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.
The Human Body Through Images
Human Structure and Development : ANHB 2212 - 2009
Dr. Avinash Bharadwaj
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
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.
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
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!
X Ray Tube Principles
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
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
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.
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
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.
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.
Understanding the Image – 2
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.
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
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
The air outside the leg is virtually
To summarise we may say :
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!
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.
A Chest X-ray as an Example - 1
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
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.
A Chest X-ray as an Example - 2
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
Key Points :
Air containing structures ‘darken’
other superimposed structures.
Thickness makes the heart as
opaque as bone!
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!
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.
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
Key Point :
A Matter of Contrast!
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
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.
By the way, the white bands in the previous picture were psoas major muscles!
Key Point :
Contrast once more!
A Matter of Contrast - 2
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.
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!
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.
Contrast Media - 1
Key Points : See next slide.
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’
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
Some of these procedures can be potentially hazardous and are done only if there are specific reasons for
Contrast Media - 2
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.
This is an oblique view of a barium swallow.
You need not worry about the details of the
Note the ribs on far side and the vertebrae at
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
Barium Meal - Stomach
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
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
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.
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
Key Points :
• In intravenous urography, the medium is
injected through a vein. It is too dilute in
• It is ‘concentrated’ in the urine by the
• This imaging method also indicates that
the kidney is functional!
Radio-opaque vs Radioactive
Positive contrast media are often described as radio-opaque (“Opaque to X-
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
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!
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.
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
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.
CT, MRI and Ultrasound
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.
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.
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
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.
Air in the stomach
As the patient is
supine, the air
the anterior side.
R. Psoas major
Inf vena cava, with left renal
vein crossing across the aorta
Key Point :
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!
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 F F
Key Point :
Fat is the key!
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