1. The Human Body Through Images
This PowerPoint presentation gives an introduction to imaging techniques. It is not designed to be a comprehensive collection of images of the entire body – rather, preliminary to study in the lab.
It is aimed primarily at students of Anatomy & Human Biology to help them understand and interpret images of normal the human body. Medical or dental students will find the basic principles useful, but as a radiological resource it is not adequate. The emphasis is on conventional radiographic images with introductory remarks on CT, MRI and other modern techniques.
Remember that in dealing with this topic some theoretical considerations have been simplified. The aim here is not technical expertise. This is especially true of the concepts in Physics. Biomedical engineering students will naturally find the physical aspects somewhat elementary! Interested students may communicate with me for further elaboration if necessary.
This PowerPoint uses large text passages and the font size used makes it easier to read off the screen. It is not appropriate for projection.
Human Structure and Development : ANHB
Dr. Avinash Bharadwaj

2. Imaging – What and Why
There was a time in the not-too-distant past when “conventional” X-ray images were the only means of visualising the interior of the human body. “Imaging” was then called radiography and the study of the normal or diseased body was radiology. Oh yes, we did sometimes have images (‘scans’) taken after injecting radioactive isotopes.
Computerised Tomography (CT) marked the beginning of the present era and was followed by a number of better techniques. The quest was mainly for less hazardous methods – both for the patient and the investigator. Apart from ultrasonography, conventional X-ray imaging still remains a common form of investigation.
Most imaging is necessarily an aid to diagnosis. Our main aim is a better understanding of normal anatomy through imaging. Technical aspects of imaging, which have been greatly simplified here, only serve to facilitate the understanding of the images.
The illustrations in this PowerPoint include only conventional X-ray images for explaining the principles. CT and MRI images will be shown in the laboratory.
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3. X Rays
X-rays are a part of the natural electromagnetic spectrum. All electromagnetic waves travel at the same speed (velocity would be more accurate, but we shall pass that!) 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 kiloHertz to tens of megaHertz).
In the middle of the spectrum are heat waves (infrared), visible light and ultraviolet rays. The shorter UV rays can be damaging to life.
Beyond UV (still shorter wavelength or higher frequency) are X-rays and the even more powerful gamma rays. Their high energy allows them to penetrate through solid matter. They can cause serious damage to the macromolecules of life.
X-rays (also called Röntgen rays after their discoverer) were discovered and artificially produced in the laboratory towards the end of the 19th century.
All electromagnetic radiation can also be considered to be particles (photons) travelling at the same speed but with different energies. While sometimes it is convenient to regard them as such, we do not have to enter that debate .
Key Points : X-rays are very high energy electromagnetic waves.
Though they are a tool for imaging, their use is not without dangers!
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4. X Ray Tube Principles
Cathode
Anode
Heater
Window
Artificially X rays are produced by decelerating high-velocity electrons. The apparatus, called X-ray tube, therefore 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 heated cathode emits electrons.
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.
Mind you, 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.
The picture shows only the basic plan of the X-ray tube to illustrate the principle.
Key Point : X-rays are produced by deceleration of high velocity electrons.
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5. 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. Nowadays an X-ray image can also be stored in a digital form on a computer.
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. Needless to say, such images are temporary. This method is called fluoroscopy. It exposes the patient to much higher doses of X-radiation and is far more hazardous.
Key Points :
Conventional X-ray images are taken by passing the rays through the body and exposing a photographic film.
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6. Understanding the Image - 1
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.
As a simple explanation we may say that 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. Remember that an X-ray image on film is seen as a negative film!
The actual shades in the image also depends on the initial energy and the ‘quantity’ of the X-rays as they emerge from the source. This is comparable to the reflectivity of the subject and the amount of available light in ordinary photography. The ‘quantity’ of X-rays is related to the electron flow from the cathode (measured in milliamperes), and the energy is related to the anode voltage (kilovolts) – the greater the anode voltage, the faster the electrons and the more energetic the X-rays when the electrons are stopped.
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.
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7. Understanding the Image – 2 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.
Remember, 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.
Remember also that an X-ray image for studying ‘soft’ tissues uses less energetic X-rays or shorter exposure than one taken for studying bone. These technical details need not worry us.
What we do need to understand is the contrast generated by different tissues.
Key Points : X-ray attenuation depends largely on the average atomic mass in a tissue, though thickness and density do have a role to play.
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8. Attenuation Patterns
Imagine that the region of the knee is being subjected to X-ray imaging. The X-ray beam passes from the source to the film as shown, with the knee joint in between. 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 labelled ‘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
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9. Image Densities Bone – calcium – greater attenuation : white image
To summarise we may say :
On films
Bone – calcium – greater attenuation : white image
Soft tissues – less attenuation – gray image
Air – least attenuation, dark areas
However … thickness also matters!
Remember that in fluoroscopy the pattern is reversed. However, we are not concerned with that. But… some textbooks do print ‘positive’ images from films, with a reversed pattern. Just keep this in mind if you do come across such images.
The knee was a good example with bones and muscles. Since we are not studying details of the knee, we shall take the familiar example of a chest X-ray. But before we do that, a bit of basic directional terminology!
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10. 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.
X-ray images are qualified by such directional terms with consideration to the direction of the X-ray beam.
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.
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. We need not worry about these details. A basic understanding of AP, PA and Lateral views is adequate.
Key Points : The “view” of an X-ray image tells us the direction of the X-ray beam through the body in directional terms.
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11. A Chest X-ray as an Example - 1B C D
This a good starting point, as we are familiar with the thorax.
This is a PA view of the thorax.
First of all, observe the blackness of the air outside the body.
Next, see the bone (a part of the clavicle) in the oval A. Can you distinguish two white bands with a darker area in this bone? The centre of the bone is “spongy”, the outer part is solid.
In the oval B, the end of the clavicle is formed by a thin solid plate with a large centre of “spongy” bone. The overall picture is therefore a bit darker.
Similarly, see the 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.
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.
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12. A Chest X-ray as an Example - 2B
In digitising this picture some contrast is lost, but you can make out the difference between the air outside the body and air in the lungs – lungs are soft tissue filled with air!
The shape of the heart is unmistakable – notice how the thick muscle wall and the blood that fills the heart create a white image – in places whiter than bone.
Structures in the hilum of the lung can be seen with variable clarity as at A. Again, a blood vessel “end-on” is more opaque than one “across” the beam.
The cervical and upper thoracic vertebrae can be seen in the oval B. The lower thoracic vertebrae are sometimes lost in the heart image, but here they are seen, along with the descending aorta, as a band running down the heart.
Key Points :
Air containing structures ‘darken’ other superimposed structures.
Thickness makes the heart as opaque as bone!
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13. A Chest X-ray as an Example - 3
Now see how thickness matters.
Observe the images of the breasts. The right breast is indicated by the curved line. Note how the 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. This also tells you that this image is that of a female subject.
Also observe how the thick musculature around the shoulder appears white.
As important, see how the 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. We thus speak of bone density, soft tissue density and so on.
Key Points :
All that is white is not bone!
Understand contrast!
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14. Cartilage
We shall study bones and joints later, but at this stage just appreciate that 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. This is an image of the elbow.
Key Points :
Cartilage is tough, but not opaque to X-rays!
Superimposed parts of two bones appear whiter.
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15. Soft Tissues
In this image of a part of the lumbar vertebral column, can you see the bands by the sides of the vertebrae? (Shown by the double-headed arrow on the right side). What do you think they are?
Key Point :
Contrast again!
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16. A Matter of Contrast! Key Point : Contrast once more!
First of all, notice the ribs. The lowest pair is the 12th. Observe the bodies of vertebrae. They are made of a thin shell of solid bone and spongy bone inside. The oval outlines (one shown by the blue arrow) are joints between the articular processes. The red arrow shows a spine. Remember that the spine of a vertebra is at a lower level than its body (check your knowledge of lumbar vertebrae!) Again, the lighter bands between the bodies of vertebrae are the intervertebral discs, which appear somewhat lighter because of the overlap of the neural arches.
And what are the dark blobs – three of them are shown by white arrows. These are 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. But then, we cannot depend on such ‘natural’ contrast to see hollow organs.
We have artificial means of introducing contrast for visualising certain organs. Let us see one more example of ‘natural’ contrast before we go to artificial contrast.
Key Point :
Contrast once more!
By the way, the white bands in the previous picture were psoas major muscles!
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17. A Matter of Contrast - 2 Key Point :
In the image labelled C1 note how the diaphragm blends with the abdominal organs below it. In the other image there is some air (black, shown by arrows) between the liver and the right dome of the diaphragm. It has outlined the thin white line of the diaphragm.
Mind you, air under the diaphragm is a very serious matter – it indicates that some abdominal hollow organ has a perforation or rupture, causing gas to escape into the peritoneal cavity. This is not exam material for you!
Key Point :
Contrast can show structures which are otherwise invisible.
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18. Contrast
Materials opaque to X-rays can be introduced in hollow organs. This means that there is ‘contrast’ between the contents of the cavity and the wall. The cavity shows up as white in an X-ray image.
In some organs we can also introduce air or a gas so that it shows up as black.
These two modes are sometimes described as positive or negative contrast. However, the concept is more important than the terms!
Materials thus introduced for this purpose are called contrast media.
A contrast medium must satisfy certain criteria :
It must be inert (non-reactive) non-toxic.
It must not be absorbed or retained by the body.
It must be easily excreted.
Key Points :
Contrast can be created artificially, by ‘contrast media’.
A contrast medium must be safe!
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19. Contrast Media - 1 Key Points : See next slide. avb
For the digestive system barium sulphate is used. It is the barium that makes it opaque to X-rays. Barium belongs to the Calcium group of elements and is much “heavier”, with an atomic mass of 137. (Do not worry, I shall not ask the atomic mass in an exam!) Barium sulphate is insoluble in water and hydrochloric acid. This is important, because this property makes it nonabsorbable even in the strong acidic environment of the stomach. It is mixed with water to form a suspension which the patient is given to drink. This is called a ‘barium meal’. For studying the oesophagus, a spoonful of barium paste is given (called a barium swallow).
In either case the radiographer watches the progress of the barium on a screen. At appropriate moments, films are exposed.
The barium stays in the stomach (a little spills into the duodenum) for a while. A study of these structures is a “stomach-duodenum” study. It is then passed on to the small intestine and the colon. A study of the small intestine and the colon is also called a “follow-through”. In a follow-through the barium is spread out and diluted by intestinal fluids. For a clearer view of the colon, barium is given through the rectum as an enema.
Key Points : See next slide.
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20. Contrast Media - 2 Key Points :
Most other contrast media are iodine containing compounds, and most of them are water soluble. Iodine is also a ‘heavy’ element (atomic mass 127). Different iodine compounds behave differently in the body.
For example, an iodine compound that is specifically excreted by the kidney is used to study the urinary system. 20 to 40 ml of this fluid, when injected in the bloodstream through a vein (intravenous) is diluted in the (approximately) 4 litres of blood, so blood vessels are not visible on an X-ray. But when excreted by the kidney in a small amount of urine, it is concentrated and the cavities of the urinary system are outlined. This is called intravenous urography. Tha fact that it is excreted by the kidneys also means that the kidneys are functioning! The compound can also be injected through a tube passed in the urinary bladder – it can reach as high as the calyces of the kidneys. This is ‘ascending’ urography.
Another iodine compound is given by mouth, is absorbed by the digestive system, reaches the liver via the portal vein and is excreted in bile, outlining the gall bladder and bile ducts.
Arteries and veins, the heart, and cavities in the brain can also be studied in this manner with other compounds.
Some of these procedures can be potentially hazardous and are done only if there are specific reasons for doing them.
Key Points :
The choice and the route of administration of a contrast medium depends on the physiology of the structure being investigated!
Barium sulphate for the GI Tract, iodine containing compounds for most other structures.
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21. Barium Swallow
This is an oblique view of a barium swallow. You need not worry about the details of the view.
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.
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22. Barium Meal - Stomach F Key Points :
The outline of the stomach is obvious. Observe the air bubble in the fundus (F).
The blue arrow shows the pylorus. This is an excellent illustration of the fact that contrast media outline the cavity. The pyloric sphincter is a small mass of muscle and therefore not visible, just a thin line of barium is seen in the narrow channel in the sphincter (point of the arrowhead).
Note the extensive spread in the intestines. This is all small intestine – in some parts you can see the breaks in the continuity of barium due to the plicae circulares.
To see the details of the colonic wall, sometimes a barium enema is given. After it is evacuated, air is introduced in the colon. The dark air contrasts with a thin layer of barium sticking to the wall of the colon. This is called double contrast. F
Key Points :
Understand how anatomical features correlate with appearances on images.
Make sure you see in the lab other images of all organs, especially double contrast images of the colon.
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23. 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!
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24. Radio-opaque vs Radioactive
Positive contrast media are often described as radio-opaque (“Opaque to X-radiation”).
Students very often call them radioactive by mistake.
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.
I hope 2212 students will never confuse contrast media with radioactive isotopes!
Key Points :
This entire slide is a key point! Don’t miss this! 
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25. Limitations
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. Tomography with conventional methods has even more limitations. (Conventional tomographic pictures are not a part of this unit.)
Modern methods of imaging have sought to overcome some of these limitations.
Key Point :
Though the limitations of X-ray imaging are outlined here, remember that a rough 3-D perspective can be obtained by taking pictures through different angles.
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26. CT, MRI and Ultrasound Key Points :
In computerised tomography (CT) the X-ray source rotates around a plane of the body, taking serial pictures with a detector (instead of a film) which are synthesized by a computer. The resulting picture created by the computer is like a section of the body and can be recorded on a film. CT pictures are therefore like X-ray images.
Magnetic resonance imaging (MRI) uses the property of protons aligning themselves in a magnetic field and their reaction to radio frequency waves. The protons ‘resonate’ to the radio frequency and revert to normal (‘decay’) when the radiation is stopped. Effectively it is the imaging of protons. The most commonly imaged proton is a hydrogen nucleus. So far it is believed that this method does not damage body tissues as X-rays do. MRI images are even more realistic than CT images.
Ultrasound on the other hand uses mechanical waves of frequencies beyond the audible range. These waves are reflected to various degrees from junctions of tissues of different nature. Ultrasound pictures require considerable skill to interpret. Ultrasound has a great advantage – it does not cause cellular damage when used in quantities required for imaging.
Key Points :
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.
All these methods illustrate structure of the body in some form of sectional view.
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27. 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. The movement of the patient can be controlled so that ‘slices’ of the body are scanned by the apparatus. X D 1 2 3
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28. 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.
Correlate this cross sectional CT image with abdominal organs as you have seen them! Remember, a CT image is seen as if one is viewing a slice from below. A P R L
Liver
Air in the stomach
As the patient is
supine, the air rises to
the anterior side.
Right kidney
R. Psoas major
R. post. Vertebral muscles
Pancreas
Left kidney
Aorta
Inf vena cava, with left renal vein crossing across the aorta
Key Point :
Recognition of major anatomical structures.
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29. MRI
The MRI apparatus looks similar to the CT machine. There is no X-ray tube, however. A strong magnetic field surrounds the area being imaged. A radio wave source and a receiver (detector) are important components of the setup (not shown in the picture).
The manner of excitation of the aligned protons and their return to normal after cessation of radio waves introduces additional terminology. A picture taken early during the ‘decay’ is described as “T 1 weighted”, one taken later during the decay is a “T 2 weighted” image. There are subtle differences of shades of grey, resolution and contrast between T1 and T2 images.
Key Point :
An MR image is not an X-ray image!
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30. The MR Image
The grey or white appearance of fat is an indicator that this is an MR image. It evident as a thick layer in the abdominal wall. It is also easily recognisable around the kidneys (perirenal fat) and in the greater omentum in front. Notice that the definition of soft tissue structures is sharper. F
Liver
Spleen K A P St
Key Point :
Fat is the key!
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31. …And Beyond
A number of highly sophisticated tools are available for imaging now. Some of them are still more of research tools, but may enter the field of routine diagnosis very soon. It is beyond the scope of this unit to describe them in detail. Those of you who study neurobiology will see illustrations of some of these.
Last Slide
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