acquisition protocol considerations for combined...

Acquisition Protocol Considerations forCombined PET/CT ImagingThomas Beyer, MD1,2; Gerald Antoch, MD2; Stefan Muller, MD1; Thomas Egelhof, MD2; Lutz S. Freudenberg, MD1;Jorg Debatin, MD, MBA2; and Andreas Bockisch, MD, PhD1

1Department of Nuclear Medicine, University Hospital Essen, Essen, Germany; and 2Department of Diagnostic and InterventionalRadiology, University Hospital Essen, Essen, Germany

Since its introduction in 1998, dual-modality PET/CT imaginghas received great attention in the medical community. For thefirst time, patients can be examined with both CT and PET in asingle examination. A whole-body survey is the standard modeof acquisition. The CT images are used for anatomic referenceof the tracer uptake patterns imaged in PET, as well as forattenuation correction of the PET data. The routine use of CT-based attenuation correction and user preferences for the qual-ity and type of the CT examination have led to the introductionof different PET/CT scanning protocols. Discussion: Two gen-eral approaches to PET/CT imaging can be distinguished today.One uses CT as a fast transmission source with little additionalinformation for anatomic labeling. The other uses CT as a fasttransmission source as well as a state-of-the-art diagnostic toolto maximize image quality using optimal acquisition parameterstogether with oral and intravenous contrast agents. Variations ofthese approaches share common concerns about image arti-facts that result from mismatches in respiration and patientpositioning between the CT and the PET examinations. Protocolrequirements for the more complex radiologic PET/CT scenarioalso include alternative contrast application schemes or modi-fications to the attenuation correction procedure to handle CTcontrast agents appropriately. Conclusion: High-qualityPET/CT studies can be provided routinely with existing PET/CTtechnology that is used efficiently by trained and motivatedtechnologists and physicians. Only then will the potential diag-nostic benefit of this new imaging modality be explored fully.

Key Words: oncology; PET/CT; PET/CT imaging protocols;clinical oncology; combined PET/CT imaging

J Nucl Med 2004; 45:25S-35S

It has been hypothesized for some time now that comple-mentary functional and anatomic imaging, such as thatperformed with PET and CT, allows for improved diagnosisand thus better patient care in clinical oncology (1,2). Soft-ware-based algorithms to align independently acquired PETand CT image sets have been available for several years,

and an alternative hardware approach has been introducedonly recently (3,4). Based on a proof-of-principle prototype,various PET/CT system designs have been developed andare available today (5). Several advantages are associatedwith combined PET/CT imaging compared with retrospec-tive or prospective software-based approaches to align com-plementary image data. Most important, the patient under-going a combined PET/CT examinations is not movedphysically (except for the translation of the bed) betweenCT and PET acquisition, thus limiting misalignment fromrepositioning. In combined PET/CT, the lengthy standardPET transmission scan is no longer needed, because trans-mission data from the CT acquisition of the combinedPET/CT examinations can be used for attenuation (andscatter) correction of the complementary emission data.This results in greatly reduced total examination times andin potentially reduced costs when standard PET transmis-sion sources are not installed in the combined tomograph.Patients further benefit from the logistic advantages of beingreferred for only one examination rather than two separatescans on 2 days. In addition, a joint report on the combinedPET/CT data, in theory, can investigate all findings with theexpertise of a radiologist and a nuclear medicine physician.

STANDARD FDG PET/CT IMAGING PROTOCOL

The main strength of PET imaging in clinical oncology isto detect primary and metastatic disease by means of awhole-body survey with a single injection of a givenamount of a radiotracer, such as 2-18F-FDG. In contrast, CTimaging has been used most frequently to image the ana-tomy of only a single organ (e.g., liver), or to reviewexamination ranges of only limited axial extent (e.g., thorax,neck). With the introduction of multislice CT technology(6) and optimized exposure management, this concept oflocoregional examinations has been changed, and extendedin vivo surveys of patient anatomy have become morepopular. The ability to acquire anatomic information overextended imaging ranges at reasonable patient exposurelevels (6) underlies the main concept of combined PET/CTimaging, which is to supplement metabolic informationfrom a whole-body PET study with detailed information onthe corresponding anatomy of the patient for improved

Received Sep. 25, 2003; revision accepted Nov. 7, 2003.For correspondence or reprints contact: Thomas Beyer, PhD, Department

of Nuclear Medicine, University Hospital Essen, Hufelandstr 55, 45122 Essen,Germany.

E-mail: [email protected]

ACQUISITION PROTOCOLS FORPET/CT IMAGING • Beyer et al. 25S

by on May 20, 2018. For personal use only. jnm.snmjournals.org Downloaded from

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diagnostic accuracy. Therefore, most, if not all, PET/CTimaging protocols are based on standard whole-body (7)PET acquisition protocols, which involve a pre- or postin-jection transmission scan followed by an emission scancovering the same axial imaging range.

Standard 18F-FDG imaging protocols with PET/CT canbe divided into 7 steps (Fig. 1). In contrast to a stand-alonePET, a topogram (overview scan) is acquired for accuratedefinition of the axial examination range. Because of theessentially nonexisting contamination from the emissionactivity of the patient, CT transmission data are alwayscollected in postinjection mode. Depending on the antici-pated use of the combined PET/CT data (i.e., initial diag-nosis or follow-up), additional requirements may be im-

posed on the acquisition protocol, such as the use of breath-hold commands or the administration of CT contrast agents.In general, however, a PET/CT protocol is similar to astandard PET protocol, with the exception that the standardPET transmission data are replaced by the CT transmissiondata. Table 1 compares important requirements for each ofthe acquisition steps in combined PET/CT and separate CTand PET.

Patient PreparationProper patient preparation for whole-body PET studies

has been described in detail, for example by Hamblen andLowe (8). These criteria apply also to patient preparation forPET/CT studies. Patients should be questioned indepen-

FIGURE 1. Standard FDG-PET/CT imag-ing protocol. The patient is positioned on acommon patient handling system in frontof the combined gantry. First, a topogramis used to define the co-axial imagingrange (orange). The spiral CT scan (graybox) precedes the emission scan (greenbox). CT images are reconstructed on-lineand used for the purpose of automatic at-tenuation correction of the correspondingemission data (blue box). Black and bluearrows indicate acquisition and data pro-cessing streams, respectively.

TABLE 1Acquisition Protocol Considerations in PET, CT, and PET/CT Whole-Body Imaging

Acquisition step PET CT PET/CT

Patient preparation Check glucose levelsInject FDG

Test patient for CTcontrast allergies

Give oral contrast

Check glucose levelsInject 18F-FDGTest patient for CT contrast allergiesGive oral contrastPractice breath-hold instructions

Patient positioning Arms downKnee rest

Remove all metal(bracelets, pants withzippers)

Arms upSpecial positioning aids

and supports

Remove all metal (bracelets, pants with zippers)Arms upSpecial positioning aids and supports

Topogram Not applicable Breath-hold Free breathing or breath-holdTransmission scan Free breathing

Ge-rod or singles-basedGive intravenous

contrastFull inspiration

breath-holdMultiple spirals

Give intravenous contrastShallow breathing or breath-holdSingle spiral

Emission scan Continuous multiple bedstudy

Not applicable Continuous multiple-bed study

Data processing Attenuation correction Postreconstructions(field of view, windowsettings, filter)

CT attenuation correctionPostreconstructions (field of view, window settings,

filter) of the CTImage analysis 3D whole-body viewer

Region-of-interest analysis2D and 3D viewer and

navigators2D viewer3D fusion of PET/CT and 3D navigatorRegion-of-interest analysis

26S THE JOURNAL OF NUCLEAR MEDICINE • Vol. 45 • No. 1 (Suppl) • January 2004



dently for allergies to iodine-based CT contrast agents ifthese are to be administered intravenously during the courseof a PET/CT study. Oral contrast agents, on the other hand,typically do not require special premedication or testing.Depending on whether and which oral CT contrast agentsare given, patients are asked to drink up to 1,500 mL of oralcontrast solution during the 18F-FDG uptake phase. Theinjected dose of 18F-FDG may vary between 300 and 700MBq, depending on scanner characteristics, such as PETdetector material and acquisition mode.

Patient PositioningBefore the examination, patients should remove all metal

(e.g., bracelets, dental braces, pants with zippers, etc.) thatcould lead to streak artifacts on the CT transmission scan.Patients must be positioned comfortably on the examinationpallet. Several PET/CT tomographs offer a uniform 70-cmgantry opening, larger than the 60-cm standard PET open-ing. Patients can be positioned in the larger opening withtheir arms raised above their heads, which is standard prac-tice in CT. Patients should be supported with adequatepositioning aids (e.g., knee, head and neck, and arm sup-ports) to limit involuntary motion that may lead to generalor local misalignment during the combined examinations.

Overview ScanPET/CT examinations start with the acquisition of a

topogram or scout scan that is an x-ray image overview scanof the patient. The topogram is acquired during continuoustable motion, with the x-ray tube/detector assembly typi-cally locked in either frontal or lateral position (or anyposition in between), thus generating an anatomic overviewimage that is similar to a conventional x-ray at a givenprojection. The topogram is used to define the axial exam-ination range of the PET/CT study. The axial extent of theCT and PET portions of the combined examinations arethereby matched to ensure fully quantitative attenuation andscatter correction of the emission data. Visual markers forthe measured transverse field of view of the CT (50 cm) andthe PET (60 cm) are displayed on the topogram. Thesemarkers guide the technologist to ensure that all body partsare positioned inside the smaller transverse field of view ofthe CT. Patients should be repositioned before the CT scanwhen truncation of the anatomy is predicted by the topo-gram.

CT AcquisitionAfter the definition of the coaxial imaging range, the

patient is moved automatically into the CT field of view forthe transmission scan. Most PET/CT users acquire a singlecontinuous spiral CT scan. The start of the CT scan isdelayed by 20–50 s if intravenous contrasts are adminis-tered according to standard CT protocols. A breath-holdcommand can be given to match the anatomy more closelyto the physiology of the patient, which is acquired in thefree-breathing emission scan.

PET AcquisitionAfter completion of the CT scan, the patient is advanced

to the field of view of the PET, to the rear of the combinedgantry, where emission scanning commences in the cau-docranial direction, starting at the thighs to limit artifactsfrom the FDG metabolite excretion into the urinary system.Depending on the axial co-scan range and the emission timeallotted for an individual bed position, the combined scan-ning is completed in 30 min (9) or less (10).

Data Processing and ReconstructionCT image reconstruction is in parallel with emission

acquisition. Because the reconstruction of a single CT im-age takes �1 s, the CT images are ready for attenuationcorrection processing before the first emission scan is com-pleted. The calculation of the attenuation correction factors(ACF) from the CT transmission data (Fig. 1) is based onthe algorithm outlined by Kinahan et al. (11).

Emission images are reconstructed consecutively with thecompletion of the emission acquisition and using the avail-able ACFs. Almost immediately after the completion of thelast bed position of the PET scan, all emission images areavailable and assembled into a whole-body volume.

Image Analysis and ReportingDepending on the reviewer and the objective of the study

(Table 2), any or all of the following image sets are needed:CT, corrected PET, and noncorrected PET. In addition,several postacquisition reconstruction tasks may be requiredto create CT image sets with alternate filter and window-level settings (e.g., lung-window). Special viewing and fu-sion tools typically are available with standard PET/CTsoftware to allow scrolling through any of the selectedindividual and fused image volumes as well as side-by-sideviewing.

OPTIMIZATION OF STANDARD 18F-FDG PET/CTIMAGING PROTOCOLS

With the routine availability of CT transmission imagesand CT-based attenuation correction, some pitfalls mayarise for PET/CT users with only limited PET or CT expe-rience. Figure 2 illustrates some concerns in combinedPET/CT imaging about the use of CT images instead ofstandard PET transmission images. Most of these concernsrelate to potential artifacts in PET/CT images that resultfrom the nature of the interaction of low-energy x-rayphotons with tissues and the subsequent propagation of CTimage artifacts into attenuation-corrected PET images. Inaddition, a number of PET/CT users believe that acquiringthe CT with standard dose protocols is inadequate in com-bined PET/CT imaging and that, therefore, PET/CT proto-cols should use reduced (if not minimized) dose settings forroutine imaging (12,13). Therefore, combined PET/CT im-aging requires addressing the often conflicting goals forPET and CT.

ACQUISITION PROTOCOLS FOR PET/CT IMAGING • Beyer et al. 27S



Today, with about 5 years of clinical PET/CT imagingexperience, comparatively little data are available on thetrue impact of PET/CT imaging on diagnostic accuracy andpatient management (2,12,14). The shortage of clinical dataand the lack of optimal protocol parameters naturally con-tribute to a dispute over the utilization of PET/CT imagingfor oncology patient work-up (15). Both clinical and scien-tific factors dictate the requirements for performing PET/CT. In practice, two general approaches can be distin-guished (Table 2).

In the first, a state-of-the-art diagnostic CT is not neededor indicated, because the patient had previously undergonea complete CT examination. The CT, as part of the PET/CT,is used instead as a fast transmission measurement forattenuation correction and for anatomic labeling of PETfindings.

In the second approach, a state-of-the-art, diagnostic CTis clinically indicated. Typically, the CT, as part of the

PET/CT, is acquired using oral or intravenous contrastagents to maximize the diagnostic information on anatomyand tumor vascularization. In addition, the CT is used as afast transmission source for attenuation correction, ana-tomic labeling, or referencing of the PET results.

In the first approach, the PET/CT is performed in additionto an independent diagnostic CT examination, whereas, inthe alternative approach, the PET/CT replaces a clinical CTand a clinical PET. A third approach to PET/CT imagingwould be to accept the CT portion of the combined PET/CTas a very fast transmission scan option only, without ex-ploring any anatomic labeling of the corresponding PETfindings. This concept of PET/CT, however, is merely a fastPET imaging procedure and will not be considered here.

In both scenarios (Table 2) and in contrast to standardPET procedures, emission data are corrected routinely forattenuation by using the available CT transmission informa-tion. Current PET/CT technology (16) addresses most of the

TABLE 2Objectives and Requirements of Clinical PET/CT Imaging

Scenario Clinical approach Focus groupDemands

on CTDemandson PET

CT for simpleanatomicorientation

PET/CT replaces PETCT for fast attenuation correction and general

anatomic orientationMostly whole-body scans

Nuclear medicine Low High

CT for state-of-the-artdiagnosticinformation

PET/CT replaces CT, or CT and PETState-of-the art diagnostic CT with contrast

agents and standard exposure levels tomaximize information on anatomy andtumor vascularization

CT for fast attenuation correctionWhole-body scans and dedicated protocols

Cross-modality, nuclearmedicine, radiology,oncologist

High High

FIGURE 2. Optimization tasks of PET/CTimaging protocols relate to clinical andmethodological concerns: What CT doselevels are adequate for diagnostic pur-poses and CT attenuation correction?Should CT contrast agents be used?Should CT standards be adopted for theposition of the arms, respiration protocols,or for the administration of CT contrastagents? Will metal implants introduce arti-facts into corrected PET images?




requirements of each scenario, which are very similar intechnical nature (Fig. 1) but differ on requirements for thequality of the CT acquisition. In practice, clinical needs, sitepreferences, and even legal concerns also affect these re-quirements. To improve overall PET/CT image quality inclinical routine and to address the requirements of diagnos-tic PET/CT imaging, several advances have been made inthe design of PET/CT acquisition protocols and in theadoption of alternative CT contrast and respiration tech-niques.

GENERAL ASPECTS OF 18F-FDG PET/CT IMAGINGPROTOCOLS

Patient PositioningWith attenuation correction based on the use of CT in-

stead of 68Ge-rod sources, whole-body scan times are re-duced by as much as a third (9) to 30 min or less (10, 16).

From our experience with about 3,500 PET/CT studies,most patients tolerate being scanned with arms raised andsupported above the head. To facilitate comfortable posi-tioning of the arms, several low-attenuation CT positioningdevices are available and can be adapted for PET/CT im-aging. By raising the arms for the duration of the whole-body scan and leaving them outside the measured CT fieldof view, scatter artifacts in the body are much reduced (Fig.3) and counting statistics of the corresponding emissionscan are increased. Conversely, for head-and-neck investi-gations, the area is scanned with arms down.

Independent of the coaxial imaging range, all patientsshould be supported with a proper knee rest for the durationof the combined scan (Fig. 4). When using foam pallets orvacuum bags, no artifacts are typically introduced into theCT transmission data or, subsequently, into the correctedemission data. When using custom-made positioning aids,

FIGURE 3. (A) Topogram scans of 3 pa-tients with different arm positions. Selectedtransverse CT images (center 50 Houns-field units [HU], width 300 HU) at level ofmidliver are shown to illustrate magnitudeof streaking artifacts. Streaks are reducedmost in case when both arms are raised(right). (B) Male patient with lymphoma.Fused image shows involvement of retro-crural lymph node (arrow). Streak artifactsoriginating from positioning of patient witharms down degrade quality of CT andfused PET/CT images.

FIGURE 4. Patient positioning with armrest (A) and knee rest (B) for whole-bodyPET/CT studies. Arm and knee rests aremade of low-density foam with CT attenu-ation close to that of air. No artifacts aregenerated from these positioning aids, asis illustrated in transverse CT images toright (window levels center, C, and width,W, in Hounsfield units).




however, phantom studies should be performed first tovalidate artifact-free positioning. This is particularly impor-tant for radiation therapy positioning set ups.

Truncation ArtifactsCareful patient positioning supported by positioning aids

is important also for reducing truncation artifacts in com-bined PET/CT imaging. Spiral CT technology today offersa measured transverse field of view of 50 cm, which is 10cm less than the transverse PET imaging field. Therefore,truncation artifacts are frequently observed in combinedPET/CT imaging of very large patients or patients imagedwith arms down (17) (Fig. 5). If the patient extends beyondthe boundaries of the transverse CT field of view, part of theanatomy is not reconstructed in CT. It will not be availableto the CT-based attenuation correction procedure, which isbased on fully reconstructed CT images. Thus, if truncationexists and is not corrected for, the reconstructed emissionimages appear to be masked by the truncated CT (18,19).The tracer distribution is then only partially recovered out-side the measured CT field of view (Fig. 5B), and some biasof the reconstructed activity distribution inside the commonfield of view is observed.

Several algorithms have been suggested to extend thetruncated CT projections and to recover truncated parts ofthe measured attenuation map (17, 18). If applied to CTimages before CT-based attenuation correction, these cor-rection algorithms will help to recover the tracer distribu-tions measured with the complementary emission data. Ad-ditional work is needed, however, to make such algorithmsavailable routinely for clinical diagnostics.

Respiration ArtifactsMismatches of breathing patterns in combined PET/CT

examinations have been described as a source of potentialartifacts in emission images after CT-based attenuation cor-rection (20). These artifacts are particularly severe whenstandard breath-hold techniques (e.g., scanning at maximuminspiration) are transferred directly from clinical CT tocombined PET/CT without further adaptation (Fig. 6).

The areas most affected by respiration are the lowerthorax, anterior chest wall, and liver (21). In the absence ofroutinely available respiratory gating options, the anatomyof the patient captured during the CT scan must be matchedto the PET images that are acquired over the course ofmultiple breathing cycles. Goerres et al. (22) have comparedthe quality of PET/CT image alignment in the thorax and

FIGURE 5. Difference (A) in maximumtransverse field of view of CT (blue) andPET (orange) may lead to truncation arti-facts in PET/CT images (B). CT images oflarge patients or patients with arms downappear truncated at sides, and derived at-tenuation maps appear to mask correctedemission activity. (B) CT, uncorrected, andcorrected PET. (C) Truncated attenuationinformation may be recovered before CT-based attenuation correction.

FIGURE 6. (A) Significant respiration mismatch between CTand PET results in severe artifacts in corrected emission im-ages. (B) Free breathing during spiral CT scan may cause arti-facts that propagate into corrected PET, which is acquired infree breathing over many respiration cycles. (C) Using specialbreathing instructions, respiration-induced artifacts and mis-matches may be reduced in majority of cases.




abdomen for breath-hold and normal breathing during theCT portion of the combined acquisition. They found normalexpiration and free breathing to provide the best match inthe thorax in 53% and 27% of patients, respectively (22).CT and PET alignment accuracy for abdominal structureswas similarly satisfactory when acquiring the CT either infree breathing or in normal expiration (23).

The applicability of the normal expiration protocol, how-ever, is limited to PET/CT tomographs with very fast CTcomponents or CT protocols that use large table feeds perrotation. Acquiring the CT in normal expiration over theentire imaging range may not be feasible when scanninguncooperative or very sick patients. Therefore, an alterna-tive, limited breath-hold protocol has been suggested (20).Patients are asked to hold their breath in normal expirationonly for the time that the CT takes to cover the lower lungand liver, which is typically �15 s. Instructing the patientbefore the PET/CT examinations on the breath-hold com-mand is essential in avoiding serious respiration artifacts.When respiration commands are not tolerated by the patientor when respiration-induced misalignment persists and ap-pears to introduce artifacts into the corrected PET, it isadvisable to also reconstruct the emission data withoutattenuation correction and to carefully review the two setsof fused PET/CT images.

CT Contrast AgentsThe utility and use of CT contrast agents in PET/CT

imaging are still the subjects of some dispute when definingclinical acquisition protocols (24). In our opinion, the ques-tion of whether to use CT contrast agents for PET/CTstudies arises only in the state-of-the art diagnostic CTimaging scenario (Table 2) that requires state-of-the-art CTscans (25). In addition to the clinical impact, the point ofconcern is whether or not CT contrast agents can be admin-istered at no expense to the combined image quality whenonly CT-based attenuation correction is available (Fig. 2).On occasion, focally increased concentrations of high-den-sity intravenous or oral contrast agents are seen on CT,which results in artificial tracer uptake patterns on thecorrected PET images of PET/CT studies. These observa-tions are further discussed by Antoch et al. (24) elsewherein this Supplement.

In the absence of reliable and automated correction algo-rithms for CT contrast, alternative contrast administrationprotocols (Fig. 7) have been proposed for PET/CT imaging(24). For example, high-density focal artifacts from intra-venous bolus injection may be avoided by applying thesame contrast volume with an adaptive pressure pump or,alternatively, a saline chaser (26) after the intravenous con-trast agent has been injected. In addition, the use of negativeoral contrast agents for the enhancement of the gastrointes-tinal tract has been proposed for PET/CT imaging (Fig. 7B)to avoid overestimation of attenuation coefficients fromhigh-Z materials (27).

Metal ImplantsMany oncology patients have artificial metal implants,

such as chemotherapy ports, metal braces in the spinalregion, artificial joints, or dental fillings. In standard PETtransmission scanning, metal implants cause little or noartifacts. However, these artifacts can be severe at CTenergies (Fig. 8), because of the significantly higher photonabsorption from high-Z materials (e.g., metals) comparedwith low-Z materials (e.g., soft tissues) at CT energies. Instandard CT, iterative algorithms are available to correct forbeam hardening and scatter from metal implants, but thesecorrective steps have not yet been implemented routinely inPET/CT protocols.

Several PET/CT users, therefore, have reported on theeffect of metal artifacts on image quality in PET/CT studiesof the head and neck and around hip replacements (28–30).Another important aspect when scanning oncology patientsis the generation of focal artificial uptake patterns of 18F-FDG from chemotherapy ports. Because of the high-densityproperties of these ports (titanium) and the low-attenuationproperties of the surrounding tissues, artificial 18F-FDG fociare likely to be generated. These foci may be misleading inthe diagnosis, particularly when true lesions are present inthe vicinity of the port (Fig. 8C). It is therefore advisable toalso reconstruct emission images without attenuation cor-rection whenever metal implants are present (28–30).

Combined Scanning and Joint ReportCurrent software packages allow the correlated review of

PET and CT data either side by side or in fused mode. Some

FIGURE 7. Alternative schemes of appli-cation of intravenous contrasts (A) andnegative oral contrast agents (B) are beingpursued to avoid contrast-induced arti-facts on PET/CT images while providingacceptable image quality for radiologists.




image-review stations also allow extensive quantitativeanalysis of the combined data as part of the clinical report-ing. State-of-the-art reporting tools, however, lack adequatecapabilities to display and review serial PET/CT studies ofpatients in therapy follow-up.

The primary concept of PET/CT is the immediate trans-formation of the functional information from PET into thespatial coordinate system of CT. The second and no lessattractive concept is to identify any relevant PET findingswith normal or pathologic morphologic background. Simi-lar to the different perspectives on the operation of aPET/CT system (Table 2), different opinions exist for thetransformation of the main findings of PET/CT imaging intoa clinical report through the image review process (31).Obviously, a joint report is necessary and is an easy taskwhen CT and PET findings are concordant. However, thereis no accepted rule for dealing with contradictory PET andCT findings. A consensus finding could be facilitated by thejoint efforts of the evaluating physicians of complementaryradiology and nuclear medicine expertise, or, in settings inwhich this expertise is provided by the same physician, bygetting the expert opinion of subspecialized medical profes-sionals (e.g., a head-and-neck surgeon). Finally, joint re-ports not only benefit from the clinical expertise of both aradiologist and a nuclear medicine specialist but also dis-cern potential image artifacts and help to eliminate imageartifacts a priori in subsequent imaging.

SPECIAL FDG PET/CT IMAGING PROTOCOLS

When combining the diagnostic power of CT and PETimaging, a straightforward application of standard whole-

body imaging protocols may not yield the best image qual-ity for all indications. For example, PET/CT studies involv-ing the head and neck frequently suffer from localmisalignment in the region of the neck (30). The cause ofthis local misalignment (Fig. 9A) is the relaxation of neckmuscles, with resulting movement of the neck within the20-min time delay between CT and PET acquisitions in astandard whole-body protocol.

To avoid patient motion in the neck, where accuratealignment of CT and PET information is critical, and tooptimize imaging parameters, we have introduced a linkedneck/torso imaging protocol with slightly overlapping co-axial imaging ranges from the neck and torso (Fig. 9B).First, a PET/CT study of the torso is performed with thepatient positioned with arms up, as described in the standardwhole-body protocol. Assuming a 5-bed–position emissionscan with a scan time of 3.5 min per bed, total examinationtime including the topogram is �20 min. The patient is thenasked to lower the arms and keep them close to the trunk forthe dedicated head-and-neck scan. A vacuum pillow is usedto support the neck and head for the duration of the secondexamination. The PET/CT study of the neck is performedfor two bed positions at 6 min per step. Total examinationtime for the torso and neck coverage, including a 2-minperiod to reposition the patient between scans, is thus about35 min, still significantly shorter than an equivalent PETstudy. The administration of intravenous contrast agents isadjusted according to the two co-axial imaging ranges cov-ering the thorax and the neck, respectively (Table 3).

There are several advantages to this protocol. First, theimaging parameters can be adjusted to torso (i.e., lung) and

FIGURE 8. High-density implants gener-ate streak artifacts on CT, which maytranslate into distortions and biases of re-covered tracer activity. PET images with-out CT-based attenuation correction (CT-AC) may help to interpret metal-inducedartifacts.




neck scanning (Table 3). This is important when utilizingcontrast agents and making use of the range of available CTscan parameters. Second, the patient can be positioned andsupported more efficiently for each scan. Third, the patientcan get off the table between the two partial examinations ifneeded. While we thus improve overall image quality, thetotal examination time is only slightly increased.

We routinely complement the standard whole-bodyPET/CT examination in patients with breast cancer with asecond, locoregional scan of the breast, with the patient inprone position and the breasts in a hanging configuration,which is standard practice in CT examinations of the breast(32). Prone positioning was also shown to be more effectivethan supine imaging for the diagnosis of breast cancer with18F-FDG PET (33). We perform the dedicated breastPET/CT examination without the administration of intrave-nous contrast and reduce the CT exposure to 24 mAs. In ourexperience, scanning the hanging breast facilitates a betterseparation of breast tissue from the chest wall.

In our hospital, combined PET/CT imaging is part of pre-and posttherapy management of patients scheduled for ra-

diation therapy of head and neck cancer and lung cancer.These patients follow the neck/torso acquisition protocoloutline described previously (Fig. 9). Care is taken whenpositioning these patients (with masks on) on the patientsupport platform of the PET/CT to approximate as closelyas possible the radiation treatment planning and treatmentposition. Upon completion and review of the scan, PET andCT Digital Imaging and Communications in Medicine (DI-COM) images are transferred directly to the radiation treat-ment planning software, where the combined imaging in-formation is entered into the treatment planning process.

DISCUSSION

After several years of clinical PET/CT experience, animportant conclusion is that diagnostic PET/CT protocolsrequire a greater amount of attention to the preparation ofthe patient and operation of the tomograph than, perhaps,for either imaging modality alone. For example, PET/CTusers without previous CT experience have quickly learnedto appreciate the sensitivity of CT image quality to patient

FIGURE 9. (A) Without efficient position-ing aids, misalignment from muscle relax-ation and involuntary patient motion maybe observed in head and neck. (B) Whole-body imaging ranges can be separated intoneck and thorax scan ranges, each with itsown optimized positioning, acquisition,and reconstruction parameters for im-proved imaging at little cost in examinationtime.

TABLE 3Diagnostic PET/CT Acquisition Parameters for Whole-Body and Combined Head/Neck–Torso Protocol

Protocol Standard whole-body Neck–Torso

Imaging range Whole-body Torso NeckTopogram 1024 mm 756 mm 256 mmCT contrast 140 mL:

90 mL at 3 mL/s, 50 mL at 1.5 mL/s90 mL:60 mL at 3 mL/s, 30 mL at 1.5 mL/s

60 mL:60 mL at 3 mL/s

CT scan Single-spiral130 kVp, 110 mAs0.8-s rotation time5-mm slice width8-mm table feed

Single-spiral130 kVp, 100 mAs0.8-s rotation time5-mm slice width8-mm table feed

Single-spiral130 kVp, 160 mAs0.8-s rotation time3-mm slice width5-mm table feed

Emission scan 3.5 min per bed2 iterations and 8 subsets on 128

matrix with 5-mm Gaussian

3 min per bed2 iterations and 8 subsets on 128

matrix with 5-mm Gaussian

6 min per bed4 iterations and 8 subsets on

256 matrix with 3-mmGaussian, zoom 2

CT postprocessing Lung window Lung window Zoomed head




motion and high-density implants, and radiology-trainedusers have become familiar with extended examinationtimes. In light of the methodologic challenges of combinedPET/CT imaging (Fig. 2), which include the intrinsic mis-match of respiration in CT and PET and the routine use ofCT contrast agents, several users prefer to operate thePET/CT with substandard CT acquisition parameters inaddition to separate clinical CT scan (Table 2). However,the increased radiation burden to the patient from repeatedCT examinations should be considered. Furthermore, scien-tific evidence is still needed to demonstrate that PET/CTwithout CT contrast is equivalent to or better than CT alonewith contrast agents.

The use of CT contrast agents in PET/CT imaging is nolonger an obstacle to clinical PET/CT imaging. The meth-ods of CT-based attenuation correction are well understood,and several modifications to the inherent scaling modelswere suggested to account for the presence of high-densitycontrast agents on CT images used for attenuation correc-tion, for example (19,20). Meanwhile, initial clinical expe-riences with alternative contrast injection schemes and neg-ative oral contrast agents (24) have shown good results andmay serve as more practical methods of choice in routinePET/CT imaging before more sophisticated yet robust cor-rection algorithms become available.

Despite advances in reducing the spatial mismatch be-tween CT and PET data and in lowering potential biases inthe reconstructed tracer distribution, the subject of optimumCT dose parameters is still in great dispute. Typically, theeffective dose in clinical CT is on the order of 3–15 mSv(35), which is about the same (9 mSv) for an average PETstudy with 370 MBq 18F-FDG (36). Several technical fea-tures are available to manage patient exposure in a varietyof CT imaging scenarios. Some CT systems offer x-raytube-current modulation, so that the photon flux is increasedsomewhat in the lateral views but reduced significantly inthe anterior and posterior direction. Tube-current modula-tions have been shown to reduce effective patient exposureby as much as 20% in adults (37) and 23% in children (38).Kalra et al. reported on a 50% reduction in x-ray tubecurrent without a degradation of clinical image quality inabdominal CT of normal-weight patients up to 90 kg (39).

If a state-of-the art diagnostic CT is not mandated by theclinician in PET/CT imaging, some users seek additionalreduction of the patient dose from CT (12–14), albeit oftenat the expense of diagnostic image quality of the CT. Iflower-quality CT images are acceptable, exposure levelscan be reduced dramatically, but little supportive data isavailable that demonstrate equivalent diagnostic accuracy oflow- and high-dose CT in combination with PET (12–14).

Together with technical improvements in both CT andPET technology (16), several updated combined tomographdesigns have been introduced recently. These designs aimprimarily at increased PET performance, such as highersensitivities and faster scan times with fast detector mate-rials and improved detector electronics, which preferentially

benefit users who administer larger doses of 18F-FDG. Byintroducing 16-row CT technology into combined PET/CTdesigns, the advantages of very fast and high-resolutionvolume coverage by CT are translated directly into thecontext of combined anatomic and functional imaging. Du-rable CT rotation speeds of 0.5 s and less help to reducerespiration-induced artifacts and to image the anatomy ofmultiple organs at peak enhancement after intravenous con-trast injection. Alternatively, combined imaging protocolsthat include multiple, contiguous spiral CT scans to coverthe extended axial imaging range for different peak en-hancements of a single intravenous contrast injection nowappear feasible.

With ever-increasing data volumes, however, there is adownside to existing PET/CT imaging scenarios. In a recentstudy (40), the logistic benefits and necessary image reviewtimes of multislice CT were compared with those of single-slice CT. The authors concluded that although the actualclinical CT examination was finished in less time with amulti-row spiral CT, the increased number of images andinformation could easily lead to longer review times unlessmore efficient visualization tools become available.PET/CT is a complex imaging technique that generatesmore information than either imaging modality alone. Italso appears to be a promising tool for monitoring therapyresponse. Therefore, novel, and probably automated soft-ware tools must be developed that allow the registration andreview of serial PET/CT image sets and the potential incor-poration of additional diagnostic studies, such as MRI. It isthen possible that in the not-too-distant future a PET/CTexamination will take less time than the actual image reviewprocess.

We already need a higher flexibility of PET/CT acquisi-tion protocols in the search for alternative imaging proto-cols. For example, the entire coaxial imaging range could becovered by a combination of a low-dose and a shorterdiagnostic CT scan, or patients could be scanned in emis-sion mode only before a decision is made on the need andextent of a complementary CT transmission scan. Withmore variable acquisition schemes at hand and with increas-ing levels of experience and expertise, PET/CT users maythen extract the most useful indications for PET/CT imag-ing. Being able to choose between various acquisition pa-rameters and to set up optimized protocols will lead to adiversification of PET/CT imaging techniques. The optimi-zation of PET/CT protocols, however, will always be drivento a great extent by the needs and preferences of the users.

CONCLUSION

Since the inception of combined PET/CT imaging inclinical oncology a few years ago, several different imagingapproaches and protocols have been proposed. Independentof the preferred imaging scenario, careful image review andconsideration of potential artifacts arising from the routineuse of CT-based attenuation correction are advised in clin-




ical PET/CT imaging. Nevertheless, with growing experi-ence and with technologic improvements of the existingPET/CT hardware, high-quality PET/CT studies can beprovided routinely in clinical practice. By using this newtechnology efficiently in the hands of trained and motivatedtechnologists and physicians, its potential diagnostic benefitcan be explored fully.

ACKNOWLEDGMENTS

We would like to thank our technologists Barbel Ter-schuren, RT; Lydia Schostok, RT; Dorothea Prosch-Plotek,RT; Sandra Pabst, RT; Gianina Markese, RT; Sandra Heist-ruvers, RT; and Slavko Maric, RT, for their encouragementin providing the best patient care to our PET/CT patients.We appreciate the support of Simone Marnitz, MD, SarahGrehl, MD, and Andrea Fluhs in preparation of our radia-tion treatment planning imaging protocols. We gratefullyacknowledge the assistance of CPS Innovations, Inc., inmaking available to us several special patient positioningaids.

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2004;45:25S-35S.J Nucl Med. BockischThomas Beyer, Gerald Antoch, Stefan Müller, Thomas Egelhof, Lutz S. Freudenberg, Jörg Debatin and Andreas Acquisition Protocol Considerations for Combined PET/CT Imaging

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