X-Ray of the Heart
, Katsushi Arisaka*a
Wing Chi Wong
Physics 89, Seminar 4
X-Ray technology is one the the most used method in diagnoses of many different types of diseases. The most common
type of x-ray is chest x-ray, which account for almost 50% of all x-ray preformed in 2004. Chest x-ray along with
angiogram (an application of chest x-ray) and computed tomography are the three x-ray technology used for the heart.
Both angiogram and CT can compensate the limitation of chest x-ray. The fundatmental of all three method is the
absoption of the x-ray photon by an atom by kicking out one electron from its orbital. When x-ray photons are emitted
into the human body, our tissue absorb some of these photons. The rest are transmitted and absorb by a radiation
detector, in many case x-ray film, to obtain a visible map of radiation absorption in the body.
X-Ray is a type of electromagnetic radiation emitted into the body. The radiation has dual characteristics as waves and
particles. For x-ray, the radiation act as photon particles. When the energy of the X-ray photon particle (for diagnostic
x-ray is between 20eV to 20,000 eV) emitted is slightly higher than the electron binding energy, the photon gives up its
energy by knocking the electron out of the orbital. This creates a photoelectron and a positively charged atom. If the
material is more dense, less x-ray photon will be transmitted through.
X-Ray technology is one the the most used method in diagnoses of heart related disease, such as heart failure, enlarged
heart, disease blood vessel, etc.They allow doctors to see the position and size of the heart.For example, figure 1 shows
the differences between a normal human male heart ans an enlarged human male heart. Figure 2 shows a blood filled
chest of a patient suffering from internal bleeding due to rupture of a artery.
The three types of x-ray technology that can be used for the heart are:
• Chest X-Ray
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• Computed Tomography (CT Scan)
Out of the three methods the most used is chest x-ray, because it has the least risk and radiation level for the human
body. However, it does have limitation which can be compensated by the two other methods (angiogram and CT Scan).
Figure 1 Source: The Franklin Institution Online
Figure 2 Source: MedlinePlus Medical Encyclopedia
In chest x-ray, a film or radiation detector is placed in front of the body, on the opposite side of the X-ray generator, as
shown in figure 3. The detector or film absorbs the photon energy that the body didn’t absorb. The film is normally
composes of silver bromide. When the x-ray photon hits the silver bromide, a photoelectron is generated. The
photoelectron is then combines with a silver ion to make a silver atom, which appear to be black. So, the x-ray film is a
map of photon absorption by the body. Darker area represents area with lower tissue density and less photon absorption.
The blackness of the film is measured in optical density, which is given by the equation D=Log10(Ii/It). Ii represent
initial intensity. It represents intensity transmitted.
Figure 3 Source: RadiologyInfo http://www.radiologyinfo.org/photocat/photos_pc.cfm?
3. HOW SAFE IS CHEST X-RAY?
Any form of ionizing radiation procedures are found to cause damage to tissues. The most important damage caused by
radiation is mutation to the DNA structure, especially to germ cells. National Council on Radiation Protection and
Measurement established in 1958 that the maximum amount allowable per person per year is 5R. The average radiation
exposure during a chest X-ray is 20 mR, which is relatively low compare to other X-ray procedures such as Dental
(650mR), Brain (250mR). It is about how much background radiation an average person is exposed to in 5 days. The
expose of radiation to the reproductive organs are lower by wearing a lead around the lower abdomen. Extra precautions
are taken for patients who are pregnant to lower the chance of mutation to the fetus DNA.
4. LIMITATIONS TO CHEST X-RAY
X-ray is a 2-dimensional representation of a 3-dimensional object. The depth information is lost. Different planes are
overlapped into one which makes detection of abnormalities very difficult. X-ray also can not differentiate between
different types of tissue with similar density. This limitation can be solved by using computed tomography which gives
a 3-dimensional cross section picture of the human body. The computed tomography can also differentiate between
some types of tissues with similar density.
Angiogram is a procedure involving inserting a flexible tube, catheter, into the heart through a small opening made in
the leg, see figure 4. The catheter ejects a water soluble dye, usually iodine, into the blood vessels. The dye absorbs the
x-ray photons and makes the blood vessels visible on a x-ray film. The tip of the catheter is a pressure sensor. It can
measure the pressure inside the blood vessel. If the pressure is about the same as the external blood pressure, it indicates
there are no narrowed or clotted vessels up to that point. If the pressure is lower then the external blood pressure, it is an
indication that the catheter passed a location of narrowed vessel. This procedure is usually performed before a bypass
surgery to see now severe is the clotting. If the clotting is not severe, angioplasty, insert of a stent, can be perform in
place of a bypass surgery. Angioplasty can be done at the same time as the angiogram, so the patient doesn’t have to
undergo another procedure.
Figure 4 Source: Medline Medical Encyclopedia
There are many benefit of angiogram. As mentioned above, the use of a catheter makes it possible to combine diagnosis
and treatment in a single procedure, such as the insertion of a stent. Angiogram provides a clear and detail picture of the
blood vessels, which cannot be done in chest x-ray. It can be use to determine diseased, narrowed, enlarged or blocked
vessels. For example, figure 5 below show an artery with a 99% blockage.
Figure 5 Source: http://www.pcrm.org/news/downloads/angiogram.pdf#search='Angiogram'
Since angiogram is an application of x-ray. The risk of angiogram is very similar to x-ray. During the process of
angiogram, more photons are emitted into the body than chest x-ray. This is because when more radiations are emitted,
more are transmitted through the body and to the detector. The higher number of photons causes the background of the
film to be whiter, by lowering the Ii/It ration. Therefore, exposure to radiation is higher for angiogram.
In addition to the radiation risk, in very rare case, the catheter can puncture the artery and cause internal bleeding, see
figure 2. Some individuals might develop severe allergic reaction to the dye.
6. COMPUTED TOMOGRAPHY (CT SCAN)
Computed Tomography is the use of computer and algebraic reconstruction to obtain a cross section view of the body
using x-ray absorption date obtain by many radiation detectors. The X-ray generator is located on the top emitting X-ray
beam while rotating (in a fan shape). The X-ray detectors are located along the fixed circle of the CT machine to receive
the transmitted photons. A typical system has around 500 to 1000 detectors, see figure 6. All the data are taken in by
the computer and reconstructed into a cross section picture of the body in as little as 2 seconds by using algebraic
Figure 9 Source: RadiologyInfo
As mentioned before, CT solved the 2-dimension limitation of X-ray by providing a 3-dimensional cross section picture
of the body. It can also differentiate between some tissues with slight differences in density. However, due to the need
of emitting radiation at different angles, CT emits a relatively large amount of radiation into the body (1000mR) in
comparison to chest x-ray (20mR). Therefore, computed tomography causes more damage to the tissues then any other
type of diagnostic x-ray. This is about how much background radiation an average person is exposed to in three years.
In this paper we talked about the three type of x-ray technology we currently used to diagnose heart related diseases.
The methods are:
• Chest X-Ray
• Computed Tomography
All three of these methods are application of the photoelectron theory or x-ray. When the energy of the X-ray photon
particle emitted is slightly higher than the electron binding energy, the photon gives up its energy by knocking the
electron out of the orbital, creating a photoelectron. The photons that are not absorbed can be absorbed by a radiation
detector to create a picture indicating the absorption pattern by the body, usually a x-ray film. The pictures allow doctor
to see the position and size of different organs, like the heart, based on the density of the different tissue. The tissue with
higher density absorbs more photon and appears whiter on the x-ray film.
The three different methods apply this theory in different ways to obtain different results. Computed Tomography used
this theory with computer reconstruction to create 3D cross section pictures of the body, while angiogram used dye and
catheter to obtain clear and detailed picture of the blood vessels.
All three methods are different application of x-ray. They all carry the risk of DNA damage associated with exposure to
radiation. The amount of radiation vary due to the amount of radiation emitted during the procedure, most for CT and
least for chest x-ray. They each also carry their own specific risk.
1. Principles of Medical Imaging - Shung, K. Academic Press, Inc. 1992 Pages 1-73
2. National Heart, Lung, and Blood Institute - http://www.nhlbi.nih.gov
3. RadiologyInfo - http://www.radiologyinfo.org
4. Medline Medical Encyclopedia - http://www.nlm.nih.gov/medlineplus/ency
5. The Franklin Institute Online - http://www.fi.edu
6. Heart Center Online - http://heart.healthcentersonline.com/angiogram