13. CAT-scan Flashcards
The index of refraction of X-ray is n = __ for every material
1
If a part of the body is placed between the X-ray source and a photographic plate, what do we get?
We get a negative shadow image corresponding to the absorption of individual tissues
What is the formula for the attenuation law?
J J0 ex .
What is the relationship between the attenuation coefficient and the density of the tissue?
They are proportional to each other
What does the contrast of the X-ray image depend on?
the density differences of the tissues
Value of the mass attenuation coefficient may be different for ___
different tissues
What is the mass attenuation coefficient ?
a function of the atomic number of the components of the absorbent material (expressed in terms of the effective atomic number, Zeff)
What does mass attenuation coefficient depend on?
- the type of the absorption process involved
- the photon energy
What is the relationship between mass attenuation coefficient and atomic number if only photoeffect occurs?
the mass attenuation coefficient is proportional to the third power of atomic number (effective atomic number, Zeff)
It can be seen from the figure that best photon energy regime for X-ray diagnostic imaging is between 10 - 100 keV, where (1)___ dominates and (2)___ plays a partial role
- photoeffect
- Compton effect
At photon energies exceeding 100 keV the mass attenuation coefficients of the tissues are almost ___.
identical
What is the role of contrast materials?
- change the absorption of some organs (e.g., stomach, intestines, and blood vessels) artificially in order to highlight them
- considerably increase the diagnostic value of the acquired image
2 types of contrast agent. How to obtain them
Depending on whether the applied contrast material absorbs better or worse than the surrounding tissues, it is called positive or negative contrast agent, respectively.
How we get a high-resolution, sharp image on the photographic plate.
- apply a point source of X-ray
- The X-ray source should be placed at the longest possible distance from the examined part of the body
- The X-ray spectrum (Fig. 3) is rather wide and of heterogeneous wavelength distribution
- The cross-sectional area of the X-ray beam can be regulated with the beam definer made of lead (Fig. 4), so that only the body parts of interest are exposed.
- Time of exposure has to be appropriately short
By applying a point source of X-ray, how do we get a high-resolution, sharp image on the photographic plate?
To achieve this, the electron beam should be focused at a very small spot (e.g., d = 0.2 mm) on the anode of the X-ray tube (W, Mo), which may overheat the anode material.
→ This technical difficulty can be eliminated by rotating the anode, as the glowing spot of the anode cools down sufficiently during one turn.
By placing the X-ray source at the longest possible distance from the examined part of the body, how do we get a high-resolution, sharp image on the photographic plate?
The half shadow effect can be reduced
→ because radiation intensity is inversely proportional to the square of the distance, to provide sufficient intensity for the examination, X-ray tube of higher power and thus larger size is required.
Characteristics of X-ray spectrum in medical diagnostics
The X-ray spectrum is rather wide and of heterogeneous wavelength distribution, containing unwanted, long-wavelength components
How can Small- energy X-ray photons affect the image and patient in CAT-SCAN? (4)
(The X-ray spectrum)
- They are mostly absorbed
- Do not contribute to the image
- Cause radiation hazard to the examined patient.
- decreases considerably the sharpness of the image
How can long wavelengths be filtered out in CAT-SCAN?
(The X-ray spectrum)
By placing an aluminum plate (Z = 13) of sufficient thickness in the X-ray beam path the long wavelengths can be filtered out (the radiation will be “harder”, Fig. 3)
How to set the optimal wavelength range that is needed for the examination?
(The X-ray spectrum)
by choosing the proper anode voltage and by using a filter (max).
What is shorter wavelength radiation (Uanode = high) used for?
(The X-ray spectrum)
It is used for the imaging of body parts containing bones as it needs larger penetration depth
(for the soft tissues longer wavelengths (Uanode = low) are sufficient.)
How do we get a high-resolution, sharp image on the photographic plate with the cross-sectional area of the X-ray beam?
The cross-sectional area of the X-ray beam can be regulated with the beam definer made of lead
→ only the body parts of interest are exposed
→ minimizes the radiation hazard of the patient and it reduces the amount scattered radiation
How to elimiate “Back scattering” effect?
“Back scattering” effect on the metal platform under the patient can be eliminated by lead shielding placed under the photographic plate.
compare the mass attenuation coefficient of bone to that of soft tissue
the mass attenuation coefficient of bone (Zeff bone ≈ 14) is much greater than that of soft tissues (Zeff soft tissues ≈ 7).
What are Principles of operation of computed tomography
We have the attenuation of a narrow X-ray beam passing through the examined body part
→ According to the attenuation law the attenuation coefficients of volume elements (voxel) of size x sum up
→ For n elements, we obtain elementary density
→ the sum of the elementary densities appears in the shadow image, the projection which is called summation image
What are Principles of operation of computed tomography
What is elementary density?
absorbance by the small volume element (voxel) of the investigated part of the body (lg J0 /J).
What are Principles of operation of computed tomography
What is summation image?
elementary densities of the volume elements along the direction of the X-ray beam are summed.
What are Principles of operation of computed tomography
If a source of a narrow X-ray beam (pencil beam, e.g., A < 1 mm2) is moved together with the detector in parallel planes (constituting a scan), and the procedure is repeated at different angles (Fig. 8), then a hundred thousand of summation intensity data can be recorded.
A computer solves the huge linear system of equations constructed from measured data and calculates ___
calculates the elementary density for every pixel in the given plane (slice)
What are Principles of operation of computed tomography
Describe the image produced by CAT-SCAN
elementary densities of the volume elements along the direction of the X-ray beam are summed.
Other names for CAT-SCAN
Computed Tomography (CT), or sometimes X-ray densitography.
How to increase the contrast of X-ray images?
By using contrast agent
→ Atomic with high numbers are used for positive contrast
→ Atoms with low atomic number are used for negative contrast
How to enhance sensitivity of X-ray image?
By using intensifier
Describe structure of X-ray intensifier (aka X-ray amplifier
- 2 luminescence screens
- Electrode
- Photocathode
Steps of operation of X-ray intensifier (aka X-ray amplifier
Body’s X ray image appears on the first luminescence screen (attenuated ration is proportional to the number of optical photons)
→ Luminescent light emitted to the photocathode produced photoelectrons (which is proportional to the intensity of the radiation)
→ Photoelectrons are accelerated and focused using high voltage to reach the 2nd luminescence screen
→ Through electron lens, a real image is projected
Advantages of X-ray intensifier (aka X-ray amplifier)
X radiation dosage needed for image is reduced
→ Digitizable optical image can be produced without using an X-ray absorption film
Disadvantages of X-ray intensifier (aka X-ray amplifier)
Spatial resolution decreases as the amplified image is made on a smaller scale (miniaturized) with respect to the original X-ray image
What is DSA?
Digital subtraction angiography is a Fluoroscopy technique that visualizes blood vessels in bone or dense soft tissue
Describe DSA (digital subtraction angiography) technique
- A native X ray image is taken without any contrast
- An additional X-ray image is taken after the contrast agent has been injected
- Finally, native image is subtracted from the contrast image → produce the contrast-image that focuses on the vessels
Contrast-native image = contrast image - native image
What is Windowing in CAT SCAN?
a CT contrast enhancement technique that….
- extreme values of Hounsfield units (HU)-range to be displayed are expanded to fit the whole range of pixel brightness
=> All areas with values falling outside designated range/window ( Hounsfield units (HU)-range) will be uniformly black or white but differences inside chosen range are visible on a grayscale
Windowing in CAT SCAN
How are images displayed according to windowing in CAT Scan
- Tissues with higher X-ray absorption are displayed as white or light gray
- Tissues with lower X-ray absorption are displayed as black
What are Hounsfield unit?
- Used to measure absorption coefficients or density values on CT-images
- Used to Magnify the small differences in density between water and soft tissues
How to calculate Hounsfield unit?
- attenuation coefficient of water as the reference
- Attenuation coefficients of different tissues components are assigned in relation to water
4 Interactions of photon that involve in X ray diagnostic
- elastic scattering
- photoeffect
- Compton scattering
- pair production (no interaction)
What is mechanism of spiral CT?
It contains
- a continuously moving X-ray tube
- A continuously and uniformly moving table
RESULT: radiation absorption of all voxels in a large cylindrical body volume can be determined
=> 3D image construction
Generations of CAT SCAN
- image can be achieved by having the patient move at a constant velocity while the X ray detector also moves at a constant angular velocity in circular motion
→ This arrangement will draw a helical path of X-ray on the surface of patient (spiraling around the patient)
→ The dimension of the beam can also be expanded along the axis of the patient, which allows multi-imaging (acquire several slide images at once)
What is effective atomic number?
A measure of average number of electrons of material that participate actively during interaction
