray images are taken as PA views. Scenario 2: Images can also show lateral views (R to L or L to R) and even oblique views which have special terms depending on whether the beam comes from the right or left side as also anterior or posterior. Some regions require special views. Key Points : The “view” of an X-ray image tells us the direction of the X-ray beam through the body in directional terms.

Chest Anatomy www.anatomyatlases.org avb

A Chest X-ray A B D C Darkness of the air outside the body Part of the clavicle bone two white bands with a darker area- the centre of the bone is “spongy”, the outer part is solid A Darkness of the air outside the body B D Appearance of the rib “flat across” the X-ray beam at C and compare with the arrowheads D where greater lengths of the ribs are across the X-ray beam, as the ribs curve around the thorax. C PA view of the thorax Key Points : Think of the anatomy of the structure being viewed. Even bone can have different appearances depending on the thickness it presents to the X-ray beam.

A Digitized Chest X-ray B Air Lungs are soft tissue filled with air! The shape of the heart is unmistakable –thick muscle wall and the blood that fills the heart create a white image –in places whiter than bone. A: Structures in the hilum of the lung with variable clarity. Again, a blood vessel “end-on” is more opaque than one “across” the beam. B: The cervical and upper thoracic vertebrae Key Points : Air containing structures ‘darken’ other superimposed structures. Thickness makes the heart as opaque as bone!

A Digitized Chest X-ray with Other Info Image of the breast is pronounced on the lateral and lower side. It is just skin, connective tissue and fat, yet the thickness casts an image. Abdominal organs also appear white. The different shades between black and white in an X-ray image are also referred to as “densities” or “shadows” in radiological jargon--bone density, soft tissue density. All that is white is not bone! Understand contrast!

Cartilage Joint-forming surfaces of bone are covered by hyaline cartilage. Even though cartilage is tough tissue, it does not have calcium, and radiologically similar to ‘soft’ tissues. The clear bands (arrows) between the bones are areas of cartilage. Image of the elbow Key Points : Cartilage is tough, but not opaque to X-rays! Superimposed parts of two bones appear whiter.

Soft Tissues Psoas major muscles! lumbar vertebral column Bands by the sides of the vertebrae lumbar vertebral column Psoas major muscles! Key Point : Understand the endogenous Contrast again!

A Matter of Contrast! Dark blobs -bubbles of gas in the colon. Ordinarily, the colon is invisible because it blends with the other viscera in an X-ray image. Gas in the colon creates contrast. Joints between the articular processes Vertebrate: Thin shell of solid bone and spongy bone inside Spine of a vertebra is at a lower level than its body

A Matter of Contrast Diaphragm blends with the abdominal organs Air between the liver and the right dome of the diaphragm Air under the diaphragm indicates that some abdominal hollow organ has a perforation or rupture, causing gas to escape into the peritoneal cavity Key Point : Contrast can show structures which are otherwise invisible

Limitations with X-ray Imaging Despite giving so much information (and being interesting!), these ‘conventional’ images have limitations. They are two dimensional images. For a 3-D perspective we have to take at least two images, one AP and one lateral. The resolution of the images is also limited. It is possible to “focus” the X-ray beam on a specific plane in the body. This is called tomography – meaning picture of a slice. avb

Computed Tomography In its simplest form, a CT imaging system consists of a finely collimated x-ray beam and a single detector. Both moving synchronously in a translate rotate mode. Translation = one rotation of source and detector

CT scanner with cover removed to show internal components CT scanner with cover removed to show internal components. Legend: T: X-ray tube D: X-ray detectors X: X-ray beam R: Gantry rotation avb

The CT Setup The X-ray tube (X), housed in a ‘wall’ (1) rotates around a hole (2) in the wall. The detector (D) also rotates diametrically opposite the tube. The patient, lying on a sliding trolley (3) or a couch passes through the hole. 1 2 3 X D The movement of the patient can be controlled so that ‘slices’ of the body are scanned by the apparatus.

Creating a cross-sectional tomographic plane of a body part Tomo = image // to long axis of the body CT = image is transverse to the body Creating a cross-sectional tomographic plane of a body part A patient is scanned by an X-ray tube rotating around the body A detector assembly measures the radiation exiting the patients

CT Imaging Overview

Voxel Each pixel in the image corresponds to the volume of tissue in the body section being imaged. The voxel volume is a product of the pixel area and slice thickness Hounsfield units: Each pixel within the matrix is assigned a number that is related to the linear attenuation coefficient of the tissue within each voxel

Hounsfield Units (HU) A relative comparison of x-ray attenuation of a voxel of tissue to an equal volume of water. Water is used because it is in abundance in the body and has a uniform density Water is assigned an arbitrary HU value of 0 Tissue denser than water are given positive CT numbers Tissue with less density than water are assigned negative CT numbers The scale of CT numbers ranges from -1000 for air to +14,000 for dense bone Only – CT # in the body are Fat, Lung & Air

Hounsfield Scale On the CRT or LCD, each pixel within the image is assigned a level of gray The gray level assigned to each pixel corresponds to the CT number or Hounsfield units for that pixel

The CT Image A CT image can be taken as a plain image or with the introduction of a contrast medium. Like conventional X-ray images, bone appears white, air black and soft tissues have intermediate densities depending on their composition and thickness. However, the contrast and resolution is better than in conventional tomography. Air in the stomach- As the patient is supine, the air rises to the anterior side. A P R L Liver Pancreas Infvena cava, with left renal vein crossing across aorta Right Kidney R. Psoas major Aorta R. post. vertebral muscles Left Kidney

Radio-opaque vs Radioactive Positive contrast media are often described as radio-opaque (“Opaque to X-radiation”). CT Contrast media are NOT radioactive! The confusion possibly arises from the fact that a radioactive isotope of iodine (atomic mass 131) is often used in diagnostic tests. Iodine is concentrated by the thyroid gland. When it is radioactive iodine, the thyroid gland emits radiation which can be used to create an image of the thyroid gland. Other radioactive isotopes are similarly used to “scan” other organs, notably the liver.

Purpose of Contrast Media To enhance subject contrast or render high subject contrast in a tissue that normally has low subject contrast.

Atomic Number Fat = 6.46 Water = 7.51 Muscle = 7.64 Bone = 12.31

Radiographic Contrast : Influenced by… Radiation Quality (KVP) Film Contrast Radiographic object (Patient)

KVP TYPE OF CONTRAST USED DETERMINES KVP RANGE BARIUM 90 – 120 kVp IODINES 70 – 80 kVp (Ionic / Nonionic Water or Oil)

Contrast Media Negative contrast (AIR OR CO2) Radiolucent Low atomic # material Black on film Positive contrast (all others) Radiopaque High atomic # material White on film

Types of Contrast Media Radiopaque- positive contrast agent- absorbs x-rays appears light Positive Contrast Agents BARIUM IODINES BISMUTH GOLD GADOLINUM Both + & - can be used in same study Radiolucent- negative contrast agent x-rays easily penetrate areas- appear dark on films Negative Contrast Media Air and gas complications emboli-air pockets in vessels lack of oxygen

Most Common TYPES OF CONTRAST material IODINE Z# 53 WATER SOLUABLE POWDER LIQUID INTRAVENOUS OR Intrathecal GI TRACT Also OIL based KVP BELOW 90* BARUIM Z# 56 NON WATER SOLUBLE GI TRACT ONLY INGESTED OR RECTALLY KVP 90 – 120*

Barium Meal This is an oblique view of a barium swallow. Note the ribs on far side and the vertebrae at lower right. At the upper end of the picture the barium paste mass is narrow, indicating that the oesophageal muscle is contracting to push the ‘bolus’ down. At lower left notice that some barium has entered the stomach and shows as a larger mass. avb

Barium Meal - Stomach F The outline of the stomach is obvious. Observe the air bubble in the fundus (F). The blue arrow shows the pylorus.

Urography These pictures show intravenous urography. Note the lumbar vertebrae, the outlines of the cavities (calyces) of the kidney and the ureters, as also the course of the ureter. In about an hour’s time all the iodine compound will be in the urinary bladder. Key Points : In intravenous urography, the medium is injected through a vein. It is too dilute in the bloodstream. It is ‘concentrated’ in the urine by the kidneys. This imaging method also indicates that the kidney is functional!

Iodinated CT Contrast Agents