oblique views in lung perfusion scanning: clinical utility

1977;18:967-972.J Nucl Med. Peter E. Nielsen, Peter T. Kirchner and Frederic H. Gerber Oblique Views in Lung Perfusion Scanning: Clinical Utility and Limitations

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The concept of using oblique views in gammaimaging of the lung was addressed in 1968 and 1969by Wellrnan et al. (1,2 ) . Using I-i 31-labeled MAAand the gamma scintillation camera, they found thatnot only perfusion defects localized more accurately with the help of oblique views, but that, inaddition, some perfusion defects were detected onlyon the oblique projections. Since overlap activitywas avoided in the oblique views, this group appeared to prefer them to lateral projections.

In spite of these reports, oblique imaging of thelung did not gain widespread acceptance, possiblydue to the inconvenience of applying the techniqueto the still frequently used rectilinear scanner. Instead, the lateral view became the standard supplement to the anterior and posterior views. Recently,Caride et al. have reviewed some practical and theo

retical advantages of oblique lung images (3). Anew atlas on lung imaging by Mandell similarly emphasizes advantages of oblique views (4).

The addition of routine anterior and posterioroblique views to the standard four-view lung scanwas instituted in our clinic in mid 1975. The presentreport is based on a quantitative analysis of over

300 patient studies. It also includes the first description of an artifact frequently found on posterioroblique views.

MATERIALS AND METHODS

Since the middle of 1975, 3 10 patients in ourclinic have received perfusion lung studies by meansof a scintillation camera. From these studies we haveexcluded 81 because some of the oblique views wereabsent or technically inadequate. Of the remaining229 studies, 77 were of patients whose clinical settingand arterial blood gases were not representative ofacute pulmonary embolization; their perfusion scanswere clearly normal or displayed only minimal, nonfocal inhomogeneities. These 77 constitute the â€œcontrolâ€•group from which we derived criteria for thenormal appearance of oblique views; they were reviewed by two observers to characterize the vanations in shape, tracer distribution, and margincontinuity for each of the oblique projections.

Received Feb. 21, 1977; revision accepted May 10, 1977.For reprints contact : Peter E. Nielsen, National Naval

Medical Center, Bethesda, MD 20014.

Volume 18, Number 10 967

Oblique Views in Lung Perfusion Scanning:

Clinical Utility and Limitations

Peter E. Nielsen, Peter 1. Kirchner, and Frederic H. Gerber

National Naval Medical Center, Bethesda, Maryland

Over 300 perfusion lung scans were reviewed to assess the clinical utilityof routinely performed anterior and posterior oblique projections. Thecharacteristics of normal anterior and posterior oblique views were analyzed

and compared with lateral projections. The four oblique images contributedmaterially toward characterization of perfusion defects in 63% of all abnormal studies; the contribution of posterior obliques was 48%, anteriorobliques, 15%. For scans displaying at least one focal defect, oblique viewswere of benefit in over 70% of cases, with posterior obliques accountingfor 55%. Oblique views contributed little when perfusion abnormalitiesinvolved both lung fields diffusely. The posterior oblique projection wasof more value than the anterior oblique because of its ability to darifylower-lobe abnormalities significantly. A scanning artifact in the posterioroblique views, due to attenuation of the near lung activity by scapula andshoul4er-girdle musculature, was seen in 90% of normal studies.

J NuciMed 18:967â€”973,1977

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NIELSEN, KIRCHNER, AND GERBER

The 152 abnormal studies were classified as showing either well-defined focal abnormalities, or significant diffuse abnormalities, or both. This was thegroup in which the clinical utility of oblique viewswas critically assessed, all studies being independentlygraded by two observers as follows:

Grade â€”1 Defect seen on standard views (antenor, posterior, and both laterals)was not shown on any of theoblique images.

Grade 0 Oblique image showed the abnormalities seen on standard views butdid not clarify or increase the certainty of any.

Grade + 1 Oblique image confirmed the presence of an abnormality that wasseen on only one of the standard

views; or, if the oblique defined thesize and character of an abnormality

better than any of the standardviews; or, if the oblique view mdicated that an area of suspicion ona standard view should be considered normal. In these instances, the

oblique image improved the localization and characterization of perfusion defects, even though it rarelyaltered the final interpretation.

Grade +2 Oblique image showed abnormalities that were not clearly apparenton standard views.

In instances where the oblique view was clinicallyhelpful (Grades + 1 and +2), each observer alsorecorded the type and location of the abnormality

A

ANT

LLAT RLAT

POST

LAO RAO

RAOLAO

HG. 1. Normalperfusionlungscintiphoto.Locationof lungsegmentson oblique projectionsin (A) are illustrated by line drawings (B). In (B), (1) = apical posterior, (2) = apical, (3) = anterior,(4) = posterior, (5) = superior, (6) = ant. basal, (7) = later, (8)= lot. basal, (9) = post. basal, (10) sup. lingular, (11) inf.lingulor, and (12) medial.LPO RPO

968 JOURNAL OF NUCLEAR MEDICINE

RPO BLPo



Number of studiesdinicallyusefulRelative

usefulnessâ€”10

+1+2Total

DIAGNOSTICNUCLEARMEDICINE

that the oblique view clarified.The control group of 77 normal radionuclide im

ages was also used for the evaluation of a frequentlyoccurring artifact seen in the scapular region of theposterior oblique projection. The artifact was gradedfor its prominence on a scale of 0 (not apparent)through 4 (markedly positive) for both LPO andRPO. The grades were related to the patient's bodybuild (height, weight, muscularity) and to right- orleft-handedness.

All perfusion lung scans were performed withTc-99m-labeled macroaggregated albumin (MAA).The average adult dose was 5 mCi of Tc-99m, contaming more than 500,000 MAA particles. All tracerinjections were performed with the patient supine,breathing normally. All lung imaging was performedwith a scintillation camera (1 0 in. field of view)

with a low-energy diverging collimator. Almost allimages were obtained with the patient in the uprightposition. Ventilation studies with xenon-i 33 weredone in about half of the patients with abnormalperfusion scans, the ventilation study always preceding the perfusion study.

RESULTS

Normal oblique projections (Fig. 1). Posterioroblique images. The near posteromedial and the farposterolateral borders were always smooth and â€œparallel,â€•as were the near and far diaphragmatic margins. Deviation of these margins from a parallelconfiguration proved to be a sensitive indicator ofdisease. The posterior bases were more sharply defined on posterior oblique than on lateral views.

To assess the potential sensitivity of the RPO viewfor right middle lobe and anterior segment RULdefects, the anterolateral contour of the RPO viewwas compared with the anterior margin of the rightlateral view. On the latter projection in normals, theRML regions was marred by a wedge-shaped areaof decreased activity in one-third of cases, showeddiffuse decreased activity in another third, and

LtAT RtAT

FIG. 2. A 38-year-oldmanwith chroniclung disease;studydone for analysis of regional pulmonary function. Perfusionabnormality at left base, probably an anterior basal defect, is best seenon LPO view. In right lung, a basal perfusion defect, well defInedon the RPO, is minimally shown on posterior view and missedonright lateral view. A xenon study confIrmed bilateral basal lungdisease.

showed a full contour of activity homogeneous withupper and lower lobes in only one-third of the cases.In virtually all RPO views, however, the anterolateralcontour of the right lung was smooth and continuousfrom apex to base. This is probably because thelateral view shows the medial segment, which projects over and is affected by cardiac mass, while theRPO projection shows the lateral segment, which isfree from cardiac interference.

On the other hand, in only one-third of the casesdid the corresponding anterolateral contour of theLPO display homogeneous activity for its entirecourse. More frequently it showed either diffuseattenuation (one-third of the cases) or a wedge

TABLE1. CLINICALUSEFULNESSOF ROUTINEOBLIQUE VIEWS IN PERFUSION STUDIES

JUDGEDABNORMALON STANDARDFOURVIEWS

Anterior obliquies 2 127 23 0 152Posteriorobliques' 2 78 72 1 152

S In five patients, posterior obliques were useful for more

thanone lesion.


@ii@POSTEQU1t



Diffuse Diffuseand focal Focalonly Total


TABLE 2. CLINICAL USEFULNESSOF ANTERIOR AND POSTERIOROBLIQUE VIEWS JUDGEDAGAINST THE TYPE OF PERFUSION DEFECT

Anterior oblique O/23 5/35 (14%) 18/94 (19%) 23/152 (15%)Posterioroblique 1/23 20/35 (57%) 51/94 (54%) 72/152 (48%)

All obliques 1/23 (4%) 25/35 (71%) 69/94 (75%) 95/152 (63%)

* Numerator of fraction indicates the number of studies in which the oblique was clinically useful (+1 or +2); denominator

indicates the total number of studies showing the indicated abnormal pattern on standard views.

shaped defect (another one-third) over the regionof the heart.

Anterior oblique views. Both left and right projections showed anterolateral and posterolateral contours of either lung to be smooth from apex to basein all normals. The anterolateral margin of the leftlung on RAO projection does not extend to the levelof the diaphragm if the heart is large. By contrast,the medial margin of both the right and left lungs(i.e., that overlying heart) in either anterior obliqueprojection is exceedingly variable in contour anddoes not permit categorization. The diaphragmaticmargins are bilaterally smooth and meet at a welldefined angle with the posterolateral margin. In allfour oblique projections the apical contours areâ€œparallel.â€•

Abnonnal studies. Table 1 summarizes our findings in 152 abnormal studies. The posterior obliqueviews were clearly more valuable than the anterioroblique views. In both P0 studies that were judgedâ€œâ€”1,â€•the perfusion abnormality was a homogeneous and diffuse decrease of activity over an entirelung field. Figures 2â€”4show â€œ-@-1â€•findings.

In order to define which types of perfusion abnormality are best detected and clarified by obliqueimages, the 152 abnormal scans were classified onthe basis of the standard views as showing one ofthree patterns: a) diffuse abnormalities, b) focalperfusion defects, or c) a combination of these. Animage was classified as showing a diffuse patternwhen multiple, small perfusion defects were scattered throughout both lung fields. A focal pattern

POST RtAT

LP0 RPOFIG. 3. A 57-year-oldmanwith clinicalsettingof recentpulmonaryembolusand infarction.RPOview aids posteriorview in con

firming that decreased right basal activity (right lateral view) is due to lack of â€˜@shine-throughâ€•from non-perfused areas of far lung.Upper lung field in LPO illustrates artifact described in text.


ANT LLAT

i@



DIAGNOSTIC NUCLEAR MEDICINE

designated one or more well-defined defects involving the two lung fields asymmetrically and nonhomogeneously. When one or two dominant defects occurred in the presence of diffuse abnormalities, thestudy was classified as a combination of focal anddiffuse patterns. Table 2 summarizes our findingsregarding the clinical usefulness of oblique views foreach of the three types of abnormal pattern. Whenperfusion abnormalities were focal in character, 18of 94 (19% ) anterior oblique images, and 5 1 of 94(54% ) posterior oblique images contributed materially to the definition and clarification of the abnormalities, yielding an improvement in lesion localization in 69 of 94 cases (73%).

In an attempt to define which areas of the lungare best examined with oblique scans, the defectsbest shown or substantially clarified by an oblique

view were classified according to location. We foundthat posterior obliques were most useful when lowerlobe abnormalities were present, and that anteriorobliques were best with upper-lobe lesions.

Presence of scanning artifact on posterior obliqueviews. In reviewing posterior oblique views of thelung, a focal area of decreased activity, projectingover the upper portion of the near lung field only onthe posterior oblique view, was present in 90% ofthe control scans. It is generally linear to triangularin configuration (Fig. 5 ), and was consistently moreprominent over the right than the left lung, especiallyin males. The patients' records and further questioning disclosed the great majority of these patients tobe right-handed, with the most pronounced exampies in men. The artifact appears to move with move

FIG. 4. A 60-year-oldwomanwithCOPD;studydonefor analysis of regional pulmonary function. Perfusion abnormality in leftupper lobe peripherally is seen poorly on anterior and left lateralviews although well shown on LAO view.

RP0

FIG. 5. Bilateralposteriorobliqueartifacts,right greaterthanleft, in young muscular male with history and clinical course indicative only of esophagitis. Standard views are essentially normal.

ment of the scapula, but radioactive markers placedon the border of the scapula project only roughlyaround the artifact (Fig. 6) , suggesting that moretissues than scapula alone are responsible for thislocal attenuation of lung activity. Review of chestx-rays did not disclose prominent pulmonary hilain patients with the artifact.

DISCUSSION

In the evolution of a format for perfusion lungscanning, both lateral and oblique projections of thelung have been shown to reveal segmental anatomybetter than anterior and posterior views. The use oflateral and oblique views also gives a much moreextensive examination of the lung contours by providing additional tangential contours. Hence, theaddition of lateral and oblique views to anterior andposterior views should theoretically improve not onlythe detection rate but also the localization and exactcharacterization of perfusion defects. The value ofthe lateral view has been reviewed in the past (5â€”7).Our study supplements both the initial 1968â€”1969reports and the recent ( 1976) reviews by documentingthe improvement in lesion characterization throughthe routine use of all four oblique projections.

Lateral views are better suited to rectilinear scan

LIat LAO


LP0

S




anterolateral margin of the RPO view for the detection of right middle lobe lesions. Oblique views servebest in the detection, confirmation, and localizationof focal perfusion defects; they rarely provide additional information in patients displaying diffuselydistributed perfusion abnormalities, as in many casesof chronic obstructive lung disease. Nevertheless, ifa patient with chronic lung disease is to be studiedfor suspected coexisting pulmonary emboli, we wouldrecommend the use of oblique views matched withposterior and possibly posterior oblique ventilationscans to provide a more sensitive examination forventilation perfusion mismatches.

The limitations of oblique lung images have atready been mentioned in the discussion of the resuits. Oblique views do not permit quantitative comparisons of right and left lung activity. Diffusely

decreased perfusion of either lung can be detectedon anterior and posterior views. Posterior obliqueimages often display the previously mentioned artifact in the scapular region over the near lung, andthis must be differentiated from a true perfusiondefect by considering the combined findings of multiple views.

On the basis of our study we recommend the routine use of oblique views for perfusion lung scanning. Since posterior oblique views are much morefrequently valuable than anterior oblique views, nuclear medicine clinics that are hard-pressed for timemay want to adopt the routine use of only the postenor oblique images. We caution against the onlyoccasional use of oblique views, since we find thatreliable interpretation oblique views is achieved onlyafter frequent exposure to both normal and abnormaloblique projections.

REFERENCES

1. WELLMAN HN, MACK JF, SAENGEREL, et al: Clinicalexperience with oblique views in pulmonary perfusion scmtiphotography in normal and pathological anatomy. I NuclMed 9:374,1968

2. MACK JF, WELLMAN HN, SAENGER EL: Oblique pulmonary scintiphotography in the analysis of perfusion abnormalities due to embolism. I Nuci Med 10: 420, 1969

3. CARIDE VJ, PURl 5, SLAVIN JD, et al: The usefulnessof the posterior oblique views in perfusion lung imaging.Radiology 121: 669â€”671,1976

4. MANDELLCH: Scintillation Camera Lung imaging,New York, Grune and Stratton, 1976

5. SURPRENANTEL: Lateral lung scanning, anatomic andphysiologic considerations. Am I Roentgenol 99: 533â€”542,1967

6. MISHKIN FS: The lateral view in lung scanning as anaid in differential diagnosis. Dis Chest 53 : 743â€”749,1968

7. Si' WM, KROL G, FAUNCEH, et al: Value and pitfallsof the lateral lung scan. Chest 67: 549â€”552,1975

FIG. 6. Radioactivemarker(Tc-99m-filledcatheterplacedalongmedial border of scapula) protects only roughly around artifact,suggestingthat more tissuesthan scapulaalone are responsibleforthis attenuation of lung activity.

ning, since rectilinear lateral views suffer less fromâ€œshine-throughâ€•activity from the far lung thangamma generated lateral images. The two techniques,however, do not differ in total crossover counts. Surprenant estimated the crossover counts from thecontralateral lung to be 15â€”30% with the rectilinear scanner (5). Using patients with unilateralabsence of perfusion as subjects, we and others (R.Secker-Walker, personal communication) have similarly estimated that with the gamma camera thecrossover counts in lateral images can amount to20 or 30% . The more important difference is thatwith the rectilinear scanner, the crossover photonsdo not project a sharp image of the contralaterallung, since these photons Originate well beyond thecollimator's focal plane. Hence, degradation of thenear-lung image occurs primarily in the form ofdiffusely increased background. The gamma camera,whose spatial resolution falls off relatively slowlywith distance, will project less blurred outlines of thecontralateral lung onto the lateral projection of thenear lung. This superimposition of the near and farlung images may not only obscure perfusion defectsin the near lung (Figs. 2, 4) but can even simulatethem (Fig. 3). With recent improvements in cameraperformance and the increasing use of cameras withlarge fields of view (which can use parallel-hole cotlimators for lung imaging), the problem of â€œshinethroughâ€•on lateral projections has worsened.

Oblique projections provide the best practicalsolution to this problem. By confirming an abnormality not clearly defined on the standard views, theoblique views enhance the observer's ability to distinguish normal from abnormal scansâ€”an observation well supported by the review of Caride et at.(3) . We found no significant difference in the usefulness of right as opposed to left oblique scans, eventhough our review suggests that the anterolateralmargin in LPO views is not as sensitive for detectionof lingular abnormalities as is the consistently smooth




oblique views in lung perfusion scanning: clinical utility

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