a measurement system for leftventricular volume

EUROPEAN JOURNAL OF CARDIOLOGY, 1974, I j 3, 259-277. EXCERPT A MEDICA

A measurement system for leftventricular volume determination~:~

Rogier P. van Wijk van Brievingh1, André Richtering Blenken1, Rube van Poelgeest2,

Jan H. J. Sneek2, Tjeerd van der Werf2, Ariaen N. E. Zimmerman2 and Frits L. Meijler2

1 Medical Engineering Group, Department of Electrical Engineering, Delft, University of Technology, 4 Mekelweg, Delft, The Netherlands and 2 Clinic for Cardiovascular Diseases, University Hospital Utrecht, 101 Catharijnesingel, Utrecht, The Netherlands

VAN WIJK VAN BRIEVINGH, R. P., RICHTERING BLENKEN, A., VAN POELGEEST, R., SNEEK, J. H . J. , ZIMMERMAN, A. N. E. and MEIJLER, F. L. (1973): A measurement system for left ventricular volume determination. Europ. J. Cardiol., 1 j3, 259-277.

A system for measurement of left ventricular volume is described, in which biplane angiocardiograms are recorded on a video disk. lnto the video frames, ECG and pressure curves are coded as weil as time references, values of 5 other quantities among which the time elapsed since the preceding R wave sampled at the moment ofthe X-ray flash, and 1000 characters of alphanumeric information. Videosubtraction is used as a preprocessing technique, after which the ventricular contours are drawn with a light-pen and retained in a digital memory. Via a video computer interface the contour data are fed to a PDP-15 computer which calculates the ventricular volume according to a geometrical model based on measurements of casts of the left ventricIe.

heartjcardiac ; ventricular volume; quantitativejmeasurement ; X-rayjsubtraction; contrastjangiography; video/television ; model j computer

Introduction

The action of the heart as a muscular pump cannot be defined completely without knowledge of its change in shape and size throughout the cardiac cyde [1]. The pump function is under the control of many factors [2,3], which can best be analysed separately, for instance in the isolated perfused heart. Results, obtained from measurements on isolated preparations, cannot be extrapolated without reserve to the function of the ventricles in the intact organism [4]. Beside the contractility - not

* A subsidy was granted by the Dutch Foundation for Fundamental Medical Research, FUNGO, for instrumentation and personnel essential to the investigation.

readily definable in physical terms - the preload, detined as the end-diastolic volume (EDV) which represents more or less the initial length of the muscle tibers, and the afterIoad, the hydraulic impedance and the pressure against which the ventricle has to pump out the blood, are determinants of the heart's performance as expressed by stroke volume (SV) or stroke work (SW) [3]. The dependence of EDV and SV on the duration of the preceding R-R interval is a major part of the investigations at the Utrecht Clinic for Cardiovascular Diseases [5]. In order to calculate SV = EDV - ESV, also the end-systolic volume (ESV) has to be measured on a beat-to-beat basis . So a measurement system had to be developed for the left ventricular volume (LVV) at these moments during the cardiac cycle; more samples are required for the construction of a pres-

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R. P. van Wijk van Brievingh et al. , A measurement systemfor left ventricular volume determination 261

sure-volume diagram of the ventricle, which gives an indication of the myocardial contractile state. Furthermore, SW may be calculated from the area enclosed by this curve. The absolute value of the first derivative of LVV during contraction as a function of time gives the blood flow into the aorta.

At this time, angiography is the standard method for determining LVV [6]. In the future, echocardiography may prove the most useful nonevasive method for dimensional analysis [7]. For a reliable calculation of the magnification in the X-ray shadowprojection, use of a biplane installation is necessary to determine the position of the heart inside the body [8]. Moreover, the ventricular cavity cannot be described accurately by a solid of revolution, so l.t least two projections are necessary to calculate ts volume [9]. Even then, reconstruction of an ir'egularly shaped volume from only two projections

is in gener al not possible; and a convex geometrical model has to be assumed [10]. A model, based on measurements on casts of postmortem ventricles, which bears a closer relationship to the anatomie shape will be proposed. Videosubtraction is a useful form of image preprocessing [11], while other video techniques make handling of contour data easy [12]. Although fully densitometric contour determination seems possible [13], an optimal task division between operator and instrumentation will be achieved when the pattern recognition and evaluation part is executed by a trained human observer. Thus quantitation of the pump function of the left heart is realized on the basis of examination procedures already indicated for diagnosis on morphological grounds.

In this article, necessary modifications of an existing catheterization room will be described, as weIl as calibration procedures, automatic control of the measurements, data conversion methods, and preliminary animal experiment results.

Methods

A general view of the instrumentation involved is shown in Figure 1, the set-up has been developed on the basis of an analysis of requirements, the results of which can be stated as follows:

1. The biplane X-ray installation, designed to obtain pictures which are interpreted visually, has

to be calibrated as it will be part of a measurement system.

2. The contrast medium is to be injected in fractions at selected moments. With a given maximum allowable amount of medium, the number of fractions has to be as large as is consistent with the detectability of the LV border.

3. All pertinent information has to be recorded on one carrier so as to preserve time relationships and patient identification. Coding of data into the video signal representing the image information has been chosen as the most flexible method.

4. Preprocessing of the video signal representing the X-ray shadow should be executed (videosubtraction, contrast enhancement) so as to facilitate contour detection.

5. Determination of the LV contour is to be based on density information as weIl as on a priori knowledge of a cardiologically trained observer. So a semiautomatic system has been chosen with regard to the accuracy of measurement required.

6. Conversion of contour and other data has to be performed in such a way, that computer manipulation is possible.

7. For calculation of LVV, a geometrical model has to be developed which is related as close as possible to the anatomie reality. For that reason the position of the ventricle relative to the coordinate system has to be taken into account also.

8. The software package should comprise also calculations with the data measured, e.g. p V loop, SW, correlation with R-R intervals.

A preliminary survey of the measurement procedure, which may be described as 'Quantitative Videoangiocardiography', has been published [14], an elaboration is given here.

1. The X-ray installation as a part oj the measurement system

Description of the processes underlying the image formation in X-ray systems can befound in literature [15,16]. The factors determining the quality of the image have been discussed frequently [17]. Different aspects of video techniques, applied in this field, have been analysed by several authors [18] ; as are the requirements for densitometric procedures [19]. Vnder the assumptions of linearity and isoplanasy, the spatial resolution of the system can be described

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262 R. P. van Wiik van Brievingh et al., A measu/'ement systemfo/'Iefl venlricular volume delerminalion

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by its spatiaI modulation transfer function, MTFs. The low-pass filter characteristic of this function poses na limitation on the transmission of the left ventricular shadow image for most systems [20].

The tempo ral resolution ean be independently de· seribed by the temporal modulation transfer fune· tion, MTFt. It is mainly determined by the persis tenee of the image-intensifier screens, the camen

R. P. van Wijk van Brievingh et al., A measuremellf systemfor left ventricular voll/me determination 263

tube, the frame rate, and the radiation dose [21,22]. Comparison of cineangiocardiographic pictures with different frame rates led to the conclusion that for proper measurement of LVV, frame rates of more than IS/sec are required [23]. Thus, by choosing television frames with 50 f/sec and 312 1/2 lines instead of complete fields (25 f/sec, 625 lines interlaced), a good compromise between temporal and spatial resolution can be found. In reference [23], the frequency content of the LVV curve is taken as a criterion. If samples of ESV and EDV are to be taken from patients with irregular R-R intervals, e.g. in the case of atrial fibrillation, [5], a higher frame rate is necessary, and elect ri cal stimulation of the heart may prove necessary.

The divergent X-ray beams produce a shadow pattern on the sensitive screen of the image intensifiers with projective enlargement. The magnification factor can be calculated if the position of the heart relative to the main axes is known [8] . As shown in Figure 2, the coordinates (Y, Z) of a point P in orthogonal projection can be derived from the measured projection coordinates (y, z) with magnification factors which are functions of

both y and zand the known distances Dl' Lj, respectively D2 , L2 of the X-ray tube foei and image intensifier input screens respective to the main axes. So a set of two simultaneous equations has to be solved for each point to eaIculate the true dimensions of the left ventricle from the shadow projections [24]. The quantities Dl ' D 2 , Ll and L2 are measured with steel bands, attaehed to the telescopic supports of the X-ray tubes and image intensifiers. In the position chosen, the main axes have to be adjusted so that they intersect perpendicularly. For this purpose, a sensitive X-ray aiming device was constructed as shown in Figure 3.

The X-ray patterns on the input screens of the image intensifiers result in two video signals which are combined into one TV frame, representing the middle halves of both projections, in the usual manner. With a light-pen system, a digital contour memory, and a video computer interface, the contour data are fed to a PDP-I5 computer. Calibrati on of this data conversion part is achieved with metal masks, placed on the image-intensifier input screens. In these masks, circular slits are machined with different radii and 0.25 mrn width. If X-ray

Fig. 3a. The X-ray-aiming device is shown which is used to adjust the X-ray souree and image-intensifier pairs so that their axes interseet perpendicularly.

264 R. P. van Wijk van Brievingh et al., A measurement system for left ventricular volume determinatioll

Fig. 3b. Perpendicular intersection ofaxes may be judged from the monitor. The lead disk covers a set of 4 smaller ring sections ofthe same shape as the outer set. The center holes in the disks should lie on the same television line.

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R. P. van Wijk van Brievingh et al., A measurement system for left ventricular volume determination 265

beams are applied, the resulting video signal is of sufficient high contrast to be accepted by the video computer interface directly. As by this procedure the coordinates in terms of computer counter units corresponding with the sampled points are known, the radius of the circle defined by these points can be computed. Thus the calibration in horizontal (number of counts per mm) and vertical direction (number of counts per TV line) is established. The results are shown in Figure 4. Together with the projective magnification factors as mentioned above, calibration of the total system is obtained. A test with a metal sphere of known diameter showed that irrespective of the position, the volume is found with an accuracy of 4%.

2. SelectÎl'e injection oj contrast medium

One of the main problems in quantitative angiocardiography is the modification in circulatory function caused by the contrast medium injected. The sudden addition of volume to the left ventricle increases ventricular size [25]. This means that the total allowable amount of contrast medium to be administered has to be injected in porti ons as small as consistent with the detectability of the contour of the LV shadow.Calculations on the basis of a simple model, phantom studies and animal experi-

ments, which were in good mutual agreement, lead 'to a number of 8- 10 fractions . A check on the extension of the bolus of contrast medium with respect to the inner waIl of the LV was made in a separate animal experiment. The use of metal markers for this purpose [32] was chosen; with application devices as shown in Figure 5, silvered cylinders of a bismuth aHoy with stainless steel springs were placed in the endocardial waH of the left ventricle during open-heart surgery in the laboratory for experimental cardiology.

AU contrast agents generate myocardial hypoxia as an amount of blood is displaced by a solution which contains no oxygen [26]. A second important cause of inffuence on the heart function is a change in osmolarity [27]. Adverse reactions to contrast agents have been reported in general [28] and with respect to hemodynamic responses [29,30]. In connection with left ventricular volume it is concluded that: "Efforts to measure left ventricular performance using angiocardiography may be focused on the first 2 and possibly 3 cardiac cycles after the beginning of left ventricular opacification. Data derived from cardiac cycles af ter this point in time may no longer reffect the previously existing state of cardiac performance" [31]. Thus, the administration of the fractions of contrast medium has to be time programmed, as the changes last for several minutes .

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R. P. van Wijk van Brievingh et al. , A measurement systemfor left ventricular volume determination 267

A system has been developed, which makes it possible to control the contrast injection with respect to the heart cyc1e in order to prevent mechanically induced extrasystoles, in the proper respiration condition to obtain pictures which may be submitted to videosubtraction, and in which synchronization requirements with regard to the HT ripple and the video system are taken into account (Fig. 6). The timing of the injections and marking of TV frames are based on the ES and EO moments. For the EO moment, an analog R wave detector is used, which contains a triggering circuit preceded by a band-pass filter, an automatic voltage control circuit, and a difl'erentiator. The ES moment is found from the first time derivative of the LVP curve; a more accurate timing is obtained from simultaneously measured left ventricular and aortic pres&ure curves, together with a time window positioned by the R wave, or from the zero crossing of pulsatile flow curve in experiments with an implanted electromagnetic flowmeter around the descending aorta. The time programming unit which governs the measurement procedure has been constructed with a reprogrammable microcomputer* as the central unit. Input and output interfaces accept and provide the necessary signais, the experiment parameters are fed in by a conventional teletape unit, which is also used to give a hard copy protocol of the experiment.

3. Combined injormation registration

Besides X-ray pictures, many other measurements are taken during catheterization. Traditionally, the recording of these is made on several carriers, e.g. paper, oscilloscope screen, analog magnetic tape. In biplane cineangiocardiography, pairing of the corresponding pictures of the two films presents a time consuming problem, as does sorting time markings on other recordings to collect the data in their proper time relationship. Furthermore, all registrations have to bear a patient identification, the date and other alphanumeric data.

With regard to the way of interpretation, this information can be divided into categories:

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1. Morphological information (X-ray pictures), 2. Analog data as a waveform (ECG, pressure

curves), 3. Momentarily values of analog measurement

results (ES and EO pressure values, pulsatile flow, time elapsed af ter last R wave),

4. Oigital values (parameters of the measurement system, frame numbers),

5. Alphanumeric information (patient identification, diagnostic data, date, time, name of physician, etc.).

As the X-ray pictures are already available in video format, devices were developed to encode all other information into the corresponding TV frame. With the 'Vidicor', momentarily retrospective curves of the last seconds of two signals (ECG and LVP) as weIl as a time scale are presented in vertical bands at the left, middle and right of the frame. From these curves interpretation of the contraction phase becomes possible. The 'Anacor' codes the values of up to four analog signals, sampled at the moment of the X-ray flash, into time intervals between pulse pairs on selected lines of the videoframe. Besides, the time elapsed since the last EO pul se (R wave) is also coded with a resolution of 1 msec. The coding has been chosen in such a way that the video computer interface is able to transfer these data directly to the computer. By means of the 'Oigicor' up to 1000 alphanumeric characters are coded as binary words into the first and last 32 lines of each video frame, which are not visible on the monitor. The characters may be displayed on a second TV monitor simultaneously or on the same screen if the other picture is switched ofl'. The Oigicor is compatible with the computer as the ASCII (teletype) code is used . A registration 'form' may be introduced into its digital memory by paper tape, the administrative and other data can be typed in with a teletype. Ouring the measurement, the text may be changed manually or automatically (frame numbering, time and date from a digital c1ock).

Thus, patient data and measurement parameters are recorded on magnetic di sc or tape simultaneousIy with the video signal as the information carrier, shown in Figure 7. High information density and flexibility of use favors this solution to other methods described [19,33]. For X-ray pictures in which

268 R. P. van Wijk van Brievingh et al., A measurement systemfor left ventricular volume determination

Fig. 7. On the monitor screen, both X-ray projections are shown, as combined in one videoframe. The 32 top and bottom lines (not visible) contain digital information in binary code. The first 10 Iines visible contain 'Anacor' information, the momentarily values of up to four analog signals and the time elapsed since the last R wave are recorded as the horizontal distance between white pulses. Each channel is recorded on three TV -Iines. In vertical bands, superimposed on the picture, ECG and L VP curves are recorded, to give information on the contraction phase of the heart at the moment of the X-ray picture.

the limited bandwidth of the TV frames offers no constraint to the spatial resolution, files on videotape, perhaps to be kept with the audiovisual department of the hospital, is an attractive prospective.

4. Videosubtraction

The subtraction technique as first described by Ziedses des Plantes in 1934 [34] is a valuable adjunct in angiography when the contrast-filled cavities are obscured and overlapped by bone structures. Application to angiocardiography has been mostly limited to demonstration of border movements [35],

with the possibility of recording cardiovascular dynamic events on a single film [36]. As soon as magnetic recording of video signals with sufficient stability became possible, it was applied to videosubtraction [37].

In angiocardiography, the cavities to be studied are marked by a quantity of material with high ab-50rption for roentgen radiation. Within Iimits, validity of Lambert-Beer's law may be assumed [19]. After nonlinearity of the imaging system has been corrected with a function generator*, the video

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R. P. van Wijk van Brievingh et al., A measurement systemfor left ventricular volume determination 269

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signal V may be regarded as proportional to the intensity of X-ray radiation I incident at the sens itive screen of the image intensifier. A simplified model is shown in Figure 8a; the difference LI V of the Iogarithmically converted video signalsV2 and Vi is proportional to the contrast at the boundary of the region (38). The registration procedure is shown in Figure 8b; use of a duaI channel video disk is required*. Respiration movement of the

thorax may be eliminated by holding the breath or by making pictures at a known phase when artificial respiration is used**. Other movement artifacts are diminished by proper fixation. The subtracted video signal, obtained according to Figure 8c is

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270 R. P. van Wijk van Brievingh et al., A measurement systemfor l~ft ventricular volume determination

taken as the basis for quantitative evaluation [39,40]. The r('sults indicate, that this analog method of preprocessing facilitates the border detection procedure considerably. Besides a more clear presentation of the LV cavity contour, subtraction of a picture of a contrast -filled aorta from one with medium injected into the ventricle, gives a good indication of the position of the aortie valves [11].

5. Contour definition

A priori knowledge on morphology has to be introduced by a competent cardiologist, either in the on-line procedure [41] or during a preliminary and subsequent adjustment of densitometric equipment [9,13,19]. The local density in X-ray pictures is only partially relevant to the determination of the ventricular outline for the following reasons: the mitral valve is not depicted when blood flows into the LV from the left atrium; during the ejection phase the aortic valve cannot be distinguished; in some pictures other cavities containing radiopaque medium overlap the LV; at the location of the papillary muscles the amount of contrast medium is so small that the 'inner' part of the contour of these muscles will be taken for the LV cavity outline. The light-pen system developed has the advantage over commercially available apparatus [12], that a lightemitting arrow is presented on the monitor screen, so that the locally different light intensity of the image gives no detection problems. The shape of the arrowas weIl as the configuration of the instrument controls around the monitor have been chosen according to the results of an ergonomic investigation. The video frames to be elaborated are presented as still pictures on the monitor; interpretation of the image can be facilitated by viewing a short sequence in move ment [42]. The contours drawn with the light-pen are retained in a digital memory with 256 X 256 elements. The memory consists of a metal oxide semiconductor static shift register with buffer and con trol circuits. The output is in the form of a video signa I in which the points detected by the light-pen are represented by puls es at the white level and of a sequence of 8-bit words (ASCII code), to be read by the computer device handIer. The possibility of keeping a set of tempI at es of manually detected contour parts, e.g. valve and papillary muscle locations, in this memo-

ry, and densitometric determination of the rest of the LV contour is under investigation as it may speed up the procedure [43]. A difficulty is the displacement of the shadow projections relative to the TV frame caused by respiration and by movement of the heart during contraction.

6. Data conversion

By means of a line selector which controls a video switch, the part of the original picture containing 'Anacor' information, and the video signal which represents the con tours drawn, are fed together to a video computer interface (VCI). A parity check is performed and guard procedure~ are carried out during the read-in. 'Digicor' information may independently be sent to the computer, as this device is connected as a separate peripheral and has a handIer of its own. By software, the data pertinent to the caIculation (e.g. parameter values of the measurement system) is selected. Data corresponding to one video frame is recorded on digital MAG tape memory as one block of information. The software package is of modular structure, a diagram is shown in Figure 9. Output is on a line printer, and contains all relevant data in a format suitable for filing with the patien status. Furthermore, a pseudo- three-dimensional picture of ventricular shape, based on the con tours drawn, is given by a scan converterjTV monitor system. By recording successive individual pictures on the videodisk, and continuous playback a 'real time' presentation ofthe ventricular cavity is possible under diffferent angles of view.

7. Geometrical model

To determine left ventricular volume and shape from angiograms, a geometrical model has to be assumed. Two types of models are to be found in literature. The first category - analytical models -refers to stereometrical bodies, the volume of which can be caIculated in analytical form if sufficient parameters are known. Proposed \vere a thickwalled cylinder with a co ne on top [44], an ellipsoid of revolution [45], a combination of two halve ellipsoids with different excentricity [46], and a general ellipsoid [47]. The axes of the ellipsoid may

R. P. van Wijk /Jan Brievingh et al., A measlIrement system for left ventriclIlar volume determil1ation 271

paper

exit

Fig. 9. The software system used is of a modular structure. Except for the 'main' module, which stays resident in core, the other modules overlay one another in the sequence in which they are used, thus saving memory space. Intermediary results or preprocessed data are recorded on DEC tape to be used in the next stages of calculation.

272 R. P. van Wijk van Brievingh et af., A measurement systemfor left ventriclifar voülIlle determination

be computed according to several principles (48]. The second category - digital models - assumes the LV to be composed of elliptical cylinders or truncated cones with elliptical parallel surfaces, the long and short axes of which are measured from diameters in biplane angiographs (42,49]. The total volume is calculated according to Simpson's rule. A comparative study (42] showed it to be more accurate than the ellipsoid modeis.

Video techniques offer the feature to digitize the LV contours by the TV scanning Iines. If Chapman's model (42] is used with slices corresponding to these scanning lines, a general ellipsoidal shape is implicitly assumed, as the direction of the main axis of the ventricle respective to the coordinate system, defined by the line from the apex to the anterior Ieaflet of the mitral valve close to the line of cleavage with the aortic valve (50], is not taken into account. It is shown (51] that this causes an error in volume of 10% if an ellipsoid of revolution is assumed, the long axis of which diverges an angle of 24 1/20 from the vertical coordinate axis by false indication of

its end points; regardless of the ellipsoid's excentricity.

So, a first requirement of the calculation procedure is that it should refer to the direction of the axis of the ventricle measured (46,51]. Furthermore it has been shown from measurements on casts (9], that rotation around this axis causes a variation in the volume computed of about 10%. A more refined model should therefore also account for the irregular shape of the left ventricular cavity. The model proposed is constructed as follows (14].

From 14 animal hearts, casts were made at the laboratory for experimental cardiology, using a two component epoxy resin, mixed with red-lead. The aortic part was sawed off at the plane of the valve, just below the coronary arteries. The end points of the 'axis' and the start point and direction for the rotational angle ljJ were marked. The casts were placed on an optical bench, on which the X-ray source was represented by a light source and the image intensifier by a transparent screen. Contours of the shadow were drawn on paper for values of ljJ

Fig. 10. (asts of ventricles are embedded in c1ear plastic, and divided mechanically into 20 equally thick slices, perpendicular to the ventricular axis. The are as LlA are determincd by videoplanimetry.

274 R. P. van Wijk van Brievingh et al., A meaSllrel11ent systel11 fol' left ventriclIlal' volume deterl11inatioll

ai (60°)

Î 1.0 , ... " (Aorta) (Apex)

Aai= __ l_

Xi . Yi

Chapman n ~ ______________ ~ __________________ ~~ ____ ~ __ ___ 4

Ol 5 10

Aorta

i=O (=20 (Aorta) (Apex)

0.5

Ol 5 10 15

Aorta

20 t i--

Apex

Chapman

20 I i--

Apex

Fig. 12a, b. From 14 casts, sliced as shown in F ig. 10, an ' index ofirregularity' was computed as the ratio ofthe planimetered area and the product of perpendicular shadow diameters. The assumption that the slices are ellipsoidaJ cylinders as made by Chapman [42 J gives a constant value for 0:1 of n/4 as depicted by the horizontal line.

made at known phases of cardiac contraction is being collected, so as to be able to give values of a at various shapes ofthe ventricle. To use this model, the human observer who draws the ventricular con tours with the light-pen should provide extra information on: .

1. The direction of the axis in both projections as shown by arrows in Figure 11 ;

2. The contraction phase ofthe heart at the frame under hand; this can be done on the basis of

the last few seconds of the EeG and L VP curves as presented by the 'Vidicor';

3. The value of ljJ pertinent, as judged from the position of the heart relative to the projection system. The line for ljJ = 0 has been defined in the model at the middle of the intraventricular septurn.

4. Pathology of the heart, e.g. hypertrophy, for which a different set of a values should be determined.

R. P. van Wijk van Brievingh et al., A measurement systemfor left ventricular volume determination 275

8. Calculatiol1

First, the coordinates of the origin (intersection of the beam axes) are determined relative to the TV frame coordinate systems; the center points of a set of circular slit phantoms as described in section 1 are used for this purpose. Then, from the projections of the ventricle axis as indicated with the light-pen, the spatial orientation of the ventricle is calculated. The intersection points of the ventricle axis projections with the corresponding contours are determined, from these the end points of the ventricle axis in space are found. On this axis, a set of equidistant points is chosen, the number of which is consistent with the number of points determining the contours. The projections of these equidistant points are determined, which in general are not equidistant because of the central projection. In these points on the axis projections, lines are calculated as projections of lines which are perpendicular to the planes through thc' X-ray focus and the ventricle axis in the equidistant points. The intersection points of the contours with these lines are used to determine the diameters X'j and Y/j which are used for calculation of slice volumes. Thereby, for each slice, the magnification factor is calculated individuaIly, and a close agreement with the geometrical model is obtained. Addition of the slice volumes is performed according to Simpson's rule.

9. Experiments

In vivo measurements were carried out to compare the model proposed with the Chapman model as weIl as the ellipsoid model using the area- length method. In the latter, the long axis L of the eIlipsoid is determined from the end points of the axes in both shadow projections. The short axes Dl and Dz are computed with: Dl z = 4· Al 2/n . Ll 2 in which Al,z = area of the ~entricular ~hadow projections, and Ll 2 = projections of the long axis L. So Dl z are the ' short axes of the equivalent eIlipses in the projections 1 and 2 respectively.

To get an impression of the absolute accuracy of the measurement, beside SV = EDV - ESV also SV = fbeatQdt was determined beat-by-beat with an implanted electromagnetic flow probe*) around the aorta. The flow signal was recorded on one of the 'Anacor' channels, as was the output of the inte-

grator. As this procedure cannot be used with patients, also thermodilution curves were made at constant heart rate, so SV = CO/HR as a mean value, measured before and after the contrast injection.

In two beagle dogs, weights 13 and 17 kg, recordings were made. After premedication with 2 1/2 mg symoron, and 1/2 mg atropine, anesthesy was performed with 10 mg/kg pentothal and 1: 2 OrN02 with 1/2- 1 1/2% fluothane. Isopaque Coronar® was injected in fractions of 5 mI, triggered by the R-wave with an electrical injector**. The heart rate was ~timulated via electrodes, sewed on the left atrium.

Results

For y = fbeat Qdt and x = EDV-ESV the linear regression formula y = 0.903x+2.87 mI was found with a correlation coefficient of 0.903. If yl = LVV as computed with the area-length method, and Xl = LVV according to the model proposed, we obtained l = 1.33xl + 0.69 mI with a correlation coefficient of 0.992.

These preliminary results indicate an accuracy which makes the method useful for clinical application.

Acknowledgements

The system described has been developed as a joint project between the Medical Engineering Group, Department of Electrical Engineering, Delft University of Technology and the Clinic for Cardiovascular Diseases of the Utrecht University Hospita!. Thanks are due to Professor L.H. van der Tweel, Department of Medical Physics, Municipal University of Amsterdam, for his guidance of the first author.

Several instrurnents were designed as M. Sc. E. E. theses. Thanks are due to the supervisors, as weIl as to J. M. Brans of the Delft Medical Engineering

* Nycotron ** Cordis, type Ir.

276 R. P. van Wijk van Brievingh et al., A measllrement system for leji ventricular voltllne determinatioll

Centre for his assistance in the literature research. In the Computer Group Cardiology, J. Heethaar programmed the geometrical modeIs, and G. Rol wrote the pseudo-three-dimensional display software.

References

[1) Soloff, L. A. (1966): On measuring left ventricular volume. Amer. J. Cardiol., i8j1, 2.

[2) Meyler, F. L. (1969) : The role of the heart in the regulation of the circulation. Acta physiol. pharmacol. neert., 15, 282.

[3) Braunwald, E ., Ross, J . and Sonnenblick, E . H. (1967) : Mechanisms of Contraction of the Normal and Failing Henrt. ChurchiIl, London.

[4) Fischer, V. J. , Lee, R. J. and Kavaler, F. (1967): The heterogeneous contractile performance of the left ventricle. In : Factors infiuencing Myocardial Contractility. Editors : R. D . Tanz, F . Kavaler and J. Roberts, Academic Press, New York.

[5) Meyler, F. L., Strackee, J., Van Capelle, F . J. L. and Du Perron, J . C. (1968): Computer analysis of the RRinterval-contractility relationship during random stimulation of the isolated heart . Circulat. Res. , 22j5, 695

[6) Sandler, H. (1970): Dimensional analysis of the heart. Amer. J. med. Sci., 260j1, 55

[7) Bom, N . (1972) : New Concepts in Echocardiography. MD thesis, Rotterdam.

[8) Dodge, H . T., Sandler, H., Ballew, D. W . and Lord, J. D. (1960) : The use of biplane angiocardiography for the measurement of left ventricular volume in man. Amer. Heart J., 60j5, 672

[9) Wood, E. H ., Ritman, E . L., Sturm, R. E., Johnson, S., Spivak, P., Gilbert, B. K. and Smith H . C. (1972) : The problem of determination ofthe roentgen density, dimensions and shape of homogeneous objeets from biplane angiographic data with particular reference to angiocardiography. In: Proceedings, San Diego Biomedical Symposium, vol. 11.

[10) Chang, S. K. and Chow, C. K. (1973): The reeonstruction of three-dimensional objects from two orthogonal projections and its application to cardiac cineangiography. IEEE Trans. Comput., C-22ji, 18 .

[11) Van Wijk van Brievingh, R . P., Richtering Blenken, A., Sneek, J . H. J. and Zimmerman, A. N. E. (1973): Subtraetion as a tooI in quantitative videoangiocardiography Digest 10th lCMBE, Dresden, 6/5, 96.

[12) Alderman, E. L., Sandler, H., Brooker, J . Z., Sanders, W. J., Simpson, C. and Harrison, D . C. (1973): Lightpen computer processing of video image for the determination ofleft ventricularvolume. Circulation, 47/2, 309.

[13) Marcus, M . L., Schuette, W. H., Whitehouse, W. C., Bailey, J. J. and Glancy, D. L . (1972): An automated method for the measurement of ventricular volume. Circulation, 45j i, 65 .

[14) Van Wijk van Brievingh, R. P. (1973) : Quantitative videoangiocardiography - A method for measuring the

heart's pumping function . Delft Progress Report, Series B i /i,lI.

[15) Kundel, H. L., Revesz, G ., Ziskin, M. C. and Shen, F. J. (1972) : The image and its influence on quantitative radiological data (A.U.R. Symposium) . In vest. Radiol. 7/4, 187.

[16) Rockoff, S. D. (1972) : Techniques of data extraction from radiological images - an overview. invest. Radioi., 7/4,206.

[17) Stieve, F . E . (ed.) (1966) : Bildguete iJl der Radiologie. Fischer, Stuttgart.

[18) Moseley, R. P. and Rust, J. H. (1969): l"elevision iJl Diagnostic Radiology. Aesculapius, Birmingham, Ala.

[19) Heintzen, P. H . (ed.) (1971) : RoentgeJl-, Cine- and Videodensitometry. Georg Thieme Verlag, Stuttgart.

[20) Herstel, W. (1968): Vision and Television in X-ray Fluoroscopy. Ph. D. thesis, Leiden.

[21) Ritman, E. L., Johnson, A. J., Sturm, R . E. and Wood, E. H . (1973): The television camera in dynamic videoangiography. Radiology, 107j2, 417.

[22) Franken, A . A. J. and Scheren, W. J. L. (1972): The influence of the camera tube on the temporal modulation transfer function in diagnostic X-ray television. Medica Mundi, 17j3, 12I.

[23) Freeman, E., Ziskin, M. C., Bove, A . A ., Gimenez, J . L. and Lynch, P. R. (1970) : Cineradiographic frame ra te selection for left ventricular volumetry. Radiology , 96/3, 587.

[24) Stewart, G. H. (1968) : High-rate Physiological Dynamics in Three-dimensional Space, their Acquisitiofl via Cineradiography. Ph. D. thesis, Temple University, Philadelphia, Pa.

[25) Hallerman, F. J., Rastelli, G. C. and Swan, H. J . C. (1964) : Effeets of rapid injection of heparinized blood into right and left ventricles in dogs. Radiology, 83/4, 647

[26] Dean, R. R. (1966): The Physiologic and Toxic Consequence of the Intracoronary Administration of Certain Radiopaque Contrast Agents. Ph. D. thesis, University of Michigan, Ann Arbor, Mich.

[27] Kloster, F. E., Bristow, J., Porter, G . A ., Induins, M. P. and Griswold, H. E. (1967): Comparative haemodynamie effects of equiosmolar injections of angiographic contrast materiaIs. In vest. Radioi., 2/5, 353.

[28] Ansel!, G. (1970) : Adverse reaetions to contrast agents - scope of problem. In vest. Radioi., 5/6, 374.

[29] Lindgren, P. (1970): Hemodynamic responses to contrast media. 1nvest. Radiol., 5/6, 424.

[30] Baron, M. G . (1973) : Angioeardiographic determination of ejection fraction in coronary artery disease. Amer. J. Cardiol., 31, 803.

[31] Carleton, R .A. (1971) : Change in left ventricular volume during angiocardiography. Amer. J. Cardiol., 27/5, 460.

[32] Carlsson, E. and Milne, M. C. (1967): Permanent implantation of endocardial tantalum screws: a new technique for funetional studies in the experimental anima!. J. Canad. Assoc. Radioi., 19, 304.

[33] Zimmerman, R ., Ameling, W., Bussmann, W. D . and Effert, S. (1972): Visual display unit for biplane X-ray videometry with simultaneous data presentation. Med. biol. Engng, JO, 784.

--------------... ......-

R. P. van Wijk van Brievingh et al., A lIleasurelllent systemfor left ventricular voftlll1e determination 277

[34) Ziedses des Plantes, B. G. (1934): Planigraphie en Subtractie. Thesis, Utrecht.

[35) Ziedses des Plant es, B. G. (1961): Subtraktion. Georg Thieme Verlag, Stuttgart.

[36) Chen, J. T. T ., McIntosh, H. D., Capp, M. P., Morris, J. J ., Canent, R. V. and Lester, R . G. (1969) : Intercalative angiocardiography: a method for recording cardiovascular dynamics on a single film. Radiology, 93/3, 499.

[37) Oosterkamp, W . J. and Schut, Th. G. (1961): Magnetische Speicherung von Roentgenbilder. Elektromedizin, 6,147.

[38) Jaeger, H. (1969) : Physikalisch-technischen Grundlagen zur roentgen-photographischen Abbildung von Weichteilen. Röntgen-BI., 22/ 7, 323.

[39) Trenholm, B. G., Winter, D. A., Mymin, D. and Lansdown, E. L. (1972): Computer determination of left ventricular volume using videodensitometry. Med. biol. Engng, 10, 163.

[40) Chow, C. K. and Kaneko, T. (1971): Computer calculation of left ventricular volumes from a cine-angiogram. In: QlIantitative Imagery in the Bio-medical Sciences, Seminar-in-Depth, Society of Photo-optical Instruments Engineers.

[41) Heintzen, P. H ., Malerczyk, V., Pilarczyk, J. and Scheel, K . W . (1971): On-line processing ofthe video-image for left ventricular volume determination. Comput. biomed. Res., 4/5, 474.

[42) Chapman, C. B., Baker, 0., Reynolds, J. and Bonte, F. J. (1958): Use of biplane cinefiuorography for measurement of ventricular volume. Circulation, 18/6, I 105.

[43) Zimmerman, R., Bussmann, W .-D., Schmidt, M .,

Ameling, W. and Effert, S. (1973): Röntgenvideometrische Verfahren zur Ventrikelvolumenbestimmung unter Verwendung ei nes Videolichtgriffels und digitaler Kontourenspeicher. Biomed. Techn., 18/4, 124.

[44) Rushmer, R . F. (1951): The mechanics of cardiac contraction. Circulation, 4/2, 219.

[45) Gribbe, P., Hirvonen, L. , Lind, J . and Wegelius, C. (1959): Cineangiographic recordings of the cyclic changes in volume of the left ventricle. Cardiologia, (BaseI), 34, 348.

[46) Arvidsson, H. (1961): Angiocardiographic determination of left ventricular volume. Acta radio I. (Stockh.), 56/5, 321.

[47) Dodge, H. T., Hay, R. E. and Sandler, H. (1962): An angiographic method for directly determining left ventricular stroke volume in man. Circlilat. Res., 11/4, 739.

[48) Sanmarco, M. E. and BartIe, S. H. (1966) : Measurement of left ventricular volume in the canine heart by biplane angiocardiography: accuracy of the method using different model analogics. Circulat. Res., 19/1, 11.

[49) Brun, Ph., Cannet, C. and Herreman, F. (1972) : La mes ure du volume endocavitaire ventriculaire gauche par méthode angiographique. Acta cardiol. (Brux.),27, 272.

[50) Bentivoglio. L. G ., Griffith, L. D., Cuesta, A. J. and Geczy, M . (1972): Radiographic evaluation of formulas for left ventricular volume using canine casts. J. appl. Physiol., 33/3, 365.

(51) Beckenbach, E. S. (1960) : Some Aspects of the Computerization of High Speed Cineangiographic Left Ventricular Volume Determination. Ph. D. Thesis, University of California, Los Angeles, Calif.

a measurement system for leftventricular volume

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