PS-2020a / part17

EEE Intravascular OCT Image (Informative)​

EEE.1 Purpose of This Annex​

The purpose of this annex is to explain key IVOCT FOR PROCESSING parameters, describe the relationship between IVOCT FOR​ PROCESSING and FOR PRESENTATION images. It also explains Intravascular Longitudinal Reconstruction.​

EEE.2 IVOCT For Processing Parameters​

EEE.2.1 Z Offset Correction​

When an OCT image is acquired, the path length difference between the reference and sample arms may vary, resulting in a shift​ along the axial direction of the image, known as the Z Offset. With FOR PROCESSING images, in order to convert the image in​ Cartesian coordinates and make measurements, this Z Offset should be corrected, typically on a per-frame or per-image basis. Z​ Offset is corrected by shifting Polar data rows (A-lines) + OCT Z Offset Correction (0052,0030) pixels along the axial dimension of​ the image.​

Z Offset correction may be either a positive or negative value. Positive values mean that the A-lines are shifted further away from the​ catheter optics. Negative values mean that the A-lines are shifted closer to the catheter optics. Figure EEE.2-1 illustrates a negative​ Z Offset Correction.​

Figure EEE.2-1. Z Offset Correction​

EEE.2.2 Refractive Index Correction​

The axial distances in an OCT image are dependent on the refractive index of the material that IVOCT light passes through. As a​ result, in order to accurately make measurements in images derived from FOR PROCESSING data, the axial dimension of the pixels​ shouldbegloballycorrectedbydividingtheA-linePixelSpacing(0052,0014)value(inair)bytheEffectiveRefractiveIndex(0052,0004)​

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and setting the Refractive Index Applied (0052,003A) to YES. Although not recommended, if A-line Pixel Spacing (0052,0014) is re-​ ported in air (i.e., not corrected by dividing by Effective Refractive Index) then the Refractive Index Applied value shall be set to NO.​

EEE.2.3 Polar-Cartesian Conversion​

FORPROCESSINGPolardataisspecifiedsuchthateachcolumnrepresentsasubsequentaxial(z)locationandeachrowanangular​

(q) coordinate. Following Z Offset and Refractive Index Correction, Polar data can be converted to Cartesian data by first orienting​ the seam line position so that it is at the correct row location. This can be accomplished by shifting the rows Seam Line Index​ (0052,0036) pixels so that its Seam Line Location (0052,0033) is located at row "A-lines Per Frame * Seam Line Location / 360".​ Once the seam line is positioned correctly, the Cartesian data can be obtained by remapping the Polar (z, q) data into Cartesian (x,​ y) space, where the leftmost column of the Polar image corresponds to the center of the Cartesian image. Figure EEE.2-2 illustrates​ the Polar to Cartesian conversion. The scan-converted frames are constructed using the Catheter Direction of Rotation (0052,0031)​ Attribute to determine the order in which the A-lines are acquired. Scan-converted frames are constructed using A-lines that contain​ actual data (I.e., not padded A-lines). Padded A-lines are added at the end of the frame and are contiguous. Figure EEE.2-2 is an​ example of Polar to Cartesian conversion.​

Polar to

Cartesian

Figure EEE.2-2. Polar to Cartesian Conversion​

EEE.3 Intravascular Longitudinal Image​

An Intravascular Longitudinal Image (L-Mode) is a constrained three-dimensional reconstruction of an IVUS or IVOCT Multi-frame​ Image.TheLongitudinalImagecanbereconstructedfromeitherFORPROCESSINGorFORPRESENTATIONImages.FigureEEE.3-​ 1 is an example of an IVUS cross-sectional image (on the left) with a reconstructed longitudinal view (on the right).​

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Current Frame Marker

Longitudinal Slice Indicator

Figure EEE.3-1. IVUS Image with Vertical Longitudinal View​

The Longitudinal reconstruction is comprised of a series of perpendicular cut planes, typically consisting of up to 360 slices spaced​ in degree increments. The cut planes are perpendicular to the cross-sectional plane, and rotate around the catheter axis (I.e., center​ of the catheter) to provide a full 360 degrees of rotation. A longitudinal slice indicator is used to select the cut plane to display, and​ is normally displayed in the associated cross-sectional image (e.g., blue arrow cursor in Figure EEE.3-1). A current frame marker​ (e.g., yellow cursor located in the longitudinal view) is used to indicate the position of the corresponding cross-sectional image, within​ the longitudinal slice.​

When pullback rate information is provided, distance measurements are possible along the catheter axis. The Intravascular Longitud-​ inal Distance (0052,0028) or IVUS Pullback Rate (0018,3101) Attributes are used along with the Frame Acquisition DateTime​ (0018,9074)Attributetofacilitatemeasurementcalculations.Thisallowsforlesion,calcium,stentandstentgaplengthmeasurements.​ Figure EEE.3-2 is an example of an IVOCT cross-sectional image (on the top), with a horizontal longitudinal view on the bottom. The​ following example also illustrates how the tint specified by the Palette Color LUT is applied to the OCT image.​

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Longitudinal Slice Indicator

Current Frame Marker

Figure EEE.3-2. IVOCT Image with Horizontal Longitudinal View​

Figure EEE.3-3. Longitudinal Reconstruction​

Figure EEE.3-3 illustrates how the 2D cross-sectional frames are stacked along the catheter longitudinal axis. True geometric repres-​ entation of the vessel morphology cannot be rendered, since only the Z position information is known. Position (X and Y) and rotation​ (X, Y and Z) information of the acquired cross-sectional frames is unknown.​

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FFF Enhanced XA/XRF Encoding Examples​ (Informative)​

FFF.1 General Concepts of X-Ray Angiography​

This chapter describes the general concepts of the X-Ray Angiography equipment and the way these concepts can be encoded in​ SOP Instances of the Enhanced XA SOP Class. It covers the time relationships during the image acquisition, the X-Ray generation​ parameters, the conic projection geometry in X-Ray Angiography, the pixel size calibration as well as the display pipeline.​

The following general concepts provide better understanding of the examples for the different application cases in the rest of this​ Annex.​

FFF.1.1 Time Relationships​

FFF.1.1.1 Time Relationships of A Multi-frame Image​

The following figure shows the time-related Attributes of the acquisition of X-Ray Multi-frame Images. The image and frame time At-​ tributes are defined as absolute times, the duration of the entire image acquisition can be then calculated.​

time

Content Date (0008,0023)

Content Time (0008,0033)

Acquisition DateTime (0008,002A)

Frame 1 Acquisition DateTime (0018,9074)

Frame 1 Reference DateTime (0018,9151)

Frame 1 Acquisition Duration (0018,9220)

Frame i ...

Frame n Acquisition DateTime (0018,9074)

Frame n Reference DateTime (0018,9151)

Frame n Acquisition Duration (0018,9220)

Image Acquisition Duration (calculated)

Acquisition Time Synchronized (0018,1800) = YES * * If Acquisition is synchronized with external time reference

Figure FFF.1.1-1. Time Relationships of a Multi-frame Image​

FFF.1.1.2 Time Relationships of One Frame​

The following figure shows the time-related Attributes of the acquisition of an individual frame "i" and the relationship with the X-Ray​ detector reading time and simultaneous ECG waveform acquisition.​

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R

time

Last R peak prior to X-Ray Frame “i”

Pre-Frame X-Ray

Frame X-Ray

Nominal Cardiac Trigger Delay Time (0020,9153)

Frame Acquisition Number (0020,9156) = “i”

Frame i Acquisition DateTime (0018,9074)

Frame i Reference DateTime (0018,9151)

Frame i Acquisition Duration (0018,9220)

Detector Activation Offset from Exposure (0018,7016)

Detector Activation Time (0018,7014)

Figure FFF.1.1-2. Time Relationships of one Frame​

Note​

1.​Positioner angle values, table position values etc… are measured at the Frame Reference DateTime.​

2.​Dose of the frame is the cumulative dose: PRE-FRAME + FRAME.​

FFF.1.2 Acquisition Geometry​

This chapter illustrates the relationships between the geometrical models of the patient, the table, the positioner, the detector and the​ pixel data.​

ThefollowingfigureshowsthedifferentstepsintheX-Rayacquisitionthatinfluencesthegeometricalrelationshipbetweenthepatient​ and the pixel data.​

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time

PATIENT positioning

TABLE movement

POSITIONER movement

X-Ray Acquisition

FOV image creation

FOV rotation

FOV Horizontal flip

Pixel Data Creation

Figure FFF.1.2-1. Acquisition Steps Influencing the Geometrical Relationship Between the Patient and​ the Pixel Data​

FFF.1.2.1 Patient Description​

Refer to Annex A for the definition of the patient orientation.​

A point of the patient is represented as: P = (Pleft, Pposterior, Phead).​

Pposterior

Posterior

Figure FFF.1.2-2. Point P Defined in the Patient Orientation​

FFF.1.2.2 Patient Position​

FFF.1.2.2.1 Table Description​

The table coordinates are defined in Section C.8.7.4.1.4 “Table Motion With Patient in Relation to Imaging Chain” in PS3.3.​

The table coordinate system is represented as: (Ot, Xt, Yt, Zt) where the origin Ot is located on the tabletop and is arbitrarily defined​ for each system.​

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+ Zt

+ Xt

Table Head

Table Left

Table Foot

+ Yt

Figure FFF.1.2-3. Table Coordinate System​

FFF.1.2.2.2 Options For Patient Position On The X-Ray Table​

The position of the patient in the X-Ray table is described in Section C.7.3.1.1.2 “Patient Position” in PS3.3.​

The table below shows the direction cosines for each of the three patient directions (Left, Posterior, Head) related to the Table co-​ ordinate system (Xt, Yt, Zt), for each patient position on the X-Ray table:​

FFF.1.2.3 Table Movement​

FFF.1.2.3.1 Isocenter Coordinate System​

The Isocenter coordinate system is defined in Section C.8.19.6.13.1.1 “Isocenter Coordinate System” in PS3.3.​

FFF.1.2.3.2 Table Movement in The Isocenter Coordinate System​

The table coordinate system is defined in Section C.8.19.6.13.1.3 “Table Coordinate System” in PS3.3 where the table translation is​ represented as (TX,TY,TZ). The table rotation is represented as (At1, At2, At3).​

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Figure FFF.1.2-4. At1: Table Horizontal Rotation Angle​

Figure FFF.1.2-5. At2: Table Head Tilt Angle​

Yt

Y

Axis of Rotation

Figure FFF.1.2-6. At3: Table Cradle Tilt Angle​

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A point (P Xt , P Yt , P Zt ) in the table coordinate system (see Figure FFF.1.2-7) can be expressed as a point (P X , P Y , P Z ) in the​ Isocenter coordinate system by applying the following transformation:​

(PX, PY, PZ)T= (R3 .R2 .R1)T .(PXt, PYt, PZt)T+ (TX, TY, TZ)T​

And inversely, a point (P X , P Y , P Z ) in the Isocenter coordinate system can be expressed as a point (P Xt , P Yt , P Zt ) in the table​ coordinate system by applying the following transformation:​

(PXt, PYt, PZt)T= (R3 .R2 .R1).((PX, PY, PZ)T- (TX, TY, TZ)T)​

Where R1 , R2 and R3 are defined in Figure FFF.1.2-7.​

Figure FFF.1.2-7. Point P in the Table and Isocenter Coordinate Systems​

FFF.1.2.4 Positioner Movement​

FFF.1.2.4.1 Positioner Movement in The Isocenter Coordinate System​

The positioner coordinate system is defined in Section C.8.19.6.13.1.2 “Positioner Coordinate System” in PS3.3 where the positioner​ angles are represented as (Ap1, Ap2, Ap3).​

A point (P Xp , P Yp , P Zp ) in the positioner coordinate system can be expressed as a point (P X , P Y , P Z ) in the Isocenter coordinate​ system by applying the following transformation:​

(PX, PY, PZ)T= (R2 .R1)T .(R3 T .(PXp, PYp, PZp)T)​

And inversely, a point (P X , P Y , P Z ) in the Isocenter coordinate system can be expressed as a point (P Xp , P Yp , P Zp ) in the po-​ sitioner coordinate system by applying the following transformation:​

(PXp, PYp, PZp)T= R3 .((R2 .R1).(PX, PY, PZ)T)​

Where R1 ,R2 andR3 are defined as follows:​

FFF.1.2.4.2 X-Ray Incidence and Image Coordinate System​

The following concepts illustrate the model of X-Ray cone-beam projection:​

The X-Ray incidence represents the vector going from the X-Ray source to the Isocenter.​

The receptor plane represents the plane perpendicular to the X-Ray Incidence, at distance SID from the X-Ray source. Applies for​ both image intensifier and digital detector. In case of digital detector it is equivalent to the detector plane.​

The image coordinate system is represented by (o, u, v), where "o" is the projection of the Isocenter on the receptor plane.​

The source to isocenter distance is called ISO. The source image receptor distance is called SID.​

The projection of a point (P Xp , P Yp , P Zp ) in the positioner coordinate system is represented as a point (P u , P v ) in the image​ coordinate system.​

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