Transcript X-ray beam

Part I: Basics of
Computed Tomographic
Imaging
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Basics of X-ray
Basics of Computed Tomographic Imaging
CT Artifacts
Next generation CT techniques
Contrast agents
• Small molecule
• Nanoparticulate
• Macromolecular
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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
Heater
Cathode
Window
Anode
• Artificially X rays are produced by decelerating highvelocity 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 Xrays, a lot is ‘wasted’ as heat. The anode is specially
designed to withstand the heat and the ‘tube’ also has
a cooling mechanism.
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
Knee
The cross section of the knee in the lower
part of the picture shows how X-rays may be
attenuated.
X-ray source
X-ray beam
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.
B
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’.
A
Skin
Muscle
Film
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
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.
Chest Anatomy
www.anatomyatlases.org
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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.
A Chest X-ray
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
C
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.
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
A
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
Image of the elbow
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.
Key Points : Cartilage is tough, but not opaque to X-rays! Superimposed parts of two
bones appear whiter.
Soft Tissues
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.
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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. Legend: T: Xray tube D: X-ray detectors X: X-ray beam R: Gantry rotation
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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.
X
1
2
3
D
The movement of the patient can be controlled so
that ‘slices’ of the body are scanned by the apparatus.
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
Right Kidney
R
L
Liver
R. Psoas major
R. post. vertebral
muscles
Pancreas
Infvena cava, with
left renal vein
crossing across aorta
Aorta
P
Left Kidney
Radio-opaque vs Radioactive
Positive contrast media are often described as radioopaque (“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
IODINES
90 – 120 kVp
70 – 80 kVp
(Ionic / Nonionic
Water or Oil)
Contrast Media
• Negative contrast
• (AIR OR CO2)
• Positive contrast
• (all others)
• Radiolucent
• Radiopaque
• Low atomic # material
• High atomic # material
• Black on film
• White on film
Types of Contrast Media
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Radiolucentnegative 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
• Radiopaque• positive contrast agent• absorbs x-rays
• appears light
Positive Contrast Agents
• BARIUM
• IODINES
• BISMUTH
• GOLD
• GADOLINUM
Both + & - can be used in
same study
Most Common TYPES
OF CONTRAST material
• BARUIM Z# 56
• NON WATER SOLUBLE
• GI TRACT ONLY
INGESTED OR RECTALLY
• KVP 90 – 120*
•
•
•
•
•
•
•
•
•
IODINE Z# 53
WATER SOLUABLE
POWDER
LIQUID
INTRAVENOUS OR
Intrathecal
GI TRACT
Also OIL based
KVP BELOW 90*
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.
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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