European Journal of

Nuclear Medicine Case report

Value of ellipsoid volume masking in myocardial tomography Miguel Rodriguez, Mercedes Borr6n

Department of Nuclear Medicine, Institute of Oncology and Radiobiology, Ciudad de la Habana, Cuba

Received 24 April 1992 and in revised form 14 June 1992

Abstract. The circumferential profile technique, used to obtain a bull's eye image from short axis slices in myocar- dial perfusion tomography, requires isolation of the re- constructed volume from other neighbouring structures that may take up the radiopharmaceutical used. If this is not possible, some regional artefacts can be introduced in the polar map that do not represent the actual myocar- dial perfusion at the corresponding level. In this case re- port we describe a method for volume masking that per- mits the myocardium inside in ellipsoidal volume to be enclosed. This technique is compared with the spherical method used in a Sophy 20P system in a case with im- paired perfusion of an inferolateral segment and an ex- tracardiac "hot" structure located in the same area. The results obtained show the usefulness of our method in such cases.

Key words: Myocardial tomography - Masking - Bull's eye image

Eur J Nucl Med (1993) 20:83-85

Materials and methods

Acquisition and processing. A Sophy camera was used to acquire the 32 projections in the patient study encompassing over 180 ° from the left posterior oblique to the right anterior oblique projec- tion, after the administration of 92.5 MBq (2.5 mCi) of 2°1T1 chlo- ride under submaximal exercise. The data were analysed in a scinti- graphic data acquisition and processing system supported by an IBM PC. A conventional algorithm of filtered back-projected ma- trices was used for the reconstruction of transaxial slices from which myocardial short and long axis slices were obtained. Projec- tion prefiltering was obtained by means of a Wiener filter, convo- luting the lines and columns of each matrix with the following vector

(1/282) (-4, -13, -15, 2, 40, 81,100, 81, 40, 2, -15, -13, -4)

Two masked sets of slices for two methods of volume masking were obtained: a spherical one (Sophy 20P 256 User's Guide, So- pha Medical, S.N.I. April 1990) and ellipsoidal masking. An algo- rithm using spherical coordinates for finding maximal circumferen- tial profiles was employed for bull's eye image generation from each set of masked slices (Miller et al. 1988). It searches maximum

Introduction

The bull's eye image or polar map, a two-dimensional representation of the myocardial perfusion using either thallium-201 or isonitriles labelled with technetium-99m and SPET (Garcia et al. 1985; DePasquale et al. 1988), is obtained using the maximal circumferential profiles extracted from the short axis slices, extended from the apex to the base, and then distributed into concentric rings from the centre to the periphery on the polar map. However, it is important to isolate the myocardial vol- ume from any other surrounding structures in order to avoid the inclusion of artefacts in the image. Several commercial systems like those from Sophy offer some masking methods for this purpose. We have developed a new masking method that improves the masking of this myocardial volume.

Correspondence to: M.R. Castillo, Calle 34 No. 9505, e/95 y 97, Cotorro, Ciudad de la Habana, Cuba

Fig. l . Non-masked short axis slices of the selected study. A hypo- perfused area in the inferolateral segment of the myocardial wall and an extracardiac "ho t" zone are seen

© Springer-Verlag 1993

84

Fig. 3. a Bull's eye image generated from spherically masked slices. An artefact is indicated by the arrow, b The same image obtained from ellipsoid volume-masked data does not show this artefact. e The artefact is better revealed in the subtracted image

Selected study. We selected the myocardial perfusion study of a patient with three-vessel ischaemic heart disease who had under- gone bypass surgery. An inferolateral hypoperfused segment and an extramyocardial "hot" area located in this direction can be seen in the short axis slices shown in Fig. 1.

Ellipsoid volume masking. The method developed for myocardial masking permits the user to predefine the minor and major axes of a revolution ellipsoid on the previously centred myocardial vol- ume: over three perpendicular planes defined by the central short axis slice and middle long axis sagittal and frontal slices, using the computer system numerical pad. In this way, the operator may adjust the volume to be masked in a more suitable form, ensuring that any undesired structure is kept out the field of interest.

Fig. 2. a Spherical masking, b Ellipsoidal masking

values in all radial directions over the entire volume, starting from the ventricular centre. Unlike the conventional method (DePas- quale et al. 1988), with this method the whole myocardial volume is sampled the same way regardless of the heart size; it is not necessary to define its limits for bull's eye image generation, and apex sampling is approximately perpendicular to this region.

Results and methods

Figu re I shows the n o n - m a s k e d shor t axis slices o f the s tudy. A h y p o p e r f u s e d a rea can be obse rved in the infer- o la te ra l wall o f these con t iguous slices. Us ing spher ica l mask ing (Fig. 2a) , the " h o t " zone loca ted in the same d i rec t ion as the defect lies inside the ad jus ted sphere. E l l ipso ida l m a s k i n g (Fig. 2b) excludes this zone f rom the r econs t ruc ted volume.

Bull ' s eye images genera ted f rom these two set o f m a s k e d slices are shown in Fig. 3 (a, b). F igu re 3 c shows the difference be tween these two images in which there is a non -ze roed area, reveal ing an a r te fac t i n t roduced

in the bull's eye image due to the peripheral extracardiac "hot" zone included in the spherical masked set of slices. These are the circumstances in which artefacts can result in erroneous data interpretation using the bull's eye tech- nique. The method our propose gives better results than the spherical method.

In conclusion, the results obtained in this case report show that the ellipsoid volume-masking method permits extracardiac "hot" zones to be excluded from the recon- structed volume used in myocardial tomography to gen- erate bull's eye images in a more accurated manner.

85

References

DePasquale EE, Nody AC, DePuey EG, Garcia EV, Pilcher G, Bredlau C, Roubin G, Gober A, Gruentzig A, D'Amato P, Berger H (1988) Quantitative rotational thallium-201 tomogra- phy for identifying and localizing coronary artery disease. Cir- culation 77: 316-327

Garcia EV, Van Train K, Maddahi J, Prigent F, Friedman J, Aree- da J, Waxman A, Berman D S (1985) Quantification of rotation- al thallium-201 myocardial tomography. J Nucl Med 26:17-26

Miller TR, Starren JB, Grothe RA Jr (1988) Three-dimensional display of positron emission tomography of the heart. J Nucl Med 29 : 530-537

Sophy 20P 256 User's Guide, Sopha Medical, S.N.I., April 1990