Medical Equipment lec 9

1. Medical Equipment IV
Nuclear Medicine
Shereen M. El-Metwally
Associate Professor,
Systems and Biomedical Engineering Department,
Faculty of Engineering - Cairo University
sh.elmetwally@eng1.cu.edu.eg

2. Nuclear Medicine
 Nuclear medicine is a branch of medicine and
medical imaging that uses a small amount of a
radioactive material called “radiotracer” or
“radiopharmaceutical” to diagnose and treat
some diseases, including many types of
cancers, heart disease and certain other
abnormalities within the body.

3. Why is it called nuclear
medicine?
 It is called nuclear medicine because we use
a small amount of radioactive material, which
is based on the nuclear properties of a
substance.

4. Nuclear Medicine
 Nuclear medicine scans a very small amount (e.g.,
nanograms) of a radioactive material called a
“radiotracer” injected intravenously into the patient
 Agent then accumulates in specific organs in the body
 How much, how rapidly and where this uptake occurs
are factors which can determine whether tissue is
healthy or diseased.
 Three different modalities under the general
umbrella of nuclear medicine
 Planar scintigraphy
 Single photon emission computed tomography (SPECT)
 Positron emission tomography (PET)

5. Which areas of the body does nuclear
medicine help treat?
 Nuclear medicine is used for all organ
systems in the body - the central nervous
system, the heart and major vessels, for
gastrointestinal tract, for the urinary system
including the kidneys, for soft tissues, for
bones and skeleton, and for the lungs.
Basically, every organ system is touched
upon by nuclear medicine.

6. Modality usefulness
 Nuclear medicine imaging is useful for
detecting:
 tumors
 aneurysms (weak spots in blood vessel walls)
 irregular or inadequate blood flow to various
tissues
 blood cell disorders and inadequate functioning
of organs, such as thyroid and pulmonary
function deficiencies.

7. Nuclear Medicine Modalities
Planar scintigraphy
 images the distribution of radioactive material in a
single two-dimensional image, analogous to a planar
X-ray scan.
 Mostly used for whole-body screening for tumors,
particularly bone and metastatic tumors.
 The most common radiotracers are chemical complexes
of technetium (99mTc), an element which emits mono-
energetic γ -rays at 140 keV.
 Various chemical complexes of 99mTc have been
designed in order to target different organs in the body.
7

8. Nuclear Medicine Modalities
Single photon emission computed
tomography (SPECT)
 produces a series of contiguous two-
dimensional images of the distribution of the
radiotracer using the same agents as planar
scintigraphy.
 There is a direct analogy between planar X-ray/CT
and planar scintigraphy/SPECT.
 Most commonly used for myocardial perfusion, the
so-called ‘nuclear cardiac stress test’.Machine Learning Spring 2014 Inas A.
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9. Nuclear Medicine Modalities
Positron emission tomography (PET)
 This involves injection of a different type of radiotracer, one
which emits positrons (positively charged electrons). These
annihilate with electrons within the body, emitting γ -rays
with an energy of 511 keV.
 The PET method has the highest sensitivity of the three
techniques, producing high quality three-dimensional
images with particular emphasis on oncological diagnoses.
 PET and CT systems have been integrated such that all
commercial units sold now are combined PET/CT
scanners.
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10. Nuclear Medicine
 Relative to most other imaging modalities, nuclear
medicine scans (in particular planar scintigraphy and
SPECT) are characterized as having:
Disadvantages:
 Poor SNR
 Low spatial resolution (~5–10 mm)
 Slow image acquisition
Advantages:
 Extremely high sensitivity to slight differences in soft tissue
contrast
 Very high specificity (no natural radioactivity from the body
and the used radiotracer has a high specific uptake in the
organ of interest and relatively low non-specific uptake in the
rest of the body)
 Intrinsic functional characteristics of the information content in
the images

11. How does nuclear medicine differ from
other types of radiology?
 Nuclear medicine differs from other
modalities, like CT or MRI, in that those
studies are anatomically-based. They look at
anatomy or structure.
 Nuclear medicine looks at physiology of the
body and all the organ systems.
 We can follow the physiological processes as they
occur in a living human using these
“radiopharmaceuticals” and through the use of
appropriate imaging systems.

12. Radioactivity
 A radioactive isotope is one which undergoes a
spontaneous change in the composition of the nucleus,
whereby unstable nuclei become more stable.
This change is termed a ‘decay’ or ‘disintegration’, resulting
in the emission of energy.
 Radioactivity, Q, is defined as the rate of decay (number of
decays or disintegrations per unit time) of a radioactive
sample.
 It depends on the number N of radioactive atoms in the
sample:

13. Radioactivity
Units of Activity
 Curie (Ci)
 Historical unit of activity, the number of disintegrations
per second from 1 gram of Radium.
1 Ci = 3.7 x 1010 disintegrations per second.
 MilliCuries (mCi): most commonly used units for doses
relevant to clinical studies.
 Bequerel (Bq)
 SI Unit of activity
 1 Bq = 1 disintegration per second
 Commonly expressed as MBq or GBq.
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14. Radioactivity

15. Radioactivity
 In nuclear medicine scans, the total
radioactive dose experienced by the
patient is limited by federal safety
guidelines.
 To calculate the dose, the biological
half-life of the radiotracer must also be
considered.
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16. Half-Life time Example
 Two patients undergo nuclear medicine
scans. One receives a dose of
radiotracer A and the other radioatracer
B. The half-life of A is 6 hours and of B
is 24 hours. If the administered dose of
radiotracer A is three times that of
radiotracer B, at what time is the
radioactivity in the body the same for the
two patients?

17. Solution
 Mathematically, we can solve for the
time t:
 The values of λA and λB are derived as
6.42 x 10-5 s-1 and 2.41 x 10-5 s-1,
respectively. Solving for t gives a value
of 7.63 hours.
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18. The ideal properties of a radiotracer for
planar scintigraphy and SPECT
i. Radioactive half-life
 should be short enough to produce significant radioactivity
without requiring a very large initial dose,
 should not be so short that there is still significant decay
before the post-injection delay required to allow the
radiotracer to clear the blood and distribute in the relevant
organs elapses.
ii. Decay should be via emission of a mono-energetic γ-ray
without emission of alpha- or beta-particles.
 Alpha- or beta-particles are completely absorbed within tissue,
therefore increasing the radioactive dose without giving any
useful image information
 A mono-energetic γ-ray allows discrimination between
Compton scattered and unscattered γ-rays, thereby improving
image contrast.

19. The ideal properties of a radiotracer for
planar scintigraphy and SPECT
iii. The energy of the γ-ray should be greater than
~100 keV, so that a reasonable proportion of γ –
rays emitted deep within the tissue have sufficient
energy to travel through the body and reach the
detector.
iv. The energy of the γ-ray should be less than ~200
keV so that the rays do not penetrate the thin lead
septa in the collimator.
v. The radiotracer should have a high uptake in the
organ of interest and relatively low non-specific
uptake in the rest of the body.Machine Learning Spring 2014 Inas A.
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21. Nuclei chart

22. Decay Types
Charged particle decay
Alpha Decay
 Mostly occurs for heavy nuclei
 is an energetic He nucleus (4
2He).
 Alpha particles of two different energies may be emitted – 4.78
MeV or 4.60 MeV.
 The alpha particle is a relatively massive, poorly penetrating type
of radiation that can be stopped by a sheet of paper.
 Therefore, alpha emitters are of little use in nuclear medicine as
they do not penetrate tissue, and they represent a severe health
hazard if ingested or inhaled.

23. Decay Types
Alpha Decay
238
94Pu144  234
92U + 
 Parent nucleus 238
94Pu
 Daughter Nucleus 234
92U
 Often the daughter nucleus is also radioactive
and will itself subsequently decay.
 Decay chains or families (e.g. uranium,
thorium decay chains).

24. Uranium Decay Chain Example
238
94Pu  234
92U + t1/2 = 88 yrs
234
92U  230
90Th +  t1/2 = 2.5 x 105 yrs
230
90Th  226
88Ra +  t1/2 = 8.0 x 104 yrs
226
88Ra  222
86Rn +  t1/2 = 1.6 x 103 yrs
222
86Rn  218
84Po +  t1/2 = 3.8 days
218
84Po  214
82Pb +  t1/2 = 3.1 min
214
82Pb  214
83Bi +  t1/2 = 27 min
214
83Bi  214
84Po +  t1/2 = 20 min
214
84Po  210
82Pb +  t1/2 = 160 s
210
82Pb  210
83Bi +  t1/2 = 22 yrs
210
83Bi  210
84Po +  t1/2 = 5 days
210 Po  206 Pb +  t = 138 days
206
82Pb is STABLE

25. Decay Types
Beta Decay
 Negatron Decay
 A neutron is transformed to a proton. β- is negatron, i.e. a
negative electron ejected from the nucleus, and ν is a
massless neutral particle that accompanies the negatron,
termed an antineutrino.
 Negatron decay results in an increase in Z (atomic no.) of
one, a decrease in N (no. of neutrons) of one, and a
constant A.
 Beta emission often leaves the daughter nucleus in an
excited state. The daughter nucleus then decays to a more
stable state by emitting a γ -ray.
β-
β-

26. Decay Types
Beta Decay
 Positron Decay
 A proton is transformed to a neutron. β+ represents a positron,
i.e. a positively charged electron, ejected from the nucleus
during decay, and ν is a neutrino that accompanies the
positron.
 Positron decay results in a decrease of one in Z, an increase
of one in N, and no change in A.
 The positron travels a short distance before it encounters an
electron. When encounters an electron, the two annihilate
thus converting their combined mass to energy in the form of
two γ –rays, each with an energy of 511 keV in opposite
directions.

27. Decay Types
 Photons
 Gamma (γ) rays
 X-rays
 Differ only in origin.
 Gamma rays are emitted from the nucleus, while
X-rays are emitted from the atom, usually the
orbital electrons.
 Uncharged.
 More penetrating power than charged
particles: few inches of Pb, many feets of air.

28. Decay Types
 Gamma (γ) rays
 An unstable nucleus rapidly and almost
instantly emits one or more gamma-rays to get
to the ground state.
 Gamma-ray energies are characteristic of the
nucleus.
 Measure the energies … identify the nucleus. (just
like atoms or molecules give off characteristic colors
of light).
 Measuring the gamma-ray is by far the best and
easiest way to measure what type of radioactive
substance is making the gamma emission.

29. Detector Types
 Range of Radiation
 Alpha: Small. Shield with a piece of paper
 Beta: Smallish Shield with a ½ inch or so
of Pb
 Gamma: Long Shield with a few inches of Pb
 To detect the radiation it has to: Get to And Get into detector

30. Detector Types
 Film Badges
 Gas-filled Detectors
 Scintillation Detectors

31. Detector Types
 Gas-Filled Detectors
 Ionization chambers – personal dosimeters, dose
calibrators.
 Geiger-Muller counters for measuring ambient
radiation.
 Principle – Incident radiation ionizes gas
particles. The ionization of gas within an
electrically charged enclosure causes the flow
of a (small) electric current that is measured.
 Gas-filled chambers are not efficient detectors
for x- and γ -ray photons.

32. Detector Types
 Scintillation Detectors
 Ionizing radiation is absorbed by the scintillator.
 (NaI) crystal:
 Emits visible light when hit by gamma ray.
 Amount of light photons produced is directly
proportional to the energy of the incident gamma ray.
 Photomultiplier tubes:
 read the light signals and translate them into electrical
signals
 light photons strike photocathode of a photomultiplier tube,
releasing photoelectrons.

33. Scintillator Detectors

34. Cross Section in Scintillator
Detectors
1. Shield Around Head
2. Mounting Ring
3. Collimator Core
4. Sodium Iodide Crystal
5. Photomultiplier Tubes

35. Technetium Generator
 The most widely used radiotracer is 99mTc which is involved in
over 90% of planar scintigraphy and SPECT studies. Tc
generator is delivered to a nuclear medicine department weekly
then replaced.
 Since there is a two-step decay process, the amount of 99mTc
increases due to the decay of 99Mo, but decreases due to its own
decay to ground state 99gTc:

36. Technetium Generator
 Technetium generator comprises an alumina ceramic column with
radioactive 99Mo absorbed on to its surface in the form of ammonium
molybdate, (NH4)2MoO4.
 At any given time the generator column
contains a mixture of 99Mo, 99mTc and 99gTc.
 Generator “Milking”: To remove the 99mTc selectively, saline is drawn
through the column to wash out most of the 99mTc which does not
bind strongly to the column and is eluted in the form of sodium
pertechnetate.
Radioactivity of 99mTc

37. Nuclear Imaging Instruments:
Gamma Camera

38. Gamma Camera
 The gamma (Anger) camera is the instrumental basis for both planar
scintigraphy and SPECT.
 In planar scintigraphy: a single gamma camera is held stationary
above the patient.
 In SPECT: either two or three cameras are rotated slowly around the
patient with data collected at each angular increment.
 The patient lies on a bed beneath the gamma camera, which is
positioned close to the organ of interest.
 Decay of the radiotracer within the body produces γ-rays, a small
percentage of which pass through the body (the vast majority are
absorbed in the body).
 The gamma camera must be capable of γ -ray detection rates of up to
tens of thousands per second.
 A two-dimensional collimator (similar to the anti-scatter grid in X-
ray imaging) is placed between the patient and the detector.
 It passes only those γ-rays which strike the gamma camera at a
perpendicular angle to be detected, and rejects those γ -rays that have
been scattered in the body therefore having no useful spatial 38

39. Gamma Camera

40. Gamma Camera:
The detector scintillation crystal
 The detector is a large single
crystal of thallium-activated sodium
iodide, NaI(Tl), approximately 40–
50 cm diameter.
 Large circular or rectangular crystals
 High linear attenuation coefficient of
2.22 cm-1 at 140 keV photon energy.
 Efficient with one light photon produced
per 30 eV of photon energy absorbed,
an efficiency of ~15%.
 Transparent to its own light emission at
a wavelength of 415 nm (visible blue
light), so little energy is lost due to
absorption.
 The 415 nm emission wavelength is
well-matched to the optimal performance
of conventional photomultiplier tubes
(PMTs).

41. Gamma Camera:
Photomultiplier Tubes
 Anger cameras have an array of PMTs (photomultiplier
tubes) attached to the back of the scintillation crystal
 # of tubes is determined by size & shape of both the
crystal and individual PMT. Arrays of 61, 75 or 91 PMTs
are typically used.
 Current PMT-hexagonal arrangement is used to cover
more of crystal area
 A PMT produces between 105 and 106 electrons for each
initial photoelectron, creating an amplified current at PMT
output.
 When a scintillation event occurs, each PMT produces an
output pulse whose amplitude is directly proportional to the
amount of light. The PMT closest to scintillation event
produces the largest output current.
 The more the number of PMT’s, the better the spatial
resolution and linearity.

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(left). The first three amplification stages in a PMT tube. (right) A commercial
PMT.

43. Gamma Camera: The Anger
position network
 Is important for the determination of the spatial location of
the scintillation event.
 Whenever a scintillation event occurs in a NaI(Tl) crystal,
the PMT closest to the scintillation event produces the
largest output current.
 If only the PMT with the largest pulse is used for (x, y)
positioning, the spatial resolution would be no finer than
the dimensions of the PMT, i.e., several cms.
 However, adjacent PMTs produce smaller output currents,
approximately inversely proportional to the distance
between the scintillation event and the particular PMT.

44.  Combining output currents from all the PMTs allows better
estimation of the location of the scintillation within the
crystal based on Centroid (center of mass) approach.
 In older analog gamma cameras this process is carried out
using an Anger logic circuit, which consists of four resistors
connected to the output of each PMT. This network
produces four output signals, X+, X-, Y+ and Y- which are
summed for all the PMTs.
 The estimated (X,Y) location of the scintillation event in the
crystal is given by:

45. Gamma Camera: Pulse Height
Analyzer (PHA)
 Role of the PHA is to discriminate whether the recorded
events correspond to γ-rays that pass directly through tissue
to the detector (primary radiation) and should be recorded, or
have been Compton scattered in the patient so do not contain
any useful spatial information and should be rejected.
 Since the amplitude of the voltage pulse from PMT is
proportional to the energy of the detected γ -ray,
discriminating on the basis of the magnitude of the output of
the PMT is equivalent to discriminating on the basis of γ -ray
energy.
 Pulse Height Analyzer selects a centerline that corresponds
to the main photopeak energy centered at 140 KeV and a
window size to set the threshold level for accepting the
‘photopeak’.
 Example: a 20% window around a 140 keV photopeak

46. Pulse Height Analyzer
 A multiplechannel analyzer (MCA), in
which the term ‘channel’ refers to a specific
energy range, uses an ADC to digitize
signal, then produce a pulse-height
spectrum
 The number of channels in an MCA can be
more than a thousand, allowing essentially a
complete energy spectrum to be produced.
 After digitization, the
upper and lower threshold
values for accepting the 𝛾 –
rays are applied. Pulse height spectrum: a plot of the number of events from
the PMTs as a function of the output voltage level.

47. Pulse Height Analyzer
 The dashed lines show the
energy resolution of the
camera, defined as the FWHM of
the main photopeak centered
at 140 keV.
 The narrower the FWHM of the
system, the better it is at
discriminating between
unscattered and scattered γ -rays.
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48. Instrument Dead Time
 If an injected dose of radiotracer is very large,
the total number of 𝛾-rays striking the
scintillation crystal can exceed the recording
capabilities of the system
 Particularly true at the beginning of a clinical scan
when radioactivity is at its highest level
 Limitation is due to the finite recovery and reset
times required for various electronic circuits in the
gamma camera
 Two types of behavior exhibited by the system
components, termed paralysableand
nonparalysable

49. Instrument Dead Time
 Paralysablebehavior is a phenomenon in which a
component of the system cannot respond to a new event
until a fixed time after the previous one, irrespective of
whether that component is already in a non-responsive
state.
 Each time a 𝛾-ray strikes the scintillation crystal it produces an
excited electronic state, which decays in 230 ns to release a
number of photons.
 If another 𝛾 -ray strikes same spot in the crystal before the
excited state decays, then it will take a further 230 ns to
decay, and only one set of photons will be produced even
though two c-rays have struck the crystal
 Very high rate of 𝛾 -rays striking detector can result in a
considerably elongated dead-time and so the number of
recorded events can actually decrease.

50. Instrument Dead Time
 Non-paralysable component cannot respond for a fixed
time, irrespective of the level of radioactivity
 For example, Anger logic circuit and PHA take a certain
time to process a given electrical input signal, and
during this time any further events are simply not
recorded: this time is fixed, and is not elongated if
further scintillation events occur.
 The overall ‘dead-time’ (s) of the system is given by:
where N is the true count rate (number of scintillations per
second), and n is the measured count rate.

51. Instrument Dead Time
 Example: Suppose that the true count rate
in a gamma camera is 10 000 per second,
but the measured rate is only 8000 per
second. What is the dead time of the
system?
 Solution:
The value of τ is given as 25 µs.
So rather than recording one count every 100 µs
(the true value), there is a 25 µs effective delay
and one event is recorded every 125 µs.Machine Learning Spring 2014 Inas A.
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52. Collimators
 There is an intrinsic trade-off between spatial
resolution and collimator efficiency.
Pinhole

53. Single Photon Emission
Computed Tomography
(SPECT) Scanners SPECT uses two or three gamma cameras
which rotate around the patient detecting γ -
rays at a number of different angles.
 Store radiotracer emission data from multiple
projections. Projections are taken every 3 or
6 degrees.
 Use CT type algorithms to reconstruct
multiple two-dimensional axial slices from the
acquired projections.
 SPECT uses essentially the same
instrumentation and many of the same
radiotracers as planar scintigraphy,
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A three-head rotating
gamma camera SPECT
system for imaging the
torso.

54. SPECT Scanners
 Image intensity is related to
the concentration and degree
of accumulation of the
radiotracer.
 The most important
application of SPECT is for
assessing myocardial
perfusion to diagnose
coronary artery disease
(CAD) and damage to the
heart muscle following an
infarction (a heart attack).
 Areas of myocardial infarction
are visualized as cold areas
on the myocardial scan.
 Used also for brain studies to detect areas of reduced blood
flow associated with stroke, epilepsy or Alzheimers.

55.  A technique that tracks biochemical and physiological
processes in vivo
 Uses radiotracer compounds labeled with positron-emitting
radionuclides.
 Considered a form of functional imaging
 How is PET different from SPECT?
 PET has between 100 and 1000 times higher SNR as well
as significantly better spatial resolution than SPECT
 The much higher SNR of PET compared to SPECT arises
from several factors including:
(i) collimation not being required, (ii) reduced attenuation
of higher energy gamma rays in tissue (511 keV vs. 140
keV) , and (iii) the use of a complete ring of detectors.
PET – Positron Emission
Tomography

56. PET – Positron Emission
Tomography Certain radionuclides
emit positrons.
 When a positron meets
an electron in tissue,
they annihilate each
other.
 This annihilation results
in the generation of two
gamma rays.
 The gamma rays travel
in opposite directions.
 The energy of these
gamma rays is 511 KeV.
 PET Imaging is based
on the detection of
these gamma rays.

57. PET Systems Event
Detection
 Several gamma-detector rings
surround the patient.
 When one of these detects a photon, a
detector opposite to it, looks for a
match.
 Time window for the search is few
nanosecs. The time-window allowed
after the first γ -ray has been detected
for a second γ -ray to be recorded and
assigned to the same annihilation is
called coincidence resolving time.
 If such a coincidence is detected, a
line-of-reconstruction(LOR) is
drawn between the two detectors.
 When done, there will be areas of
overlapping lines indicating regions of
LOR – Line of Response

58. Coincidence Events
 Three Types:
 True
 The event we are after
 Scatter
 At least one Compton scatter event
 Wrong line-of-reconstruction(LOR)
 Random
 Unlucky break
 Current hot topic:
 Time-of-flight PET
 Estimate the arrival time at the two
detectors.
 Picoseconds electronics
 This allows localization within the LOR.

59. PET Scanner Components
 Scanner - to perform the
clinical exam
 Includes a tomographic
reconstruction
 The detectors (typically many
thousands) consist of small
crystals of bismuth germanate
(BGO), coupled to PMTs
whose output voltages are
then digitized.
 Includes also annihilation
coincidence circuitry.
 Cyclotron – for on-site
synthesis of PET

60. PET/CT and SPECT/CT
scanners Since 2006 no stand-alone commercial PET
scanners have been produced, all are now
hybrid PET/CT systems. The hybrid system
use a single patient bed to slide between the
two scanners.
 The reasons for combining PET and CT in a
hybrid imaging system are essentially identical
to those for SPECT/CT, namely:
 The fusion of high-resolution anatomical
(CT) with functional (PET or SPECT)
information, allowing the anatomical location of
radioactive ‘hot’ or ‘cold’ spots to be defined
much better.
 Improved attenuation correction using CT
data: γ-rays from radiotracers located in the 61
SPECT/CT
PET/CT

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62. What are the benefits and risks of nuclear
medicine?
 Benefits
 helping with diagnosis and especially more recently
helping with therapy in many of the conditions that
humans suffer. For example, thyroid cancer and more
recently lymphoma.
 Risks
 Radiation risk but that's also very small, and it's
approximately the same level you receive from natural
sources.
 Very little side effects associated with the administration
of radiopharmaceuticals, if any, and therefore the benefit
to risk ratio for nuclear medicine is tremendous.

63. References
 Chapter 3 from the book: "Introduction to
Medical Imaging Physics, Engineering and
Clinical Applications“ by N. Smith et. Al., 2011.
 Chapter 3 from the book “Medical Imaging
Physics”, by W. Hendee.
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