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nhancing Lung Scintigraphy Withingle-Photon Emission Computed Tomography

aul J. Roach, MBBS, FRACP,* Dale L. Bailey, PhD,* andenjamin E. Harris, MBBS, PhD, FRACP†,‡

Although widely used for many years in the assessment of pulmonary embolism, planarventilation-perfusion (V/Q) scintigraphy has well-recognized limitations. Single-photonemission computed tomography (SPECT) imaging, which can be readily performed in mostmodern nuclear medicine centers equipped with multihead gamma cameras, overcomesmany of these limitations through its ability to generate 3-dimensional imaging data. V/QSPECT has been shown to have a greater sensitivity and specificity than planar imaging andhas a lower nondiagnostic rate. For reporting clinicians who may be reluctant to abandonconventional planar V/Q images, planar-like images can also be readily obtained from V/QSPECT with the use of postacquisition techniques. The use of SPECT can also facilitateadvances in V/Q imaging, including the generation of parametric V:Q ratio images, coreg-istration with computed tomography, respiratory gating, and more accurate quantificationof regional lung function. Although direct comparisons in the literature are limited innumber, V/Q SPECT appears to have comparable, or greater, sensitivity than multidetectorcomputed tomography pulmonary angiography and is not associated with contrast-relatedcomplications such as allergy and nephropathy. It also involves significantly less radiationdose to breast tissue, an important consideration, particularly in young women. For the V/Qscan to remain relevant in the evaluation of patients with suspected pulmonary embolism,it is essential that image data are obtained so as to maximize their accuracy and diagnosticusefulness. V/Q SPECT can achieve this and, furthermore, may have a role in conditionsother than pulmonary embolism, including both clinical and research fields.Semin Nucl Med 38:441-449 © 2008 Elsevier Inc. All rights reserved.

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he accurate diagnosis of pulmonary embolism (PE) con-tinues to be a challenge for both clinicians and imaging

pecialists. Misdiagnosis is problematic because untreated PEs reported to have a mortality rate of up to 30%, and unnec-ssary treatment with anticoagulation places the patient atisk of bleeding.1-3 Historically, ventilation-perfusion (V/Q)ung scan and digital subtraction pulmonary angiographyave been used as imaging investigations in the diagnosis ofotential PE.4 More recently, radiographic computed tomog-aphy pulmonary angiography (CTPA) has been increasinglysed.4 Although pulmonary angiography has previouslyeen considered the gold standard investigation for PE, it is

Department of Nuclear Medicine, Royal North Shore Hospital, St. Leo-nards, NSW, Australia.

Department of Respiratory Medicine, Royal North Shore Hospital, St. Leo-nards, NSW, Australia.

Woolcock Institute of Medical Research, Sydney, Australia.ddress reprint requests to Paul J. Roach, MBBS, FRACP, Department of

Nuclear Medicine, Royal North Shore Hospital, St. Leonards, NSW

e2065, Australia. E-mail: proach@nsccahs.health.nsw.gov.au

001-2998/08/$-see front matter © 2008 Elsevier Inc. All rights reserved.oi:10.1053/j.semnuclmed.2008.06.002

erformed less frequently today because of its limited avail-bility, requirement for operator expertise, and invasive na-ure.4 To add to this, pulmonary angiography has recentlyeendemonstrated tohave less-than-optimaldiagnostic accu-acy.5 Consequently, the V/Q scan and CTPA are the 2 mostidely available and used investigations to image patientsith suspected PE today.Although V/Q scintigraphy has been used for more than 30

ears in the assessment of patients with suspected PE, thisechnique is widely recognized as having limitations.6-10

hen the lungs are imaged in only two dimensions (2D), asccurs with planar imaging, there is significant overlap ofnatomical segments, hence accurate assignment of defects topecific lung segments is difficult. Embolic defects may note detected if there is “shine-through” occurring from under-

ying lung segments with normal perfusion.9 The size andhape of each lung segment varies and accurately determin-ng the extent of embolic involvement in each individualegment can be problematic.9,11,12 In addition to these inher-

nt technical limitations of planar lung scintigraphy, there

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re the problems posed by the widely used probabilistic PIO-ED reporting schema.13-17 PE is not a condition in whichlinicians welcome “indeterminate” results and it is desirableo reduce such reports as much as possible.

SPECT is widely used in many areas of radionuclide imag-ng today because of its ability to image in three dimensions3D). It has been shown to be superior to planar imaging inhe evaluation of many conditions, such as assessing myocar-ial perfusion and brain and liver imaging.18 In contrast tolanar imaging, SPECT avoids the problems introduced byegmental overlap and “shine-through” of adjacent lung,aking it better able to image all segments of the lungs andore accurately define the size and location of perfusionefects.9 For these reasons, it would be expected that SPECT/Q scintigraphy should be superior to planar imaging, and it

s perhaps surprising that SPECT has not been more widelydopted for lung scanning. Furthermore, with the wide-pread availability today of multidetector gamma camerasnd improved computing power allowing faster processing,ung scintigraphy is ideally suited to SPECT acquisition.

ow to Optimallyerform V/Q SPECT

PECT can be used to image both ventilation and perfusion,owever, this requires the use of appropriate imaging agents.

entilationor imaging ventilation, several alternatives exist. These in-lude inert radioactive gases such as 81mKr and 133Xe, radio-abeled aerosols such as 99mTc-diethylene triamine penta-ace-ic acid (99mTc-DTPA), and the ultrafine carbon suspension9mTc-Technegas.19 Although the gases are considered toost accurately represent regional ventilation, several prob-

ems exist with their use. The use of 81mKr requires a kryptonenerator that is expensive and needs to be replaced daily. Asresult, 81mKr ventilation imaging can be problematic to per-

orm, especially outside of routine working hours. In addi-ion, 81mKr gas must be continuously administered duringmage acquisition due to its short half life.20 Although 133Xeas has the advantage of a longer half life, errors result fromts recirculation due to clearance into the pulmonary circula-ion.21,22 Given that SPECT assumes a static distribution ofracer for the duration of the data acquisition, these in vivoynamics impair 133Xe’s ability to be used for SPECT venti-

ation imaging. Further compounding these issues, the lowernergy of 133Xe results in poorer spatial resolution, making itess than ideal as an agent to image ventilation.20

Given these limitations, 99mTc-labeled particulate aerosolsuch as 99mTc-DTPA or the carbon labeled nanoparticle9mTc-Technegas tend to be more widely used due to theirreater availability, low cost and good image quality.20 Al-hough the choice of agent depends on factors such as localvailability, both have been reported to produce SPECT ven-ilation scans of good diagnostic quality. The most widelyvailable is 99mTc-DTPA, which can be used with doses of just

.8 mCi (30 MBq).23 However, because of the relatively larger v

ean particle mass, problems may arise from central airwayeposition, particularly in patients with chronic obstructiveulmonary disease. Technegas, with a smaller particle size,enerally has greater alveolar penetration than 99mTc-DTPA.his results in less impaction in the central airways, withechnegas being demonstrated to have a similar distribution

o that of an inert gas.24-28 Together with its lack of lunglearance during image acquisition, this would appear toake Technegas an ideal agent for ventilation SPECT.Typically, the doses of 99mTc-based imaging agents admin-

stered are identical to those used in conventional planarmaging, however, some authors have proposed a slight in-rease to the administered dose in an attempt to improvemage quality.29,30 In our institution, 13.5mCi (500 MBq) of9mTc is added to a Technegas generator, with the aim ofelivering a dose of approximately 1.35 mCi (50 MBq) to theatient. This equates to a posterior count rate of approxi-ately 2.0 to 2.5 kcps.

erfusions with planar imaging, 99mTc-macro-aggregated albumin

99mTc-MAA) is generally used to assess perfusion.19 The dis-ribution of MAA, which is proportional to regional bloodow, will be reduced distal to vascular occlusions in theulmonary arteries. Thus, it can be considered that perfusion

maging performed in this fashion has an inherent “amplifi-ation,” as even a small embolus can cause a large section ofung to be underperfused.

The dose of 99mTc-MAA used is dependent on the ventila-ion agent used. In the case where a radioactive gas is used,he dose of perfusion agent is typically lower than if a tech-etium-based ventilation agent is used. This is because theignal from the radioactive gas can be separated from that ofhe perfusion agent based on the energy level of the emittedhotons. Additionally, in the case of 81mKr, the short half-lifeesults in negligible gas remaining in the lungs during perfu-ion imaging. If a technetium-based agent is used for bothentilation and perfusion imaging, the typical approach is todrown out” the underlying ventilation signal by administer-ng a substantially greater dose of perfusion agent. A perfu-ion-ventilation dose ratio of �4:1 is generally required.19 Atur institution, the standard administered activity of 99mTc-AA is 6 mCi (220 MBq), resulting in an effective radiation

ose for the combined ventilation and perfusion scan of �2.5Sv. Other authors have proposed the use of lower activity.23

Another approach to perfusion imaging is to use MAAabeled with a different radionuclide. Sanchez-Crespo andoworkers have used 111In-MAA for perfusion.31 By combin-ng this with 99mTc-Technegas for ventilation, the authorsere able to simultaneously acquire both ventilation and per-

usion data. Although the limited availability and high cost of11In make this approach more expensive than 99mTc-basederfusion imaging, this approach has the advantage of reduc-

ng overall imaging time and producing inherently registered

entilation and perfusion image data.

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amma Cameraardware and Image Acquisition

o perform SPECT in an efficient fashion, multiheaded gammaameras (either dual or triple head) are required. A typical pro-ocol that uses a multiheaded camera requires 25 to 30 minutesf total acquisition time for a ventilation and perfusion dataset.ur acquisition protocol uses 3° radial steps over 360° with the

entilation study acquired for 12 seconds per projection and theerfusion study acquired for 8 seconds per projection. Otherenters have reported adequate SPECT quality in as little as 6inutes.30 Although it is possible to perform SPECT with a

ingle-head camera, the acquisition time becomes prohibitiveor standard clinical practice. SPECT has the added advantagever planar imaging that multiple repositioning of the detectorsnd patient arms is not required, resulting in easier data acqui-ition for the technologist.

When using 99mTc radionuclides, low-energy, high-reso-ution collimators should ideally be used. These optimizemage quality, although at the expense of reduced countsompared with low energy all-purpose collimator. If aigher-energy radionuclide such as 81mKr is used for ventila-ion, a medium energy collimator may be required. A matrixize of 128 � 128 (or greater) is appropriate for today’samma cameras, although some reports have described using64 � 64 matrix with acceptable image quality.29

mage Reconstructionncreasingly, iterative reconstruction techniques, such as the or-ered-subset expectation-maximization algorithm (OSEM)32

re replacing filtered back-projection in many areas of imageeconstruction in nuclear medicine. These algorithms permithe inclusion of many physical aspects of the imaging processn the system model, such as attenuation, Compton scatter-ng, and resolution degradation. Consequently, they offeretter control of signal-to-noise in the event that a study is

ow in counts.33 For V/Q SPECT reconstruction, we use anrdered-subset expectation-maximization algorithm (8 iter-tions, 4 subsets) smoothed with a postreconstruction 3Dutterworth filter using a cut-off of 0.8 cycles · cm�1 with anrder of 9. Traditionally, corrections for photon attenuationnd scatter are not routinely applied to V/Q SPECT, althoughhey would be required for any quantitative analysis (eg,ndividual lobar function as discussed herein).

mage Display and Reviewingfter coregistration of the ventilation and perfusion data sets, theaired SPECT data are best viewed simultaneously in transverse,oronal, and sagittal planes. Although several computer soft-are options exist that allow one SPECT study to be manually

ligned with the other, some packages allow for automatic reg-stration of the studies to each other.34 Although images can berinted to film, given the amount of data to be considered,PECT data are generally best reviewed directly on a worksta-ion. This allows the reporter to interactively examine the linkedentilation and perfusion SPECT studies in each of the threerthogonal imaging planes. An example of a normal V/Q SPECT

can is shown in Figure 1. t

In addition to tomographic display of SPECT images, fur-her data processing can also be performed. In the case inhich 99mTc is used for both ventilation and perfusion imag-

ng, perfusion data can be corrected for the background ac-ivity of the preceding ventilation scan using image subtrac-ion of coregistered data sets.23,35 Although this ventilationubtraction enhances perfusion defect contrast, it is not cur-ently in routine use. The use of SPECT also facilitates novelays of displaying V:Q quotient data to assist image reporting.almer and coworkers have described a technique where these

mages can be presented as either 3D surface shaded images ors tomographic sections in each of the orthogonal planes.35

hese so-called “quotient images” can be helpful in facilitatingmage reporting and are a useful way of demonstrating the loca-ion and extent of mismatched defects. Figure 2 shows an ex-mple of an abnormal SPECT study and corresponding selected:Q quotient images in a patient with multiple PE.

ow Does V/Q SPECTompare With Planar Imaging?

he advantages of SPECT over planar lung imaging haveeen demonstrated repeatedly over many years. In a studyerformed 25 years ago in which subsegmental and segmen-

igure 1 An example of a normal V/Q SPECT. Ventilation (V) anderfusion (Q) images (using Technegas and 99mTc-macro-aggre-ated albumin) are aligned and displayed in transverse, coronal andagittal planes.

al clots were induced in dogs, SPECT was shown to be more

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ensitive than planar imaging.36 Similar results were de-cribed by Bajc and coworkers,37 who compared SPECT andlanar imaging using 99mTc-DTPA aerosols and 99mTc-MAA

n a porcine model. They induced artificial emboli labeledith 201Tl and demonstrated SPECT to have an increased

ensitivity (91% versus 64%) and specificity (87% versus9%) over planar imaging. Using Monte Carlo simulation of

ungs containing “deficits” to mimic PE, Magnussen and co-orkers demonstrated that SPECT was more sensitive thanlanar imaging (97% versus 77%).38

Several other authors have investigated the performance ofPECT and planar imaging in clinical studies. In a study of 53

igure 2 (A) Example of a patient with multiple bilateral PE. Venti-ation (V) and perfusion (Q) images are aligned and displayed inransverse, coronal and sagittal planes. Multiple perfusion defects inreas with normal ventilation can be seen. (B) Representative coro-al, transverse and sagittal ventilation (V), perfusion (Q) and V:Quotient images from the patient shown in (A). Areas of pulmonarymbolism correspond to dark areas on the V:Q quotient images,ndicating a high V:Q ratio value.

atients with suspected PE, Bajc and coworkers23 found p

PECT to be more sensitive than planar imaging (100% ver-us 85%). In addition, the authors concluded that SPECTemonstrated better delineation of mismatched defects, and

ess interobserver variation compared with planar imaging.ollart and coworkers, in a study of 114 patients, also dem-nstrated that SPECT was more specific than planar imaging96% versus 78%) and had better reproducibility, both in-raobserver (94% versus 91%) and interobserver (88% versus9%).39 Taken together, these data suggest that SPECT has areater sensitivity and specificity, and improved reproduc-bility compared with planar imaging.

ow Does V/Q SPECTompare With CTPA?

ultidetector CTPA has evolved to the point in which it isrequently used as an initial investigation of patients withotential PE. Recent literature suggests that although it isighly specific, its sensitivity is somewhat less than desirable.ith the multidetector CT scanners used in the large PIO-

ED II study, CTPA had a reported sensitivity of 83%, indi-ating that emboli were not detected in 1 in 6 patients.40

lthough the accuracy of CTPA appears to be high in cases inhich the scan result is in keeping with the pretest clinical

uspicion, this is not true of cases in which there is discor-ance between these results. CTPA, because of its anatomicalature, has an advantage of potentially diagnosing other pa-hology (eg, pneumonia or aortic dissection). However, thiss at the expense of exposing the patient to increased radia-ion (something that is particularly concerning in the case ofoung women) and to the potential risks of contrast admin-stration such as allergy or nephrotoxicity.

Unfortunately, little literature exists making a direct com-arison between SPECT V/Q and CTPA. In 2004, Reinartznd coworkers29 compared the performance of V/Q SPECTusing Technegas) with multidetector (4-slice) CTPA. In theireries of 83 patients with a 45% prevalence of PE, they de-ermined that SPECT was more sensitive (97% versus 86%)ut less specific (91% versus 98%) than CTPA. Interestingly,owever, both modalities had comparable overall accuracy94% versus 93%). In 2007, Thomas and coworkers pre-ented preliminary results of a prospective comparison of/Q SPECT with CTPA performed using a 16-slice scanner in00 patients with suspected PE.41 They concluded that theverall accuracy of both examinations was comparable, sug-esting that SPECT V/Q and CTPA could be used inter-hangeably.

Although further studies are needed to better examine theelative strengths of SPECT and CTPA in the same popula-ion, these data suggest that both SPECT and CTPA performith similar overall accuracy. It appears that the strength ofPECT is its relatively high sensitivity, whereas the strengthf CTPA is its relatively high specificity. Consequently, bothPECT V/Q scintigraphy and CTPA have a role in investigat-ng patients with suspected PE. It is through recognizing the

otential benefits and limitations of these two modalities that

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oth V/Q and CTPA can be best incorporated into the diag-ostic pathway of patients with potential PE.

ow Should V/QPECT Results Be Reported?

he optimal way to report V/Q SPECT has yet to be clearlyefined, and the supporting literature is sparse. Although theraditional way to report planar scintigraphy has revolvedround the use of PIOPED probability categories, the adop-ion of these criteria to SPECT reporting seems inappropriate.his is particularly so, given that these criteria were derivedsing planar perfusion imaging and single view 133Xe venti-

ation, a very different imaging technique to V/Q SPECT.Reinartz and coworkers found that although the modified

IOPED criteria were generally applicable to SPECT report-ng, counterintuitively, more patients had PE in the low prob-bility category (64%) than in the intermediate probabilityategory (50%).29 Newer, and simpler, reporting criteria havelso been proposed.23,29,30 In the same study, Reinartz andoworkers29 reported a high sensitivity (97%) and specificity91%) using a simple reporting scheme that regarded all mis-atches as PE. Other authors have also used alternative re-orting criteria such as considering patients as positive for PE

f they have more than 1 segmental or subsegmental mis-atched defect23 or if they have any clear-cut mismatched

ascular-type perfusion defect, regardless of size.30 Whenpplied to SPECT reporting, these simplified schema haveesulted in low rates of inconclusive studies (typically5%).23,30,42,43 With its superior contrast resolution com-

ared with planar imaging, SPECT will reveal small perfusionefects29 and it remains to be determined how defects of thisize are best reported. While further studies are needed toetter define the most accurate reporting criteria for SPECT,ur general approach is to regard segmental and moderate orarge subsegmental mismatched defects as embolic (espe-

igure 3 Example of true planar im-ges compared with SPECT-derivedngular summed images and re-rojected images in a normal patient.or display purposes, ventilation (V)nd perfusion (Q) scans are shownnly in the anterior and left anteriorblique projections.

ially when multiple). t

hat New Applicationsan V/Q SPECT Offer?eneration of Planar Images From SPECT

or reporting specialists, viewing scintigraphic data in tomo-raphic planes represents a significant change compared withraditional planar reporting. A good ability to relate imageata to the underlying segmental lung anatomy is required,omething that is helped by having access to an accurate lungtlas. During the transition phase to SPECT imaging, thereay be a need for reporting clinicians to view both planar

nd SPECT data on each patient. Although the acquisition ofoth data sets can be performed, this approach significantly

engthens the acquisition time and may not be tolerated byany patients. Another approach is to generate “planar-like”

mages from SPECT data. Reinartz and coworkers29 have de-cribed such a technique, which takes the acquired projec-ion data from any point in a SPECT dataset and combines itith the frames obtained from the 1 or 2 projections either

ide of it. By summing these 3 to 5 data projections togethertypically covering a 10° to 15° arc), “angular summed” pla-ar images can be generated for any angle, including theraditional anterior, posterior, and oblique views.

Although this approach produces images that approximatehe traditional planar images in most cases, images can belurred and small defects may not be well visualized.44 Anxample of this can be seen in Figure 3. An alternative ap-roach proposed by Bailey and coworkers, is to use a re-rojection technique. By forward projecting a reconstructedPECT volume through a synthetic attenuation map derivedrom the lung SPECT emission data alone, planar images cane produced.45 This technique has the advantage of produc-

ng images that use all of the counts from the SPECT datasettypically 8-12 � 106 cts for each planar image), and can besed to produce planar-like images from any aspect. Al-hough images generated with the use of this approach dis-lay subtle differences compared with traditional planar im-ges, it has been shown that this has no significant impact on

he final clinical interpretation.44

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PECT/CT Fusioniven the tomographic nature of both SPECT and CT data,

he potential exists to combine the display of the imagessing image fusion. This can be achieved either by fusingata obtained from separate SPECT and CT scanners usingoftware registration,34 or by using data acquired in a sin-le scanning session on a combined SPECT/CT scanner.46

eliable, accurate registration of the SPECT study and sep-rate CT data using software fusion can be problematicue to differences in the scanning bed, arm positioningnd breathing protocols. Greater registration accuracyould be anticipated with hybrid SPECT/CT scanners.46

ome authors have postulated that SPECT/CT image fu-ion may be useful in cases of inconclusive CTPA imag-ng.47,48 This approach may potentially combine the highensitivity of SPECT with the high specificity of CTPA tomprove the diagnostic accuracy of these investigations.nother option for combined scanners is to perform alow-dose” CT study (typically using a 30-80 mA beamurrent) in conjunction with V/Q SPECT. This may pro-ide anatomical information, such as vascular, parenchy-al and pleural abnormalities, which may explain the

ause of perfusion defects seen on the V/Q SPECT scan,hus altering the final SPECT interpretation and improvingpecificity (see Fig. 4).49-51 In a preliminary study usingoftware fusion of SPECT and CTPA in 30 patients withuspected PE, Harris and coworkers demonstrated the po-ential for reducing the nondiagnostic rate of V/Q SPECTcans using this approach.47

bjective Analysisor many years, attempts have made to reduce the interob-erver variability of V/Q scintigraphy by the use of objectivenalysis, rather than subjective image interpretation.52-58 This

as been problematic for planar imaging due to high levels of a

mage noise, difficulties in image registration and poor spatialontrast.59-61 SPECT has the ability to provide a more accu-ate determination of the ventilation and perfusion to eachoxel which has been shown to correlate well with the tradi-ional physiological measures of the pulmonary ventilation-erfusion relationship.53,62

In areas of normal lung, regional ventilation is closelyatched to perfusion giving a ventilation-perfusion ratio

lose to 1. However, in areas of lung affected by PE, per-usion to the lung is typically impaired whereas ventilationemains largely unaffected, resulting in an increase in theentilation-perfusion ratio. On this basis, Harris and co-orkers have developed a methodology that determines

he overall ventilation-perfusion histogram of the lung andhen uses iterative curve-fitting to determine functionalubpopulations of the lung likely to represent PE.63 Afterpplying this to a training population, objective measuresf ventilation-perfusion heterogeneity were derived andpplied to subsequent populations of patients with poten-ial PE. This performed with impressive overall accuracy,chieving an area under a receiver operator characteristicROC) curve equal to 0.93. Furthermore, in a subset of 36atients who had intermediate probability V/Q scans usingraditional subjective interpretation, 86% were able to beeclassified as PE positive or PE negative, with an overallccuracy of 90%.63 Given that PE is a disease where de-reased perfusion is determined by the distribution of thenderlying pulmonary vasculature, Harris and coworkersave further developed this methodology to allow objec-ive analysis to be performed at a lobar level, rather than atwhole lung level, by using the anatomical informationrovided by SPECT/CT.64 Objective analysis of SPECTcintigraphy has the potential to reduce the number ofondiagnostic scan results in PE diagnosis and may havepplicability in other pulmonary disorders, such as asthma

Figure 4 SPECT/CT fusion in a 36-year-old man with right chest pain,sarcoidosis, and bilateral hilarlymphadenopathy. Coronal SPECTimages of ventilation (V) and perfu-sion SPECT (Q) show matched de-fects at the apices corresponding tothe parenchymal opacities (arrowed)seen on the CT due to sarcoidosis. ACTPA performed to investigate po-tential PE demonstrated heteroge-neous opacification of the right lowerlobe pulmonary arteries (arrowed),and was considered to be inconclu-sive. The corresponding perfusionSPECT (Q) and fused perfusionSPECT/CT image demonstrated nor-mal perfusion to this region, thus ex-cluding PE.

nd emphysema.63

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espiratory Gatings SPECT data are acquired over minutes, during which time

here is normal tidal breathing, respiratory motion results inlurring of the acquired data. One approach to overcome this

s to use respiratory gating. This has been previously reportedsing both SPECT and PET data.22,65 By only choosing datarom a specific lung volume, it is suggested that the ability toesolve defects can be improved. Despite the potential advan-ages of this approach, to compensate for the use of only aimited portion of the total dataset, either a higher dose ofadiotracer must be administered or patients must becanned for a longer period of time. In addition, the imple-entation of respiratory gating is dependent on hardware

ssues, such as the need for a physiological respiratory syn-hronizer and list mode acquisition, which may not be gen-rally available. Ideally, all of the data should be used but thisill require sophisticated affine transforms of the lung shapeuring coregistration between different images captured atifferent phases of the respiratory cycle to “warp” it into onearticular phase of the respiratory cycle.

pplications Other Than PElthough the main clinical indication for V/Q scintigraphy is

n the evaluation of PE, there are other indications in whichPECT may have an important role. For patients with lungancer being evaluated for lung reduction surgery, it is usefulo know the relative contribution to total ventilation anderfusion of the lobe(s) to be excised.66 Planar V/Q scanning

igure 5 Anterior (A) and posteriorB) planar images in a patient with aight lung carcinoma (arrowed). Thexact lobar location of the tumor can-ot be determined on the planar im-ging. Fused SPECT/CT perfusionmages in the coronal (C), transverseD), and sagittal (E) planes show theumor and corresponding perfusionefect (indicated by cursors). Corre-ponding CT scan slices in the coro-al (F), transverse (G), and sagittalH) planes (with patient-specific lo-ar region-of-interest derived fromhe CT) shows the lesion to be lo-ated in the right upper lobe (ar-owed). The SPECT/CT allowed ac-urate determination of each lobe’selative contribution to overall venti-ation and perfusion.

as been used for this purpose. However, because of the t

natomical overlap of the lobes of the lung, this approach isnherently inaccurate. Neither of these limitations affectsPECT imaging. Furthermore, hybrid SPECT/CT scannersllow each lobe of the lung to be accurately identified andapped back onto the functional data, thereby facilitating

natomically accurate assessment of individual lobar contri-ution to lung function46 (see Fig. 5).Other potential applications for the technique include

uiding thoracic radiation treatment planning to avoid irra-iating functioning lung, estimating regional lung function inatients with interstitial pulmonary disease, and using 133XePECT to assess obstructive lung disease.22,67-71 Techniquesuch as fractal analysis as well as coefficient of variation of theixel counts also have been used to objectively evaluate ven-ilation inhomogeneity with Technegas SPECT.72-74 In addi-ion, several authors have used SPECT imaging as a physio-ogical tool to provide invaluable information regarding theegional distribution of ventilation and perfusion.20 Givenhe relative ease with which these data can be collected andhe power of the topographical information SPECT provides,his aspect promises to be an exciting emergent applicationor V/Q imaging.

onclusionlthough it has been used in the diagnosis of PE for manyears, it is evident that planar V/Q scintigraphy has someimitations. If the V/Q scan is to remain relevant as an imaging

ool, it is important that it be optimized so that the diagnostic

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nformation it provides is maximized. V/Q SPECT can beeadily performed in most nuclear medicine centers todayith no increase in patient imaging time. It has been shown

o have superior sensitivity, specificity, reduced interob-erver variability, and greater overall accuracy comparedith planar imaging. Unlike CTPA, V/Q SPECT is not asso-

iated with contrast-related complications, such as allergynd nephropathy, a high radiation dose to the breast, orechnical difficulties such as the need for a sustained breathold or highly synchronized injection timing to acquisition.n the basis of the published literature, the accuracy of V/Q

PECT is similar to that reported for multi-detector CTPA.urthermore, the higher sensitivity of V/Q SPECT makes it

deally suited to excluding potential PE. In the future, it isikely that V/Q SPECT imaging will be further improved byespiratory gating, image fusion and quantification. In addi-ion, there are further emerging clinical and research appli-ations that are likely to benefit from the use of V/Q SPECT.

cknowledgmentshe authors acknowledge the assistance of Elizabeth Baileyho helped to generate the images for this article.

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