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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5255_Библиотеки_им_академика_М_И_Перельмана
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Image Quality in Nuclear Medicine
G.S. Pant
Image is the representation of variations in physical properties of an object. When the
variations in physical properties are spatial the image is said to be static whereas temporal
variations in physical properties refer to a dynamic image. The image quality refers to how
closely the image represents the object. This definition is quite subjective therefore to avoid
observer bias it is better to express it in terms of some physical parameters, which could
either be measured or estimated. In nuclear medicine, the following three parameters
characterize the image quality.
(a) Spatial resolution
(b) Contrast
(c) Noise
All these parameters are either measured or estimated. A good quality image requires high
(numerically low) spatial resolution, high contrast resolution and low noise. For good quality
image the system sensitivity should be very high. The sensitivity in gamma camera system is
hampered by the collimator, which is an essential component for image formation. Information
density (counts/area) can however be increased with proper data acquisition. To achieve this the
radioactivity administered to the patient should be high enough that provides high count rate.
Secondly the time duration of image acquisition should be long enough to acquire adequate
information. Both the amount of radioactivity (dose) and time of acquisition are limiting factors.
The primary objective in diagnostic imaging with radiation is to get good quality image in
reasonably short period with minimum radiation dose to the patient. Thus the amount of
radioactivity that is administered to the patient is limited to reduce the radiation burden. The
time of acquisition is also a limiting factor for more throughput and patient comfort. Imaging
systems with very high system sensitivity help to a great extent in reducing the time of image
acquisition. Pure gamma emitters of short physical half-life (such as
administered in relatively larger quantity to get better information density without any appreciable
increase in radiation dose to the patient. Further the spatial resolution of the imaging systems in
nuclear medicine is also a limiting factor due to various physical phenomena involved in image
formation.
99m
Tc) can be conveniently
20 2
Image Quality in Nuclear Medicine 203
In addition to these physical parameters, there is an observer who is involved in evaluation
of the image quality. A general term for such evaluation is called observer performance study.
This not only tests the ability of the imaging device to detect the object but also tests the ability
of the observer to detect them. The most commonly used method for this purpose is the receivers
operating characteristic (ROC).
System Spatial Resolution
Spatial resolution refers to the ability of imaging devise to resolve two closely spaced points in
the image. The smaller the numerical value of spatial resolution (SR), the better it is. With good
(smaller) SR the image sharpness improves. The nuclear medicine imaging systems are poor in
spatial resolution. The imaging instrument has its intrinsic resolution (Ri) added to which is the
collimator resolution (Rc). The system resolution (Rs) is estimated from them using the following
equation.
22
)(
RRR
is
c
In figure 1, a lead bar phantom with equal bar spacing is shown. When a bar phantom with
equal spacing is placed in front of the imaging system and exposed to gamma rays, image of the
bars and bar spacing may be seen. The number of bars per unit length represents the spatial
frequency in the object. The spatial frequency can be increased or decreased depending upon the
number of bars per unit length. These bars can be imaged in a gamma camera by exposing them
with photons from a radionuclide. The image profile at right angles to the bars will appear as
sinusoid with decreased amplitude than the true (ideal) image. The object or image contrast may
be expressed in terms of their modulation (Figure 1).
Figure 1: Object and image profile of an equidistant bar pattern. The object and image modulation
can be measured from these profiles.
Image Quality in Nuclear Medicine204
The modulation of the object (which is measure of its contrast) is expressed as:
M
objectobject
ob
objectobject
)(
minmax
)(
minmax
Similarly the image modulation would be expressed as:
M
image
imageimage
imageimage
)(
minmax
)(
minmax
The modulation of the object and the image signals for an imaging system can be understood
clearly in terms of the spatial frequency as shown in figure 1. The ratio of object to image
contrast is an important parameter and is called modulation transfer function (MTF) at a given
spatial frequency.
MTF
contrastimage
contrastobject
The line spread function (LSF) or point spread function (PSF) may also be used to calculate
MTF for an imaging system. The LSF is the count profile through a line source of radioactivity
in a direction perpendicular to the line source. Mathematically it is expressed as:
dxxxLSF
)2cos().(
dxxLSF
)(
MTF
This is the one dimensional Fourier transform (FT) of the LSF. Similarly 2D FT of PSF will
provide 2D MTF, which determines the frequency response of the system. The MTF is a parameter
that depends on spatial frequency and in an imaging system like gamma camera its value
decreases with increasing frequency. Beyond certain frequency the MTF becomes so low that it
becomes impossible to resolve the image (bars in our example). This is not the case with
imaging by a photographic camera where MTF is nearly constant (dotted line in figure 2). The
deterioration in image (if any) is so small that it is not perceptible to human eye. The imaging
devices in nuclear medicine are unable to support high frequency components of the object
(Figure 2). The different imaging systems may have different MTF value as can be seen in
figure 3. Its value is nearly one at low frequencies and drops as the spatial frequency increases.
Image Quality in Nuclear Medicine 205
Figure 2: MTF of a gamma camera imaging device. The value of MTF decreases with increasing
frequency. The image may not be retrieved beyond certain frequency (curve is not based on any
measurement)
Figure 3: MTF of two imaging systems. System B is better than system A in terms of MTF
(spatial resolution).
In gamma camera the system MTF is the product of collimator MTFc and intrinsic MTF
(Figure 4). Thus
MTFMTFMTF
cisys
Contrast
The contrast in nuclear medicine image depends upon the uptake of radiopharmaceutical in the
target organ/tissues compared to that in the surrounding normal tissues, which is measured in
terms of difference of counts in respective areas or pixels. Better the difference in uptake of
i
Image Quality in Nuclear Medicine206
Figure 4: MTF of a system is the product of intrinsic MTF and collimator MTF.
The system MTF is less than either of them.
abnormal and normal tissues better is the object contrast. Good image contrast can be expected
only if the object contrast is good. Quality of radiopharmaceuticals plays an important role in
creating good object contrast.
Object contrast (Co) can be expressed as:
RRC
o
nl
Where
R
n
R is the count rate in the lesion and nR is the count rate in the
l
surrounding normal tissue with the same area or volume as in the lesion. The value of the object
contrast may be positive or negative with maximum numerical value as +1 or –1.
Noise
Noise is an unwanted signal in the image. In medical imaging it refers to the statistical
variations in the signal, which arise primarily due to random nature of physical phenomena
(radioactivity and radiation detection etc) involved. The random noise in nuclear medicine is
usually governed by Poisson distribution where variance of counts in a given region is taken as
the mean counts in that region. The relative noise in that region (ratio of Standard deviation to
mean) decreases with increase in counts in that region.
Re
noiselative
dsdeviationdards
countsMean
More counts results in relatively less noise and therefore a better quality image. Scatter,
attenuation and counts from under lying or overlying organs (with radioactive uptake) also
deteriorate the image quality. Correction methods are used to minimize the errors caused by
these physical processes.
N
N
1.).(tan
N
Image Quality in Nuclear Medicine 207
Observer performance
Most of the images in nuclear medicine are noisy and resolution limited as discussed above,
there is likelihood of observer variation in image interpretation. The observer performance can
be studied using receiver operating characteristic (ROC) curves. Looking at the images the
observer decides whether a lesion is present (positive image) or absent (negative image). The
confidence levels of the observer are numbered for a) the lesion definitely present b) the lesion
probably present c) lesion probably not present d) lesion definitely not present. Based on these
values ROC curves are plotted. The details of ROC curve are given in a chapter on “counting
statistics and ROC curves” in this book.
1. Physics in Nuclear Medicine, SR Cherry, JA Sorenson, ME. Phelps, 3rd edition, Saunders, Philadelphia,
USA, 2003.
2. Introductory Physics of Nuclear Medicine, R Chandra, Lea & Febiger, publisher, Philadelphia,USA,
1992.
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Part – II
Radionuclides/Radiopharmaceuticals
in Nuclear Medicine
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Radionuclides in Medicine and
Research
N. Ramamoorthy, S. Padmanabhan and M. Venkatesh
Radionuclides are being extensively used in medicine for diagnosis, treatment and research
since several decades. It is well known that the radioisotope 60Co is the common source in
teletherapy for treatment of deep seated tumors, while sealed sources of
suitable forms (seed, needle, wire) are used in brachytherapy for interstitial/intracavitary
treatment of some cancers. On the other hand, the specialty of nuclear medicine, which uses
radionuclides in the form of unsealed sources that are directly administered into the human
body through oral or parenteral route, has been steadily gaining popularity on account of its
diverse applications in a variety of areas in medicine, such as endocrinology, oncology,
cardiology, neurology, nephro-urology, gastroenterology etc. The rise in popularity could be
attributed to the availability of a wide spectrum of radionuclides to choose from. The radiation
characteristics of the radionuclides (half-life, decay mode and energy etc.) ultimately decide
the end use of the product for diagnosis or therapy (1,2). The emergence of nuclear medicine
as a medical specialty has thus brought forth the utility of many radionuclides. The gamma
emitters and positron emitters are used in in-vivo diagnostic procedures for imaging organs
to elicit the functional integrity (Table 1). Radioisotope imaging scores over CT/MRI in its
ability to pick up changes in physiology/function, which manifest much earlier than
morphological changes. Radionuclides having particulate emissions (alpha, beta and Auger
electron emitters), on the other hand, are used for therapy, wherein the cytotoxic properties
of these radiations are harnessed (Table 2 and 3). Recent years have witnessed a changing
trend to seek newer vistas in the therapeutic applications of radionuclides, with the growth
of radionuclide therapy (RNT) using radiopharmaceuticals. An attempt is made in this
presentation to discuss various radionuclides of such importance, with special reference to
the recent trends.
137
Cs and
192
Ir in
21 1
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