Medical Physics
Chapter 17 Physics of Nuclear Medicine
Chapter 17 Physics of Nuclear Medicine
(Radioisotopes in Medicine)
l
l
l
Natural radioactivity (Table 17.1)
m Becquerel (1905 Novel Prize)
m Curie: radium
m Alpha ray
Ÿ Nuclei of helium atoms
Ÿ A few centimeters in air
Ÿ Positively charges
Ÿ Fixed energy
m Beta ray or negatron (β-)
Ÿ High-speed electrons
Ÿ A few meters in air
Ÿ Negatively charged
Ÿ Spread of energies
m Gamma ray
Ÿ Very penetrating
Ÿ Physically identical to x-ray but much higher energy
Ÿ Fixed energy
Isotopes: single radioactive element
m Nuclei of a given element with different numbers of neutrons
m Stable isotopes: not radioactive, 12C, 13C
m Radioisotopes: radioactive, 11C, 14C, 15C
Radionuclide: several radioactive elements
1. Review of Basic Characteristics and Units of Radioactivity
m Radioactive element ⇒ daughter (radioactive) ⇒ daughter ⇒ …⇒ final daughter
(Table 17.2)
m Radionuclide (Table 17.3)
Ÿ > 1000
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Ÿ
Ÿ
Ÿ
Ÿ
Most of them are man-made
Characteristics: radioactivity, type and energy of its emitted particles of rays
Beta-emitting radionuclide: 3H, 14C, 32P are used in medicine
Gamma rays: usually > 100 keV
Ÿ
Positron (β+): positive beta ray, only from man-made radionuclide, physically
identical to electron with positive charge
Decay
Ÿ
ù
A = Ao e − λt (A is in disintegrations/s, Ao: initial activity, λ: decay constant)
ù
A = λ N (N is the number of radioactive atoms)
ù
0.693
: Fig. 17.1
λ
Average or mean life time, τ = 1.44T1/2
ù
Ÿ
Half-life, T1/2 =
Unit of radioactivity (Table 17.4)
ù
ù
1 curie (Ci) = 3.7×1010 disintegrations/s
SI unit: Becquerel (Bq), 1 Bq = 1 disintegrations/s
2. Sources of Radioactivity for Nuclear Medicine
m Radioactive drugs or radiopharmaceutical
Ÿ Long-lived: may be delivered from a factory
Ÿ Short-lived: produced by radiopharmacist in local medical centers
Ÿ Need calibration before use
3. Statistical Aspects of Nuclear Medicine
m Counting the number of gamma rays detected from a patient in 1 min: Fig. 17.5
m Net count, N net = N g − N b and
Ÿ
Ÿ
Ng = gross count with radioactive source
Nb = background count without radioactive source (noise)
Ÿ
Standard deviation of net count, σ net = N g + N b
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m Net count rate,
Chapter 17 Physics of Nuclear Medicine
N net N g N b
=
−
min t g
tb
and standard deviation of net count rate,
σ net = σ g2 + σ b2
4. Basic Instrumentation and Its Clinical Applications
m Types of counting
Ÿ Amount of radioactivity in a given sample or volume
Ÿ Distribution of radioactivity in the body (imaging)
m Detector
Ÿ
Ÿ
Scintillation counter: alpha particle ⇒ crystal of zinc sulfide ⇒ flash or
scintillation
Geiger-Mueller counter (Geiger counter or GM counter): Fig. 17.6
ù Beta ray ⇒ ionization ⇒ electric pulse
ù No information about the amount of ionization
ù Not efficient for gamma ray
Ÿ Photomultiplier tube (PMT): Fig. 17.7 and 17.8
ù Scintillation detector
ù
ù
ù
Gamma ray ⇒ crystal ⇒ light photon ⇒ electron from photocathode ⇒
more electrons and acceleration through dynodes ⇒ anode
Usually 10 dynodes ⇒ 105 ~ 106 times electron multiplication
High voltage source of 1000 V
Scintillation crystal (NaI): gamma ray ⇒ light flash (photon)
Collimator
Pulse height analyzer (PHA) determines the energy of gamma ray: Fig.
17.9
ù Multichannel analyzer (MCA): Fig. 17.10
Ÿ Solid-state semiconductor detector: Fig. 17.11
ù
ù
ù
ù Gamma ray ⇒ electron-hone pair generation
m Collimator: lead shield with one or more holes: Fig. 17.12
Ÿ Open or flat field collimator: gamma rays from large volume (thyroid or
kidney)
Ÿ Focused collimator: gamma rays from small volume (imaging)
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Chapter 17 Physics of Nuclear Medicine
m Applications
Ÿ 24-h uptake of radioactive iodine by the thyroid: thyroid uses iodine in the
production of hormones (Fig. 17.13)
Ÿ Kidney function evaluation using 131I-labeled hippuric acid: hippuric acid is
removed from blood by the kidney (Fig. 17.14 and 15)
Ÿ In-vitro counting: well counter (Fig. 17.16)
5. Nuclear Medicine Imaging Devices
m Distribution of radioactivity ⇒ image
Ÿ Rectilinear scanner
Ÿ Gamma camera
m Rectilinear scanner (Fig. 17.17)
Ÿ Focused collimator
Ÿ Two detectors for scanning both sides of a patient simultaneously (Fig. 17.19)
Ÿ Long scan time ⇒ motion artifact
m Gamma camera or Anger camera (Fig. 17.20 and 21)
Ÿ Large scintillation crystal ⇒ many PMTs ⇒ image (Fig. 17.22)
Ÿ Resolution of 5 mm
Ÿ Shorter imaging time of 1 ~ 2 min
m Positron camera (Fig. 17.24)
6. Physical Principles of Nuclear Medicine Imaging Procedures
m Cancerous nodules in the thyroid (Fig. 17.25)
m Liver scan (Fig. 17.26)
Ÿ Normal liver: filter radioactive particles from blood
Ÿ Tumor in the liver: no filtration of radioactive particle
m Brain tumor (Fig. 17.27)
Ÿ Brain tumor takes up some radioactive materials better
m Bone scan (Fig. 17.28)
Destroyed bone due to cancer ⇒ take up more radioactive materials to rebuild
bone
m Pulmonary embolism (blood clot blocking a major artery in the lung)
Ÿ
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Ÿ Radioactive material in blood to detect blockage
m Air circulation system in the lung using radioactive gas
m Heart scan using radioactive material that will be concentrated in infarct region
7. Therapy with Radioactivity
m Radioactive drugs
m See Chapter 18
8. Radiation Doses in Nuclear Medicine
m Critical organ: an organ receiving the largest dose (Table 17.6)
m Gonad dose to estimate the possible generic effect of the procedure
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