1. Introduction to Medical Imaging Week 2: X-ray and CT
Course
Introduction to Medical Imaging Week 2: X-ray and CT
Guy Gilboa

2. X-Ray
First imaging modality (Discovered by Röntgen in 1895).

3. X-ray discovery Wilhelm Rontgen
Discovered and detected X-rays
Used it as a first medical imaging modality.
Nobel 1901, first Noble prize in physics was awarded to him.
First x-ray image. Hand of Anna Rontgen (wife)

4. X-ray machines
Standard machine C-arm

5. X-ray tube

6. X-ray tube diagram [Board (1)]
Taken from

7. X-ray on the spectrum

8. X-ray energy spectrum
[Board (2)]

9. Energy units
eV = electron- Volt 1𝑒𝑉≅1.6× 10 −19 𝐽 It is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt. Photon energy: 𝐸= ℎ𝑐 𝜆 1𝑒𝑉 𝜆=1240𝑛𝑚 kVp = peak kilo-voltage. The maximum voltage applied across an X-ray tube. Determines the kinetic energy of the electrons accelerated in the X-ray tube and the peak energy of the X-ray emission spectrum. Noise model – Poisson (see details in CT part)
h – Planck’s constant 𝜆 - wavelength
c – speed of light

10. X-ray measures the total attenuation across a line
[Board (3-4)]
Linear attenuation coefficient
Photoelectric and Compton absorptions.
I – X-rays intensity (transmitted through a certain thickness L of tissue):
𝐼= 𝐼 0 𝑒 −𝜇𝐿

11. Attenuation coefficients
Linear attenuation coefficient
N – number of X-rays transmitted through a certain thickness x of tissue
𝑁= 𝑁 0 𝑒 −𝜇 𝐸 𝑥
𝜇 𝐸 = 𝜇 𝐸 𝑝ℎ𝑜𝑡𝑜𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 + 𝜇 𝐸 𝐶𝑜𝑚𝑝𝑡𝑜𝑛

12. X-ray detectors Previously – film, analog radiography.
Today – digital radiography
Indirect conversion
X-ray to light using a scintillator (CsI:T1)
Light to voltage using photodiodes
Direct conversion
Thin-Film-Transistors
Cadium-Teloride, Cadium-Zinc-Teloride – technology not mature yet.

13. Signal to Noise Ratio
Noise – mainly Shot noise - Poisson distribution.
Variance is
SNR
𝜎 2 =𝜇≈𝑁
𝑆𝑁𝑅≝ 𝑆 2 𝑑𝑥 𝑛 2 𝑑𝑥 = 𝑐𝑁 𝜎 ∝ 𝑁

14. Standard optics constraints affect the quality of the X-ray image
Taken from [1]

15. Popular for checking chest (lung problems) and bone fractures

16. X-Ray Summary Advantages: Drawbacks: Does not give 3D info.
Cheap and simple
Low radiation (compared to CT)
Drawbacks:
Does not give 3D info.
Bones can occlude significant diagnostic data.

17. Digital Mummography
Used to detect small tumors or microcalcifications in the breast.
Very high spatial resolution and Contrast to noise ratio (CNR) are need for these type of pathologies (often <1mm in diameter)
Low radiation is important – avoid tissue damage and allow frequent usage.
Low energy (e.g. 26 keV) is used – high contrast, low radiation, low penetration.

19. Computed Tomography (CT)
3D imaging using X-ray radiography

20. History - Invention of CT
Sir Godfrey Hounsfield (English Electrical Engineer), built first CT 1971, scanned head.
Allan McLeod Cormack - math framework.
Nobel prize for both in for the invention of CT.
1975 – first full body scanner.
Hounsfield sketch

21. CT scanner

22. CT is based on the physics of X-ray
Generation by an X-ray tube.
X-ray spectrum (higher energies)
Attenuation in the body (mu)
Poisson noise model.

23. CT – operation principle
Taken from

24. Hounsfield unit Water: 0HU Air: -1000HU
What is displayed in CT images?
Typical medical scanner display:
[-1024HU,+3071HU],
Range:
12 bit per pixel is required in display.

25. Hounsfield units of tissues
Taken from [1]

26. Rendering based on different HU thresholds
Taken from

27. Helical CT
Taken from [1]

28. Gantry

29. Scanning Geometry of a CT System
From [1]

30. Multi Slice Detectors
Taken from

31. Visualization (extra): X-ray tube movie
(3:26 min.)

32. We learned today
Basic principles of generating X- rays: braking and characteristic radiations.
How X-ray is absorbed in the body – photoelectric and Compton absorptions.
Linear attenuation coeffient
Poisson noise model with SNR proportional to square-root of intensity.
CT - Hounsfield Units
CT system basic structure
Next week: CT recon and clinical uses.