Pet Cancer

REVIEW
Annals of Nuclear Medicine Vol. 20, No. 4, 255–267, 2006

PET/CT today: System and its impact on cancer diagnosis
Eriko TSUKAMOTO and Shinji OCHI

Medical Cooperation Teishinkai Central CI clinic

Over the past six years, PET/CT has spread rapidly and replaced conventional PET. Although PET/
CT is a combination of PET for functional information and CT for morphological information, their
combination is synergistic. PET/CT fusion images result in higher diagnostic accuracy with fewer
equivocal findings. This results in a greater impact on cancer diagnosis. With attenuation correction
performed by the CT component, PET/CT can provide higher quality images over shorter
examination times than conventional PET. As with all modalities, PET/CT has several characteristic artifacts such as misregistration due to respiration, overattenuation correction due to metals,
etc. Awareness of these pitfalls will help the imaging physician use PET/CT effectively in daily
practice.
Key words: positron-emission tomography; tomography scanners, X-ray computed; 18F-FDG;
radionuclide imaging

INTRODUCTION
ALTHOUGH we think of Positron Emission Tomography as
a new modality of recent value in cancer diagnosis, the
basic technique of PET is about 50 years old.1 It was used
primarily for academic investigations until the advent of
18F-Fluorodeoxyglucose (FDG), which is taken up selectively by cancer cells.2,3 Since cancer has become a
leading cause of death all over the world, FDG-PET has
gained popularity for its ability to diagnose cancer. Today, PET is not only a tool of academic investigations but
also a tool of clinical practice. Over the past ten years,
FDG-PET has been a focus of interest for clinical studies
involving cancer diagnosis. Many papers about cancer
diagnosis using FDG-PET have been published. It is now
generally accepted that FDG-PET is useful for the evaluation of solitary pulmonary nodules,4–6 staging of malignant lymphoma,7–9 detection of local recurrence of colon
cancer,10–12 and many other indications.
With an increasing demand for a PET camera that is
suitable for clinical practice, PET cameras have gone
Received May 8, 2006, revision accepted May 8, 2006.
For reprint contact: Eriko Tsukamoto, M.D., Medical Cooperation Teishinkai Central CI clinic, Sapporo Medi-Care center
building, 1–27, Nishi 17, Odori, Chuo-ku, Sapporo 060–0042,
JAPAN.
E-mail: [email protected]

Vol. 20, No. 4, 2006

through several evolutions.13 The progressively increasing speed of computers dramatically shortened the amount
of time needed to process an image. New reconstruction
algorithms resulted in image quality improvement. However, given the difficulties with anatomic resolution and
the long duration of the transmission scan needed for
attenuation correction, stand-alone PET still had serious
limitations.
The advent of PET/CT solved these problems. A nearsimultaneous acquisition of transmission and emission
images, with diagnostic-quality CT images used for both
attenuation correction and anatomic localization, PET/
CT is more than its separate modalities.14
DEVELOPMENT OF PET/CT
Researchers realized early on that the morphologic information from CT and the functional information from PET
would be ideal if they were combined.15–17 Software
registration techniques for the separate modalities provided more accurate localization than that provided by
visual coregistration. However, several problems regarding erroneous registration resulted. Especially in the fusion of body images, deformable organs caused various
errors. Time differences in imaging created other problems. For example, the contours of the abdomen are
dependent on the pallet design; also, gastrointestinal
organs move over time. This made accurate registration

Review 255

Table 1 Main characters of the CT scanners integrated in the PET/CT system available in Japan

Number of detector
per row
360° full scan time
(sec/rot)
Maximum scan field
of view
Tube voltage (kvp)
Tube current (mA)
Heat capacity (HU)
Slice width
Auto exposure control

GE
Discovery ST Elite

Siemens
Biograph

Toshiba
Aquiduo16

Philips
Gemini GXL

Shimadzu
Eminence SOPHIA
SET-3000GCT

8, 16

6, 16

16

6, 10, 16

1

0.5

0.5

0.5

0.5

50
(CTAC70)
80–140
10–440
6.3 M
0.625, 1.25, 2.5,
3.75, 5, 7.5, 10
yes

50

50

50

0.75
(option 0.4)
50

80–140
28–500
5.3 M
0.6–10

80–135
10–500
7.5 M
0.5, 1, 2, 3,
4, 6, 8
yes

90–140
20–500
8.0 M
0.6–12

80–135
10–300
4.5 M
1, 2, 3, 5, 7, 10

yes

no

yes

Table 2 Main characters of PET scanners integrated in the PET/CT system available in Japan

Acquisition mode
Scintillator
Detector size (mm)
Number of detectors
Trans FOV (cm)
Axial FOV (cm)
Max length of scan
(cm)
Number of slices
Slice pitch (mm)
Trans resolution
(center, mm)
Axial resolution
(center, mm)
Sensitivity (cps/kBq)
Scatter fraction (%)
NECR (kcps·kBq/ml)
Reconstruction

Matrix size
Reconstruction time
(/frame)
Attenuation correction

GE
Discovery ST Elite

Siemens
Biograph

Toshiba
Aquiduo16

Philips
Gemini GXL

Shimadzu
Eminence SOPHIA
SET-3000GCT

3D & 2D
BGO
4.75 × 6.2 × 30
13,440
70
15.7
160

3D
LSO
4.2 × 4.2 × 20
24,336
58.5
16.2
180

3D
LSO
4 × 4 × 20
24,336
58.5
16.2
180

3D
GSO
4 × 6 × 20
17,864
57
18
190

3D
GSO
2.45 × 5.1 × 30
39,600
60
26 (20.8, 15.6)
180

47
3.27
5.1

81
2
4.2

81
2
4.2

Brain 90, WH 45
Brain 2, WH 4
4.1

99
2.6
3.5

4.8

4.5

4.5

4.5

4.2

2.0 (2D), 8.5 (3D)
5.7
4.8
8
19 (2D), 36 (3D)
19
36
35
80 kcps·12 kBq/ml 93 kcps·29 kBq/ml 85 cps·24 kBq/ml 70 kcps·11 kBq/ml
2D VUE Point, 2D OSEM, DIFT, FBP
FBP, OSEM
3D-RAMLA,
FBP, 2D OSEM,
3D-LOR
3D VUE Point, 3D
Rp, 3D FORE-IR
64, 128, 256
128, 256, 512
128, 144, 256, 288
128, 256
Less than 2 minutes
1 minute
2 minutes
Less than 1 minute
X ray

X ray

extremely difficult.
PET/CT was designed to provide the solution to these
shortcomings. The first PET/CT prototype was introduced by David Townsend and colleagues at University
of Pittsburgh in 1998.18,19 In this prototype, PET detector
electronics were mounted to the rear portion of a CT
gantry. The detectors consisted of 2 arrays of bismuth
germinate (BGO) blocks covering an axial field of 16 cm

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X ray

X ray

19
50
60 kcps·9.8 Bq/ml
FBP, OSEM,
DRAMA

1–5 minutes
137Cs

with 24 partial rings of detectors, providing full 3D data.
The CT and PET cameras were placed close together in
the same gantry housing, with different consoles controlling the operation of the CT and the PET. In spite of
separate operation, accurate image fusion and attenuation
correction with CT data were possible with the two
systems intrinsically aligned on the same mechanical
support.

Annals of Nuclear Medicine

Hundreds of patients were imaged in clinical trials
using this prototype PET/CT.20–23 These clinical trials
revealed the superiority of PET/CT image over either
modality alone.
Encouraged by these data, various manufacturers made
PET/CT scanners commercially available in Europe and
the United States in 2001.
PET/CT scanner design
The first PET/CT system approved by FDA was marketed
by Sidemen’s (Siemens Medical Solutions, Erlangen,
Germany) as the Biograph.24 It incorporated a highperformance CT, a dual slice Somatom Emotion, and PET
with fixed complete rings. CT and PET were placed
tandem within the gantry housing. Instead of the old PET
transmission sources, CT-based attenuation correction
was standard on this system. The couch was changed so
that vertical deflection of the pallet was eliminated.
At present, several vendors are producing PET/CT
systems. Their PET/CT designs are basically the same.25
PET/CT scanners consist of three main components: 1) a
PET scanner, 2) a CT scanner, 3) a patient bed (Fig. 1).
Currently, each PET/CT system consists of its latest high
performance PET scanner and high performance multidetector 2-16-slice CT. In addition to the older NaI and
BGO scintillators, lutetium oxyorthosilicate (LSO), and

gadolinium oxyorthosilicate (GSO) are available now
and are used in the bulk of new installations. Acquisition
mode is usually in 3D; only General Electric Medical
Systems provide operation in 2D. Most of the PET/CT
systems have a patient port with a larger diameter than
stand-alone PET systems to match the CT port. Each
vendor developed unique patient handling systems to
reduce vertical deflection of the bed so that serious misalignment between PET and CT images would not occur.
A single bed moves axially into the scanner and usually
the patient undergoes the CT scan and then the PET scan.
Characteristics of major PET/CT systems available in
Japan are shown in the next section.
PET/CT in Japan
Although PET/CT has spread rapidly in Europe and the
United States, approval of the PET/CT scanner took a
while in Japan. The first PET/CT approved in Japan was
the Discovery LS manufactured by General Electric (GE
Medical Systems, Milwaukee, Wisconsin, USA) in December 2003 followed by the Discovery ST in July 2004.
Since then, several PET/CT cameras produced by other
vendors have become available. Those cameras have
different characteristics as shown in Tables 1 and 2. All
the CT scanners have multiple detectors, up to 16. Each
PET camera has its own unique characteristics in terms of
scintillator materials, detector concepts, and reconstruction algorithms. At present, the PET/CT systems of five
vendors are commercially available. Figure 2 shows their
latest cameras.
Now, as in other countries, new cameras purchased in
Japan are mostly PET/CT. Although there is no official
data about PET/CT systems sold in Japan, the number
could reach more than 150 by the end of 2006.
TECHNICAL CONSIDERATIONS

Fig. 1 Basic PET/CT scanner design.

Image Fusion
Although PET and CT detectors are placed within the
same gantry housing in PET/CT, its image fusion is not a

Fig. 2 Recent PET/CT systems commercially available in Japan.

Vol. 20, No. 4, 2006

Review 257

Fig. 3 Registration errors due to respiratory motion. A: Deformity of dome of the liver is noted (arrows).
B: Separated liver dome activity is noted in the lung (arrows).

Fig. 4 Recurrence of tumor within left mainstem bronchus detected by FDG-PET. A: PET MIP image
of the chest. B: Accumulation in the bronchus is noted on image taken with tidal breathing. C: With 30
second breathhold, accumulation of FDG is noted in tumor in the bronchus. D: CT image shows tumor
inside of the bronchus (arrow).

Fig. 5 Phantom attenuation images with CT and 137Cs sources under various conditions. Graphs show
relationship between coefficient of variation of the counts in the region of interest and transmission time
with 68Ge-68Ga rod source or CT tube current. CT provides the same quality of transmission images even
with the lowest tube current.

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Annals of Nuclear Medicine

Fig. 6 Overcorrection of attenuation by pacemaker. A: a fusion image, B: image with attenuation
correction, C: image without attenuation correction.

Fig. 7 Attenuation error in a liver lesion. A: PET MIP image of at the level of diaphragm, B: transaxial
image of PET, C: CT image of same level of PET. No lesion is noted.

Fig. 8 Truncation error from arms. Photon deficient area is noted (arrowheads) at the level of the elbows,
which are positioned outside of CT FOV.

true hardware fusion. They are operated separately and
images taken by each camera are fused with software.
They are, however, acquired with the same mechanical
support and with the separate scanners positioned in-line
at a fixed distance, which makes image fusion much more
accurate than previous software fusion with images
taken by separate camera systems at different times.26,27
PET/CT solves the problem caused by body contour
deformability and time difference, but does not routinely
correct for breathing artifacts or inadvertent positioning
changes between the two scans. CT scans are usually
acquired during a breath hold while PET scans are acquired during free tidal breathing because of the long
duration of scan. This difference in breathing techniques
can cause misregistration (Fig. 3), especially in the organs

Vol. 20, No. 4, 2006

most affected by respiratory motion. One of the available
solutions is acquisition of CT scan during shallow breathing.28,29 The other option is CT scanning during breath
hold in a fixed position and best match with PET scan in
unforced expiration,30,31 which is not very easy to do for
most patients. The ideal solution may be acquisition with
respiratory gating, which is not widely used currently.
Gated acquisition is, however, going to be a standard in
the future and many systems for it are now being developed.32–35 In our clinic, we are performing the PET scan
with a 30-second inspiratory breath hold per bed position
when misregistration is observed. Figure 4 is a patient
with suspected lung cancer recurrence. FDG was taken up
in the left mediastinum but it was difficult to localize the
lesion on CT. With the 30 second breath hold, we can

Review 259

Fig. 9 Images reconstructed using OSEM. Subset and iteration numbers affect image quality and SUV.

Fig. 10 CT images acquired with various tube currents.

clearly recognize the intense accumulation of FDG in the
tumor inside of the left mainstem bronchus.
Arm position is another very important factor for image
quality; sometimes patients move their arms because it is
very tiring to keep the arms up for long time. Patients also
move their heads, especially when they fall asleep during
the examination. It is very important to use comfortable
arm rests and to try to keep the patient awake during the
examination.
CT-based attenuation correction
In the stand-alone PET systems, the effect of attenuation
is corrected using external sources. Conventional PET
uses either 68Ge-68Ga rod sources for 511-keV annihilation photons or 137Cs point sources of 662-keV singlephoton gamma rays for acquiring the transmission scan.
This conventional method of transmission scanning needs
significant time to collect sufficient photons; a transmission image obtained this way usually contains high levels
of noise. A CT scan, on the contrary, needs a short time to

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Eriko Tsukamoto and Shinji Ochi

scan the whole body and can provide a comparatively
noiseless transmission image.36,37 The multi-detector CT’s
which are used in current PET/CT models take less than
1 minute to scan whole body. It would take 60 minutes
per bed position with 68Ge-68Ga rod sources to obtain a
transmission image of CT quality. High-quality attenuation correction images are easily achievable with a lowdose CT scan.38 Figure 5 shows phantom transmission
images of CT and 137Cs sources under various parameters.
CT attenuation values at a given energy depend on both
the density and the relative element composition of the
tissue. Attenuation coefficients are energy-dependent.
Therefore the attenuation coefficients obtained by original CT images acquired at an energy of 60–80 keV needs
to be scaled to those of 511 keV pixel by pixel.18,39
CT attenuation correction also has several disadvantages. One of them is overestimation of 511 keV attenuation values. It is usually associated with iodine- or
barium-based contrast agents and metallic implants such
as pacemakers, chemotherapy ports, dental fillings, ortho-

Annals of Nuclear Medicine

Fig. 11 Typical example of brown fat. A: Symmetrical uptake
is noted in supraclavicular area and the vicinity of spines. B:
Transaxial images of PET, fusion and CT show uptake in the fat.

Fig. 12 Mediastinal brown fat. A: PET MIP image chest shows
uptake in the right mediastinum (arrow). B: Fusion image
reveals uptake in the fat between right and left atrium (arrow).

A

B

Fig. 13 Fusion images show FDG uptake in normal adrenal glands. A: FDG uptake in normal right
adrenal gland. B: FDG uptake in normal left adrenal gland.

Fig. 14 Symmetrical facial muscle uptake (arrows). A: PET MIP image of face. B: Transaxial fusion
image. C: Transaxial PET image.

pedic metal implants, etc.40–43 As described earlier, the
attenuation coefficients measured by CT scanning at an
energy from 60–80 keV need to be scaled to those expected at 511 keV pixel by pixel. Most metals and contrast
agents exhibit strong photoelectric absorption of X-rays

Vol. 20, No. 4, 2006

but, like bone and other tissues, interact with 511-keV
gammas primarily via Compton scattering. The CT-AC
scaling algorithm does not account for this effect and
causes overcorrection of PET images.44,45 To avoid misinterpretation of the images, it is necessary to reconstruct

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Fig. 15 A patient with follicular lymphoma. A: PET MIP image
of head, neck, and chest. B: Fusion images show faint uptake in
small lymph nodes (arrows).

PET images without attenuation correction when focal
uptake might occur near structures with high CT numbers.
Recently, several camera systems have methods such as
the tissue conversion table used in the GE Discovery ST
PET Scanner to reduce this artifact. Figure 6 shows
overcorrection caused by a pacemaker.
The mismatch of PET and CT images due to patient
respiration can have a marked effect on CT-derived attenuation correction as well as on image fusion.46,47 The
anatomic regions most affected by breathing artifacts
include the diaphragm, lung bases, and liver dome. Figure
7 shows typical misregistration artifact at the level of the
diaphragm. The possible solutions for this artifact have
been described earlier.
Truncation error also causes an artifact.48–50 In some
cases, the arms may extend outside the transverse FOV of
the CT scanner. Because the transverse FOV of the PET
scanner is usually larger than that of the CT scanner, this
results in missing data for the CT-based attenuation
correction. This discrepancy in imaging FOV results in
artifacts on the PET images (Fig. 8). Most of the recent
PET/CT systems have algorithms for extrapolating the
inconsistent CT projections to mitigate the truncation
errors within the FOV of the CT scanner, and reduce the
bias in the attenuation-corrected PET images.

Fig. 16 Ovarian cancer patient with carcinomatous peritonitis
A: Whole body PET MIP image. B: PET transaxial images. C:
Transaxial fusion images clearly show metastatic nodules in
peritoneal space.

ally diffusely low. Patients are requested to drink water
for hydration prior to the study in some PET centers.
Patient positioning
Prior to the examination, all metal such as dental braces,
belt buckles, clothes with zippers and so on should be
removed to avoid artifacts on both CT and PET scan. After
bladder emptying to reduce radioactivity in the bladder,
the patients are positioned supine on the bed with either
arms up or down. They are usually positioned with the
arms raised over the head to avoid CT beam hardening
artifact. Additionally, because recent CT systems have
automatic exposure control, the radiation dose increases
in patients with their arms at their sides. As it usually takes
around 30 minutes to complete the scan, comfortable arm
rests are needed to reduce patient discomfort.52 In contrast, the arms should be placed along the body in patients
with head and neck tumors. Other devices such as comfortable pillows, foam pallets or vacuum bags for immobilization of the patients are also preferable for accurate
fusion of images.

OPTIMAL PROTOCOL
Patient preparation
The patient preparation for PET/CT is the same as that for
conventional PET. Patients are fasted at least 4 hours prior
to the injection of FDG. Diabetic patients should be in a
stable state of disease management. Insulin levels greatly
affect the biodistribution of FDG.51 Therefore, insulin
should not be administered before PET scan to lower the
blood sugar. When control of blood sugar is difficult,
examination is better performed in the patient with hyperglycemia. In that situation, accumulation of FDG is usu-

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PET Protocol
PET protocol for PET/CT is basically the same as that for
stand-alone PET. With the increased sensitivity of the
latest PET cameras, the administered dose is getting lower
and the acquisition time is getting shorter. It is, however,
important to validate the optimal dose and acquisition
time in each PET center.53 Especially in obese patients,
longer acquisition time is necessary to acquire an image of
comparable quality.54,55 Often, PET imaging extends
from the base of the skull to the pubic symphysis. It is,
however, important to cover all the extremities in patients

Annals of Nuclear Medicine

with malignant melanoma and in any other cases where
involvement of the extremities is expected. Careful evaluation of the patients’ history is always necessary.
In most PET cameras, an iterative attenuation-weighed
ordered-subset expectation maximization algorithm
(OSEM), using CT-based attenuation and scatter correction, is employed for the reconstruction of PET data. The
optimal number of iterations and subsets is usually proposed by camera vendors and so are the filters. However,
they should be evaluated by each PET center to confirm if
they are optimal for that center’s patient population.
These parameters of PET/CT differ from those of conventional PET. They also influence standard uptake values
(SUV) (Fig. 9).56
CT Protocol
Whole-body PET/CT examinations involve more radiation exposure to the patient than conventional PET examinations. Therefore, the added CT protocol for PET/CT
must be justified in each case to avoid overexposure of
patients to radiation for image quality that may not be
needed.57–60 Figure 10 shows CT images with varying
tube currents, resulting in different levels of exposure.
In practice, for the CT protocol, one of two options may
be chosen. One is CT as part of the PET/CT, used as a fast
transmission measurement for attenuation correction and
for anatomic labeling of the PET findings. In this case, the
patient will have previously undergone a complete CT
examination. Therefore, the radiation dose should be
limited to reduce radiation exposure to the patient even
though CT image quality will be suboptimal. The other is
state-of-the-art CT, where diagnostic CT is clinically
indicated. In this case, the examination should be performed with adequate radiation dose, and with contrast
agent when necessary. When contrast agent is administered, timing of the CT scan will be determined depending on the body parts to be explored and the underlying
disease being investigated. 61–63 Effect of contrast
agent on attenuation correction also should be kept in
mind.64–66
In Japan, because reimbursement of CT as a part of
PET/CT had not been determined, CT was not performed
as a diagnostic exam. However, reimbursement of PET/
CT has been approved in April 2006 in Japan, and the CT
protocol in PET/CT in Japan will be adjusted accordingly.
IMAGE INTERPRETATION AND CLINICAL
SIGNIFICANCE
Physiologic accumulation
Image interpretation of PET/CT is not very different from
that of conventional PET. Nonphysiologic uptake is recognized as abnormal. Compared with conventional PET,
it is easier to distinguish physiological foci from abnormal
foci with PET/CT. PET/CT has been shown to reduce the
false-positive interpretation of physiological uptake.67,68

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One of the confusing examples of physiological uptake, which is correctly recognized by PET/CT but not by
PET, is brown fat.69–71 There are two types of adipose
tissue: white fat that stores energy and brown fat, which
plays an important role in cold-induced and diet-induced
thermogenesis. With stand-alone PET, increased symmetrical uptake in the neck had been interpreted as physiological uptake in muscle. However, fusion PET/CT
images subsequently revealed that, in many cases, these
accumulations were localized to brown adipose tissue.
Cohade et al. first reported these accumulations as USAFAT (Uptake in Supraclavicular Area) (Fig. 11).72 Actually, brown fat exists in other locations as well, and it is
often not symmetrical.73,74 It is important not to confuse
these physiologic accumulations with malignancy. A
typical example is brown fat in the mediastinum, which
was reported by Trung et al. (Fig. 12).75
The wide range of normal uptake in the adrenal glands
is also clarified by PET/CT while it is difficult to recognize by conventional PET. Earlier papers on adrenal
uptake by conventional PET described that most uptake in
the adrenal gland was due to malignancy.76–78 Now there
is increasing awareness of the spectrum of normal adrenal
uptake (Fig. 13)79 as well as uptake in benign adrenal
adenomas.80
Focal uptake in normal muscle (Fig. 14) and colon,
often difficult with conventional PET, is also easy to
recognize as physiological with PET/CT.
Impact on cancer diagnosis
Although PET/CT is a combination of PET and CT, its
results are not additive but highly synergistic. Side-byside analysis of CT and conventional PET is well-established. It is often difficult to achieve sufficient diagnostic
clarity using conventional PET without the aid of CT.81
Buell et al. reported that side-by-side reading is sufficient
for diagnosis in most cases and PET/CT is needed in a few
cases.82 Antoch et al. concluded that tumor staging with
PET/CT is significantly more accurate than CT alone,
PET alone, and side-by-side reading of PET and CT.83
Lardinois et al. analyzed the additional value of PET/CT
in patients with non-small cell lung cancer.84 PET/CT
allowed for more exact localization of metastases and
more precise determination of the local extent of primary
tumor in about 40% of patients compared with conventional side-by-side reading of PET and CT. There are
several discussions about this issue, but it is evident that
it is much easier to reach an accurate diagnosis with PET/
CT than with side-by-side reading of separately performed PET and CT examinations.
PET/CT has a slight advantage over side-by-side reading of PET and CT in characterization of known malignant lesions. Because the location of lesions is already
known by other modalities, PET findings are the most
important thing to be considered. A separate diagnostic
CT scan can play an adequate role in precise diagnosis.

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In restaging, localization of recurrence, and unknown
primary tumors, PET/CT has a greater advantage over
side-by-side reading of PET and CT.85–88 It allows accurate and precise location of lesions. PET/CT increases
reader confidence, reduces equivocal findings, and helps
achieve an accurate diagnosis. PET/CT sometimes detects unsuspected foci which are difficult to identify on
CT. Even with side-by-side reading it is often difficult to
identify the anatomic area taking up the agent. Small
lymph nodes are one example.89 It is often important to
identify their exact location, especially when biopsy of
the lesion is necessary. Small lymph nodes are often
difficult to identify with conventional PET because the
uptake is usually very faint and difficult to distinguish
from background, but it is not very difficult to localize
even faint uptake in small lymph nodes by PET/CT by
recognizing the uptake is superimposed on the visible
lymph nodes. Especially in low-grade malignant lymphoma, thanks to PET/CT, we now can identify these
small lymph node lesions with faint uptake, which is
essential for staging (Fig. 15).90,91
The advantage of PET/CT over PET may be even
greater in the abdomen than in the head, neck, and thorax.
The peristalsis of the gastrointestinal tract and the variable
filling states of hollow organs cause problems on side-byside reading of PET and CT acquired at different times.
Especially in the case of peritoneal spread, where lesions
are often difficult to distinguish from normal gastrointestinal tract activity, PET/CT confers a large advantage
(Fig. 16).92–94
Radiation therapy planning
PET/CT does not only improve diagnostic accuracy but
has an impact on therapy planning by more exact and
accurate location of the lesions. PET/CT may play an
important role in radiation therapy planning. PET/CT
imaging provides molecular information about a tumor in
addition to morphological information to aid planning.
Prior to using PET/CT in the radiation therapy planning
process, the data transfer between the PET/CT system and
the therapy planning system must be validated and quantitative accuracy of displayed values should be verified.95
It is very important to put the patient in the position
planned for later radiation therapy when PET/CT examination is performed. The same flat radiation pallet and
fixation devices planned for radiation therapy are necessary for the PET/CT examination.45 A preliminary report
described that PET/CT significantly improves the measurement of gross tumor volume, one of the key volumes
to be identified in radiation therapy planning.96 Many
centers of radiation oncology are already starting to use
PET/CT for radiation therapy planning.97–100 However,
data regarding radiotherapy planning and PET/CT are
still sparse and several issues remain to be solved before
PET/CT is routinely used for radiotherapy planning.

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Eriko Tsukamoto and Shinji Ochi

CONCLUSION
In only 6 years, PET/CT systems have become widespread throughout the world. PET/CT has made it possible to acquire functional and anatomical data at the same
time. It has improved PET interpretation dramatically,
resulting in improved cancer diagnosis. PET/CT may
become a major diagnostic tool for cancer diagnosis in the
near future. PET/CT imaging is also potentially useful in
radiation therapy planning, adding functional information to anatomical information.
There are still several problems to be solved in performing PET/CT. PET/CT has characteristic artifacts and
other disadvantages different from those that arise with
conventional PET. We must understand the unique characteristics of PET/CT and make an effort to use this
revolutionary modality effectively.
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