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24
EE
..
Incident
particle
g
β+
Photon of 511 keV
https://t.me/med1917
T. Wendler et al.
bremsstrahlung, from the German “braking radiation” (Fig. 2.11).
The likelihood (percentage P
Bremsstrahlung
) that
bremsstrahlung will occur relative to collisions or
excitation is roughly approximated by an equation involving the maximum energy of the β particle (Eβ) and the atomic number Z of the material
through which the β particle passes [9]:
ZE
P
Bremsstrahlung
≈
3000
b
MeV
2.6.3 β+ Annihilation
If a β+ loses its complete kinetic energy (or comes
close to doing so), it will combine with an electron in an atomic matter–antimatter reaction.
This reaction will convert both particles to pure
energy. Due to the law of conservation of energy
and momentum, the energy will then be dissipated in the form of two 180° oriented photons,
each with the energy equivalent to the mass of the
electron/positron, i.e., 511 keV (Fig. 2.12).
If the positron has not been completely
stopped, the two photons may display a small
deviation from the 180° orientation, canonically
explained in physics books.
2.6.4 Penetration of β Particles
Tissue
in
Unlike γ rays, β particles do not follow an exponential law in terms of penetration. Rather, owing
to the complexity of the interactions and the fact
that their emission energy is not a line but a spectrum, empirical formulas have to be used.
A common approach is the use of the formula
given by Katz and Penfold [10], where the tissueindependent range R in g/cm2 of a β particle of an
energy E in MeV is given by
E
−
1 265 0 0945
0 412 00125
R
=
0 530 0 106 25
..ln
−<
.. .
EE
<<
MeV
.
MeV
β
Bremsstrahlun
Fig. 2.11 Bremsstrahlung caused by a strong change in
the trajectory of a β particle passing close to a nucleus
particle
Fig. 2.12 Annihilation
of β+ and orbital
electron. The energy of
the mass of both
particles is emitted in
the form of two
photons at almost 180°
to each other
2.7 Detection of Radiation
In the previous sections, the basic principles of
radioactivity and its interaction with matter have
been discussed. These principles govern the way
in which radiation is emitted and “comes out of
the patient.” They also explain the process of
detection, a key part of radioguided surgery.
Within this section, only the most relevant technologies for detection will be considered.
Photon of
511 keV
photo
reader
photo
cathode
time
source
2 Physics of Radioguided Surgery: Basic Principles and Methods of Radiation Detection
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25
2.7.1 Scintillator Crystals and Light
Detection
During the early days of X-ray imaging, it was
realized that some crystals will shine if they are
placed close to a radioactive source. This effect
was termed scintillation and crystals behaving in
this way were called scintillators.
In scintillation, a γ ray or a β particle interacts
with electrons by the effects described above in
such a way that either free electrons or photons
are emitted. They on their own interact again and
eventually a significant amount of the energy is
deposited in the material and then re-emitted in
the form of light. The total generated light intensity is proportional to the energy deposited in the
crystal. If the emitted light is visible, humans will
be able to see the scintillation.
If used for detection of radiation, this effect is
very convenient as photon-counting techniques
are widely available. The ones most commonly
used in radioguided surgery are explained below.
2.7.2 Photomultiplier Tubes
Photomultiplier tubes (PMTs) are still the most
used technology in nuclear medicine for photon
counting.
Their mechanism of action consists first in
converting an incident light photon into an
electron using a photocathode, a thin-layer vapordeposited conducting material. The reason why a
photocathode emits an electron is explained by
the previously described photoelectric effect.
The photoelectron is amplified by the use of
a strong electric field (voltages in the range of a
few thousand volts) and so-called dynodes
(Fig. 2.13). Dynodes are nothing more than
electrodes each at a higher potential than the
previous one. They enable an iterative process.
In practice the first photoelectrons are accelerated to the first dynode, which they hit, generating secondary electrons. These are then
accelerated to the next dynode, where they generate more secondary electrons as they arrive.
After several steps the initial light and small
amount of photoelectrons reaches several orders
of magnitude so that when the last electrode (the
anode) is hit, a clear electrical peak can be
measured.
The amplification process of a PMT takes
~50 ns. A normal design of a PMT has ~10 dynodes, each generating ~5 secondary electrons per
incident electron. As a result, a typical amplification factor is 107 [11].
It is important to note that due to variations in
construction, voltages, temperature, inpurities,
PMTs coupled to scintillators only reach energy
resolutions in the range of 8.5–50 % at 122 keV
[11]. On the other hand, due to the large differences in crystals, their area of application is wide.
Fig. 2.13 Schematic representation of a PMT. The incoming photon is converted into a photoelectron in the photocathode. This is then accelerated to the first dynode. When
the dynode is hit, several secondary electrons are gener-
electron
incoming
photon
voltage
dynodes
anode
resulting electrical pulse
voltage
pulse
ated. They are then accelerated to the next dynode and the
process is repeated. Once the cascade of electrons reaches
the anode, a pulse is detected
26
T. Wendler et al.
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Table 2.3 The most relevant scintillating crystals in radioguided surgery: thallium-doped sodium and cesium iodine
[NaI(Tl) and CsI(Tl)], sodium-doped cesium iodine [CsI(Na)], bismuth germanate (BGO), and lutetium-yttrium oxyorthosilicate (LYSO)
3
Density [g/cm
Hygroscopic Yes Slightly Yes No No
Light yield [photons/MeV γ]38
Primary decay time [μs] 0.25 1 0.63 0.3 0.041
Data taken from [
] 3.67 4.51 4.51 7.13 7.15
12]
NaI(Tl) CsI(Tl) CsI(Na) BGO LYSO
× 10
3
54 × 10
3
41 × 10
3
9 × 10
3
3
32 × 10
2.7.3 Silicon Photomultipliers
Silicon photomultipliers (SiPMs) have been
developed during the past two decades. They
essentially combine several avalanche photodiodes (APDs) in arrays for linear light amplification. Their main advantages over PMTs are a
reduced size, the need for significantly lower
voltages (<100 V), a higher speed, and immunity
to electromagnetic fields. Gains are in ranges
similar to those achieved by PMTs [8].
A SiPM consists of up to 1000 APDs per
square millimeter working in Geiger mode. This
makes it possible to detect up to that amount of
photons simultaneously per area unit. The
moment a photon hits an APD, it will fire, resulting in a pulse. If more photons arrive at the same
time, the pulse will be proportional to the amount
of incident photons [8].
2.7.4 Scintillating Crystals
PMTs and SiPMs are only one way to convert
light into electrical signals. They always require a
scintillator, a scintillating crystal, to convert radiation to light. A commonly used nomenclature
for crystals employs their chemical formula, e.g.,
NaI – sodium iodine, followed by any type of
doping used to change their basic characteristics
in parenthesis, e.g., NaI(Tl) – thallium-doped
sodium iodine.
The most important characteristics of these
crystals are
1. Density: The denser a crystal, the higher the
likelihood that it will detect higher energy
radiation. As a result, in order to detect the
511 keV annihilation photons, crystals like
BGO or LYSO are commonly used, whereas
for the 140 keV of Tc-99m, NaI(Tl) is normally sufficient. The thickness at which the
crystal can be manufactured also plays an
important role here.
2. Hygroscopic character: Some crystals tend to
absorb moisture from the air, resulting in a
deterioration of the crystal in the long term.
NaI(Tl) is a material for which this property is
problematic; as a result, NaI(Tl) crystals are
only used when well sealed from the exterior
world.
3. Light yield: The greater the light yield from a
crystal, the higher will be the sensitivity and
the better the energy resolution. As a result,
crystals like CsI(Tl) and LYSO are preferred
for SPECT and PET, respectively, as they
have a better light yield than comparable
materials.
4. Primary decay time: If many photons are to be
detected simultaneously or if coincidences are
to be measured, a fast response is desirable.
A list of the main characteristics of the most
important scintillators is displayed in Table 2.3.
2.8 Direct Radiation Detection
Semiconductors can also be used for radiation
detection. As radiation passes through a semiconductor, it ionizes the material, which can be measured directly (Fig. 2.14).
In radioguided surgery in particular, CdTe and
CdZnTe (CZT) alloys are commonly used. These
2 Physics of Radioguided Surgery: Basic Principles and Methods of Radiation Detection
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27
Fig. 2.14 Charges are distributed relatively homogeneously within the semiconductor; however, as a γ ray
passes through it, negative and positive charges (electrons
are very versatile and possess a major advantage
over scintillator detectors in that they have a significantly higher energy resolution (~5 % versus
~9–10 % for properly tuned scintillator detectors
[13]). In practice this plays a role in image quality and scatter rejection. A well-calibrated CZT
detector will not detect much of a Co-57 source if
it has been tuned for Tc-99m, while a scintillator
detector will detect most γ rays emitted from
Co-57 in its Tc-99m window.
2.9 Collimation and Radiation
Shielding
Radiation detectors are not particularly useful for
radioguided surgery if they are not properly shielded
and collimated. Shielding entails surrounding part
of the detector with a strong absorbing material
such that no or only a little radiation is detected
from regions at which the detector is not pointing.
For γ rays, the materials most commonly used
for shielding are W (wolfram) and Pb (lead). The
choice between these materials depends mainly
on cost, weight, thickness, shape to be built, and
the fact that Pb is toxic, so that it needs to be
properly covered for use in medicine. Table
compares the two materials.
Shielding is much easier for β particles: only a
few millimeters of Al (aluminium or aluminum)
are sufficient to stop most β particles used in
radioguided surgery.
Collimation is used to shape the radiationsensitive region in front of a radiation detector.
2.4
and holes) are separated and flow to the anode and cathode, respectively. This results in an electrical pulse, as
explained in the section on PMTs
Table 2.4
materials
Density [g/cm
Half-length [mm] @ 140
Half-length [mm] @ 511
Bulk modulus [GPa] 310 46
Data taken from [14, 15]
W has a stronger “stopping power”(as seen from the halflengths in Table 2.4, a thinner layer of tungsten is needed
to stop the same amount of γ rays than lead) than Pb; it is
also harder and denser. However, it is significantly more
expensive and harder to process. Half-length is the depth
that a γ ray has to pass through the said material before
losing half of its intensity
Comparison of W and Pb as shielding
3
] 19.25 11.34
keV 0.23 0.3
keV 2.6 3.7
W Pb
Depending upon whether the detector to be used
is a single detector (a probe) or a 2D array of
detectors (a camera), the collimation can have
different shapes and different aspects need to be
considered. Collimation is used in radioguided
surgery in the context of the detection of γ rays,
since β particles follow random paths and as such
cannot be “collimated” (i.e., it is almost impossible to determine their origin, and shaping the
region where they are detected is highly
complex).
For gamma probes the choice of collimator
entails a trade-off between sensitivity and spatial
resolution. The sensitivity is commonly given as
the counts per second (CPS) detected by the
gamma probe from a source of 1 MBq situated at
a certain distance. The spatial resolution is
frequently given as the full-width half maximum
28
100 CPS 50 CPS
distance
distance
m
60
25000
Full width half maximum in
Sensitivity in CPS/MBq
T. Wendler et al.
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to source
Fig. 2.15 Definition of spatial resolution in terms of full-width half maximum at a certain distance between source and
probe
50
40
30
mm
20
10
0
0 5000 10000 15000 20000
Fig. 2.16 Spatial resolution (in terms of full-width half
maximum of a point source: the larger this number, the
worse the resolution) as a function of sensitivity (the higher
the better) for a standard gamma probe and a source at
30 mm. For this plot a cylindrical crystal of 6 mm diameter
and 8 mm length and 2 mm tungsten shielding were
assumed. If a high resolution is desired (e.g., 10
sensitivity is very poor, at 1000 CPS/MBq. On the other
hand, a high sensitivity of 18,000 CPS/MBq yields a resolution of only 46 mm. A good trade-off (and common design
in commercial gamma probes) would be a sensitivity of
10,000 CPS/MBq which generates a resolution of 33.5 mm
to source
full width
half maximu
mm), the
of a source at a certain distance, i.e., the displacement that the source has to have on the perpendicular axis for the detector to report half the
counts (Fig. 2.15).
An example of the relation between spatial
resolution and sensitivity for a standard gamma
probe optimized for Tc-99m detection is given in
Fig.
2.16.
In the case of gamma cameras, collimation is
a far more complex issue owing to the availability
of different collimator options. In general, two
types are used: parallel hole collimators and pinhole collimators.
Pinhole collimators use the concept of a
“camera obscura”; as a result, they produce
high- resolution images of close objects and magnify them, while far objects are depicted as small
spots (Fig. 2.17). The main problems with these
collimators are their lack of sensitivity and the
distortion effect, which may induce difficulties in
interpretation.
Parallel hole collimators are used more frequently as they do not introduce any distortion.
On the other hand, they have a limited field of
view as only objects directly in front of them are
seen, and these objects are “clipped” if they are
partly outside this field of view (Fig.
As with gamma probes, in gamma cameras,
the collimator characteristics are also dependent
on each other. Here, spatial resolution and sensitivity again play a role. In the particular case of
parallel hole collimators, since only parallel
incoming rays are considered, sensitivity is
almost not a function of distance.
2.10 Measuring Radiation
in the Human Body
In order to avoid injuries to incorrect structures
and unnecessary extension of the search for hot
lesions, it is vital that surgeons have an
2.18).
d
pinhole
far source
parallel hole
far source
2 Physics of Radioguided Surgery: Basic Principles and Methods of Radiation Detection
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29
Fig. 2.17 Image produced
by a gamma camera with a
pinhole collimator. The
close square object is
depicted magnified as it is
very close to the pinhole.
The round object, on the
other hand, is only a small
dot in the camera image
Fig. 2.18 Image produced
by a gamma camera with a
parallel hole collimator.
The objects are depicted in
their original shape, but
there is clipping of the
round object
understanding of the major effects of radiation in
the body and the potential pitfalls arising from
the underlying physics.
2.10.1 Interactions of Radiation
with Tissue
detectorimage produced
detectorimage produced
collimator
close source
collimator
close source
necessary for the surgeon to know approximately
how deep he/she needs to go as small faint
sources of activity may be detectable only at a
close distance. This effect is purely geometric
and is called, in the radioguided surgery jargon,
the “solid angle effect.”
As described in the section on the interactions of
radiation with matter, different interactions may
occur, primarily depending on the type of radiation. Since the task of radioguided surgery is to
detect radioactive sources in the human body, the
effects to be considered can be distinguished
according to the type of emission traced.
The effect of geometry is, however, common
for all types of radiation. Approximately, the
amount I
of radiation that a detector of diameter
D
D receives from a source of intensity I0 placed at
a distance d is proportional to the quotient of
their squares:
Put another way, the farther away the source is,
the less radiation will be detected. It is still
II
µ´
D
0
2.10.2 γ or X-Ray Detection
Almost all radioguided procedures are based on γ
or X-ray detection, since β particles have a short
penetration in tissue. Independent of the origin of
the γ or X-rays (γ decay, radioactive-related
X-ray emissions, bremsstrahlung, or annihilation
of β+ particles) or the type of detector (γ probe or
γ camera), the main interactions of γ or X-ray
radiation with matter are absorption and scatter.
Absorption due to the photoelectric effect and
geometry explains why sources in tissue farther
away from the detector seem less active than
2
D
2
closer sources with the same activity and solid
angle. In tissue, attenuation due to this effect is
not as high as in shielding material, collimators,
or even the detector itself (the density of tissue is
significantly lower than the densities of the
30
3000
0
Counts
Energy in keV
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T. Wendler et al.
2500
2000
1500
1000
500
0
50 70 90 110130 15
-500
Fig. 2.19 Typical energy spectrum of a Co-57 source as
seen by a CZT detector (here averaged over 256 pixels). The
major peak consists mostly of “true” counts of the 122-keV
peak of Co-57. Counts at lower energies result from electronic noise and, mainly, scatter (at lower energies). Counts
collimator and detector materials), but it still
exerts a major impact on the search for radioactive sources.
In tissue, Compton scatter is the predominant
contributor to scatter in the energy range considered, causing photons to change trajectory and
lose energy. In practice this means that if a γ
detector is pointed at a structure which is not
radioactive, it may still show a response to radiation due to photon scatter from a nearby structure. Appropriate choice of the energy window
may reduce this effect but it cannot be completely
eliminated (Fig. 2.19).
2.10.3 β Detection
If β particles are to be detected next to the solid
angle effect, the sum of all interactions that the
particle undergoes can be seen as the penetration.
As mentioned in a previous section, penetration
of β particles cannot be modeled easily in tissue,
but a good approximation is the empiric formula
of Katz and Penfold.
It needs to be borne in mind that β particles
have a continuous energy spectrum. In most
at higher energy may also be attributable to electronic noise
but mainly with the 136-keV peak of Co-57. An appropriate
choice of the energy window would be to set the limits
slightly below and slightly above the main peak to reduce
most of the influence of electronic noise and scatter
cases, this spectrum will have an average energy
at approximately one-third of the maximum
energy. As a consequence, for most relevant
nuclides the penetration will be only a few millimeters. A thin layer of tissue can thus cover a
radioactive source, so care should be taken not to
overlook radioactive target tissue if a complete
resection is needed.
2.11 Pitfalls during Radioguided
Surgery
2.11.1 Fast Measurements, Fast
Movements
First, there is one effect common to both γ or
X-ray detection and β detection which influences
detection, namely, the statistical nature of the
radioactive decay coupled with the speed of
detection (the speed of the electronics and especially the integration time of the detector).
As explained above, radioactive decays are
random in nature and are independent of all physical effects around them. This forces to integrate
measurements over a period of time, i.e., all
2 Physics of Radioguided Surgery: Basic Principles and Methods of Radiation Detection
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field of view of detector
true source
non-radioactive
structure
31
Fig. 2.20 Example of shine-through and how to avoid it.
Left: Two structures are within the field of view of the
detector, but only the deeper one is radioactive. Right: In
order to avoid potential resection of a nonradioactive
events detected within a sampling time are added.
This is relevant to radioguided surgery as
normally only minimal amounts of radioactivity
are used and integration times are in the range of
0.5–10 s.
In practice, the structures for which the sur-
geon is looking commonly have count rates in the
range of 10–30 CPS. In this case, a faint source
can easily be overlooked if the detector is moved
rapidly above it. For example, if a source emitting 30 CPS is pointed at the source for 1 s and
there is a background activity of 20 CPS, there is
an almost 50 % chance that a surgeon will miss it
if he/she is using a discrimination threshold of 10
CPS. Even if the discrimination threshold is only
5 CPS, the likelihood of missing the source is
about 15 %.
On top of this problem, it must be considered
that rapid movement can also “smear” the radioactivity over an area, making it impossible for the
user to distinguish a faint spot over a larger lowradioactivity area.
The solution is to move the detector slowly, if
possible remaining for a few seconds above
structures that potentially have activity. If it is
expected that a low activity will be detected, then
a longer integration time should be used and the
detector should be held steady in the same position for the duration of this integration time.
structure, the angle of the detector has to be changed such
that it points away from potential sources, thereby ensuring that the structure is indeed radioactive
2.11.2 Shine-Through
When performing a sentinel lymph node biopsy
in a breast cancer patient, a procedure in which a
radioactive lymph node is extracted from the
axilla, it is not uncommon for the surgeon to
remove a lymph node that he/she believes contains radioactivity only for measurement outside
the body to reveal that this is not the case.
Frequently the real radioactive lymph node lies
directly behind it and slightly deeper.
This problem is called “shine-through.” It is
explained by the fact that due to improper handling of the detector, one structure is thought to
be radioactive while the true radioactive structure
is shining through it (Fig. 2.20).
The structure that shines through does not
have to be the target structure. In procedures like
sentinel lymph node biopsy, it is also possible for
the injection site to be the source of the counts.
The way to avoid this pitfall is to try to point the
gamma detector at the structure being analyzed from
different angles so as to ensure that the radioactive
structure is detected (cf. Fig. 2.20). Additionally, use
of preoperative imaging, e.g., planar scintigraphy or
SPECT/CT, or 3D intraoperative imaging such as
freehand SPECT may help the surgeon to obtain a
clearer view of the source of the radiation and thus
avoid resection of a nonradioactive structure.
32
field of view of detector
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relevant weak
source
strong source
Fig. 2.21 Example of shadowing and how to avoid it.
Left: Two structures are within the field of view of the
detector, but only the closer, smaller one is relevant. Right:
In order to avoid missing this structure, the angle of the
2.11.3 Shadowing
Almost the opposite effect of shine-through is the
shadowing effect. Here, a faint radioactive source
is missed by the surgeon since it is too close to a
hot radioactive source which “shadows” it.
This is an extremely common occurrence in
radioguided surgery as the biological uptake of
structures such as the liver or activity at the injection site (in the case of sentinel lymph node
biopsy) may disguise small structures that contain less radioactivity and are potentially of more
importance than the shadowing structure.
In this case, proper use of the gamma detector,
as well as the use of more highly collimated detectors, freehand SPECT, or preoperative information, may help the surgeon to avoid overlooking a
structure of clinical relevance (Fig. 2.21).
Conclusions
Within this chapter we have tried to explain all
relevant aspects of the physics underlying
radioguided surgery to provide a sound basic
understanding for users of γ probes, β probes,
or γ cameras.
Pitfalls, recommendations, and content
have been chosen based on our experience
gained over many years in radioguided surgery
detector has to be changed such that it points away from
the strong source (the strong source should not be in the
field of view of the detector) before abandoning scanning
of the anatomy of interest
in different organs, with different nuclides, and
a wide variety of radiation detectors.
In general, a deeper understanding of the
physics of radioguided surgery enables a good
surgeon to become a good radioguided surgeon, ensuring excellent performance to the
benefit of patients.
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