Annex U Ophthalmology Use Cases (Informative)

U.1 Ophthalmic Photography Use Cases

The following use cases are examples of how the DICOM Ophthalmology Photography objects may be used. These use cases utilize the term “picture” or “pictures” to avoid using the DICOM terminology of frame, instance or image. In the use cases, the series means DICOM Series.

U.1.1 Routine N-spot exam

An N-spot retinal photography exam means that “N” predefined locations on the retina are examined.

A routine N-spot retinal photography exam is ordered for both eyes. There is nothing unusual observed during the exam, so N pictures are taken of each retina. This healthcare facility also specifies that in an N-spot exam a routine external picture is captured of both eyes, that the current intraocular pressure (IOP) is measured, and that the current refractive state is measured.

The resulting study contains:

  1. 2N pictures of the retina and one external picture. Each retinal picture is labeled in the acquisition information to indicate its position in the local N-spot definition. The series is not labeled, each picture is labeled OS or OD as appropriate.

Note: DICOM uses L, R, and B in the Image Laterality Attribute (0020,0062). The actual encodings will be L, R, or B. Ophthalmic equipment can convert this to OS, OD, and OU before display.

  1. In the acquisition information of every picture, the IOP and refractive state information is replicated.

  2. Since there are no stereo pictures taken, there is no Stereometric Relationship IOD instance created.

The pictures may or may not be in the same Series.

U.1.2 Routine N-spot exam with exceptions

A routine N-spot retinal photography exam is ordered for both eyes. During the exam a lesion is observed in the right eye. The lesion spans several spots, so an additional wide angle view is taken that captures the complete lesion. Additional narrow angle views of the lesion are captured in stereo. After completing the N-spot exam, several slit lamp pictures are taken to further detail the lesion outline.

The resulting study contains:

  1. 2N pictures of the retina and one external picture, one additional wide angle picture of the abnormal retina, 2M additional pictures for the stereo detail of the abnormal retina, and several slit lamp pictures of the abnormal eye. The different lenses and lighting parameters are documented in the acquisition information for each picture.

  2. One instance of a Stereometric Relationship IOD, indicating which of the stereo detail pictures above should be used as stereo pairs.

The pictures may or may not be in the same Series.

U.1.3 Routine Flourescein Exam

A routine fluorescein exam is ordered for one eye. The procedure includes:

  1. Routine stereo N-spot pictures of both eyes, routine external picture, and current IOP.

  2. Reference stereo picture of each eye using filtered lighting

  3. Fluorescein injection

  4. Capture of 20 stereo pairs with about 0.7 seconds between pictures in a pair and 3-5 seconds between pairs.

  5. Stereo pair capture of each eye at increasing intervals for the next 10 minutes, taking a total of 8 pairs for each eye.

The result is a study with:

  1. The usual 2N+1 pictures from the N-spot exam

  2. Four pictures taken with filtered lighting (documented in acquisition information) that constitute a stereo pair for each eye.

  3. 40 pictures (20 pairs) for one eye of near term fluorescein. These include the acquisition information, lighting context, and time stamp.

  4. 32 pictures (8 pairs for each eye) of long term fluorescein. These include acquisition information, lighting context, and time stamp.

  5. One Stereometric Relationship IOD, indicating which of the above OP instances should be used as stereo pairs.

The pictures of a) through d) may or may not be in the same series.

U.1.4 External examination

The patient presents with a generic eye complaint. Visual examination reveals a possible abrasion. The general appearance of the eyes is documented with a wide angle shot, followed by several detailed pictures of the ocular surface. A topical stain is applied to reveal details of the surface lesion, followed by several additional pictures. Due to the nature of the examination, no basic ophthalmic measurements were taken.

The result is a study with one or more series that contains:

  1. One overall external picture of both eyes

  2. Several close-up pictures of the injured eye

  3. Several close-up pictures of the injured eye after topical stain. These pictures have the additional stain information conveyed in the acquisition information for these pictures.

U.1.5 External examination with intention

The patient is suspected of a nervous system injury. A series of external pictures are taken with the patient given instructions to follow a light with his eyes. For each picture the location of the light is indicated by the patient intent information, (e.g. above, below, patient left, patient right).

The result is a study with one or more series that contains:

  1. Individual pictures with each picture using the patient intent field to indicate the intended direction.

U.1.6 External examination with drug application

Patient is suspected of myaesthenia gravis. Both eyes are imaged in normal situation. Then after Tensilon® (edrophonium chloride) injection a series of pictures is taken. The time, amount, and method of Tensilon® (edrophonium chloride) administration is captured in the acquisition information. The time stamps of the pictures are used in conjunction with the behavior of the eyelids to assess the state of the disease.

Note: Tensilon® is a registered trademark of Roche Laboratories

The result is a study with one or more series that contains:

  1. Multiple reference pictures prior to test

  2. Pictures with acquisition information to document drug administration time.

U.1.7 Routine stereo camera examination

A stereo optic disk examination is ordered for a patient with glaucoma. For this examination, the IOP does not need to be measured. The procedure includes:

  1. Mydriasis using agent at time t

  2. N stereo pictures (camera pictures right and left stereo picture simultaneously) of the optic disk region at the time t+s

The result is a study with:

  1. N right and N left stereo pictures. These include acquisition information, lighting context, agent and time stamps.

  2. One Stereometric Relationship SOP Instance, indicating that the above OP images should be used as stereo pairs.

U.2 Typical Sequence of Events

The following shows the proposed sequence of events using individual images that are captured for later stereo viewing, with the stereo viewing relationships captured in the stereometric relationship instance.

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The instances captured are all time stamped so that the fluorescein progress can be measured accurately. The acquisition and equipment information captures the different setups that are in use:

  1. Acquisition information A is the ordinary illumination and planned lenses for the examination.

  2. Acquisition information B is the filtered illumination, filtered viewing, and lenses appropriate for the fluorescein examination.

  3. Acquisition information C indicates no change to the equipment settings, but once the injection is made, the subsequent images include the drug, method, dose, and time of delivery.

U.3 Ophthalmic Tomography Use Cases (Informative)

Optical tomography uses the backscattering of light to provide cross-sectional images of ocular structures. Visible (or near-visible) light works well for imaging the eye because many important structures are optically transparent (cornea, aqueous humor, lens, vitreous humor, and retina – see Figure U.3-1).

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Figure U.3-1 Schematic representation of the human eye.

To provide analogy to ultrasound imaging, the terms A-scan and B-scan are used to describe optical tomography images. In this setting, an A-scan is the image acquired by passing a single beam of light through the structure of interest. An A-scan image represents the optical reflectivity of the imaged tissue along the path of that beam – a one-dimensional view through the structure. A B-scan is then created from a collection of adjacent A-scan images – a two dimensional image. It is also possible to combine multiple B-scans into a 3-dimensional image of the tissue.

When using optical tomography in the eye it is desirable to have information about the anatomic and physiologic state of the eye. Measurements like the patient’s refractive error and axial eye length are frequently important for calculating magnification or minification of images. The accommodative state and application of pupil dilating medications are important when imaging the anterior segment of the eye as they each cause shifts in the relative positions of ocular structures. The use of dilating medications is also relevant when imaging posterior segment structures because a small pupil can account for poor image quality.

U.3.1 Anterior Chamber Tomography

U.3.1.1 Anterior Chamber Exam for Phakic Intraocular Lens surgery planning

Ophthalmic tomography may be used to plan placement of a phakic intraocular lens (IOL). A phakic IOL is a synthetic lens placed in the anterior segment of the eye in someone who still has their natural crystalline lens (i.e. they are “phakic”). This procedure is done to correct the patient’s refractive error, typically a high degree of myopia (near-sightedness). The exam will typically be performed on both eyes, and each eye may be examined in a relaxed and accommodated state. Refractive information for each eye is required to interpret the tomographic study.

A study consists of one or more B-scans (see Figure U.3-2) and one or more instances of refractive state information. There may be a reference image of the eye associated with each B-scan that shows the position of the scan on the eye.

U.3.1.2 Anterior Chamber Angle Exam

The anterior chamber angle is defined by the angle between the iris and cornea where they meet the sclera. This anatomic feature is important in people with narrow angles. Since the drainage of aqueous humor occurs in the angle, a significantly narrow angle can impede outflow and result in increased intraocular pressure. Chronically elevated intraocular pressures can result in glaucoma. Ophthalmic tomography represents one way of assessing the anterior chamber angle.

B-scans are obtained of the anterior segment including the cornea and iris. Scans may be taken at multiple angles in each eye (see Figure U.3-2). A reference image may be acquired at the time of each B-scan(s). Accommodative and refractive state information are also important for interpretation of the resulting tomographic information.

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Figure U.3-2 Tomography of the anterior segment showing a cross section through the cornea.

Note in the Figure the ability to characterize the narrow angle between the iris and peripheral cornea.

U.3.1.4 Corneal Exam

As a transparent structure located at the front of the eye, the cornea is ideally suited to optical tomography. There are multiple disease states including glaucoma and corneal edema where the thickness of the cornea is relevant and tomography can provide this information using one or more B-scans taken at different angles relative to an axis through the center of the cornea.

Tomography is also useful for defining the curvature of the cornea. Accurate measurements of the anterior and posterior curvatures are important in diseases like keratoconus (where the cornea “bulges” abnormally) and in the correction of refractive error via surgery or contact lenses. Measurements of corneal curvature can be derived from multiple B-scans taken at different angles through the center of the cornea.

In both cases, a photograph of the imaged structure may be associated with each B-scan image.

U.3.2 Posterior Segment Tomography

U.3.2.1 Retinal Nerve Fiber Layer Exam

The Retinal Nerve Fiber Layer (RNFL) is made up of the axons of the ganglion cells of the retina. These axons exit the eye as the optic nerve carrying visual signals to the brain. RNFL thinning is a sign of glaucoma and other optic nerve diseases.

An ophthalmic tomography study contains one or more circular scans, perhaps at varying distances from the optic nerve. Each circular scan can be “unfolded” and treated as a B-scan used to assess the thickness of the nerve fiber layer (see Figure U.3-3). A fundus image that shows the scan location on the retina may be associated with each B-scan. To detect a loss of retinal nerve fiber cells the exam might be repeated one or multiple times over some period of time. The change in thickness of the nerve fiber tissue or a trend (serial plot of thickness data) might be used to support the diagnosis.

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Figure U.3-3 Example tomogram of the retinal nerve fiber layer with a corresponding fundus image.

In the Figure, the pseudo-colored image on the left shows the various layers of the retina in cross section with the nerve fiber layer between the two white lines. The location of the scan is indicated by the bright circle in the photograph on the right.

U.3.2.2 Macular Exam

The macula is located roughly in the center of the retina, temporal to the optic nerve.  It is a small and highly sensitive part of the retina responsible for detailed central vision. Many common ophthalmic diseases affect the macula, frequently impacting the thickness of different layers in the macula. A series of scans through the macula can be used to assess those layers (see Figure U.3-4).

A study may contain a series of B-scans. A fundus image showing the scan location(s) on the retina may be associated with one or more B-scans. In the Figure, the corresponding fundus photograph is in the upper left.

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Figure U.3-4 Example of a macular scan showing a series of B-scans collected at six different angles

U.3.2.3 Angiographic Exams

Some color retinal imaging studies are done to determine vascular caliber of retinal vessels which can vary throughout the cardiac cycle. Images are captured while connected to an ECG machine or a cardiac pulse monitor allowing image acquisition to be synchronized to the cardiac cycle.

Angiography is a procedure which requires a dye to be injected into the patient for the purpose of enhancing the imaging of vascular structures in the eye. A standard step in this procedure is imaging the eye at specified intervals to detect the pooling of small amounts of dye and/or blood in the retina. For a doctor or technician to properly interpret angiography images it is important to know how much time had elapsed between the dye being injected in the patient (time 0) and the image frame being taken. It is known that such dyes can have an affect on OPT tomographic images as well (and it may be possible to use such dyes to enhance vascular structure in the OPT images), therefore time synchronization will be applied to the creation of the OPT images as well as any associated OP images

The angiographic acquisition is instantiated as a multiframe OPT Image. The variable time increments between frames of the image are captured in the Frame Time Vector of the OPT Multi-frame Module. For multiple sets of images, e.g. sets of retinal scan images, the Slice Location Vector will be used in addition to the Frame Time Vector. For 5 sets of 6 scans there will be 30 frames in the multi-frame image. The first 6 values in the Frame Time Vector will give the time from injection to the first set of scans, the second 6 will contain the time interval for the second set of 6 scans, and so on, for a total of 5 time intervals.

Another example of an angiographic study with related sets of images is a sequence of SLO/OCT/"ICG filtered" image triples (or SLO/OCT image pairs) that are time-stamped relative to a user-defined event.  This user-defined event usually corresponds to the inject time of ICG (indocyanine green) into the patients blood stream.  The resultant images form an angiography study where the patient’s blood flow can be observed with the "ICG filtered" images and can be correlated with the pathologies observed in the SLO and OCT images which are spatially related to the ICG image with a pixel-to-pixel correspondence on the X-Y plane.

U.3.2.4 3D Reconstruction Exam

The prognosis of some pathologies can be aided by a 3D visualization of the affected areas of the eye.  For example, in certain cases the density of cystic formations or the amount of drusen present can be hard to ascertain from a series of unrelated two-dimensional longitudinal images of the eye.  However, some OCT machines are capable of taking a sequence of spatially related two-dimensional images in a suitably short period of time.  These images can either be oriented longitudinally (perpendicular to the retina) or transversally (near-parallel to the retina).  Once such a sequence has been captured, it then becomes possible for the examined volume of data to be reconstructed for an interactive 3D inspection by a user of the system (see Figure U.3-5).  It is also possible for measurements, including volumes, to be calculated based on the 3D data set. 

A reference image is often combined with the OCT data to provide a means of registering the 3D OCT data-set with a location on the surface of the retina (see Figures U.3-6 and U.3-7). 

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Figure U.3-5 Example 3D reconstruction

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Figure U.3-6 Longitudinal OCT Image with Reference Image (inset)

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Figure U.3-7 Superimposition of Longitudinal Image on Reference Image

U.3.2.5 Transverse Imaging

While the majority of ophthalmic tomography imaging consists of sets of longitudinal images (also known as B scans or line scans), transverse images (also known as coronal or “en face” images) can also provide useful information in determining the full extent of the volume affected by pathology.

Longitudinal images are oriented in a manner that is perpendicular to the structure being examined, while transverse images are oriented in an “en face” or near parallel fashion through the structure being examined.

Transverse images can be obtained from a directly as a single scan (as shown in Figures U.3-8 and U.3-9) or they can also be reconstructed from a 3D dataset (as shown in Figures U.3-10 and U.3-11). A sequence of transverse images can also be combined to form a 3D dataset.

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Figure U.3-8 Transverse OCT Image

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Figure U.3-9 Correlation between a Transverse OCT Image and a Reference Image Obtained Simultaneously

Figures U.3-8 through U.3-10 are all images of the same pathology in the same eye, but the two different orientations provide complementary information about the size and shape of the pathology being examined. For example, when examining macular holes, determining the amount of surrounding cystic formation is important aid in the following treatment. Determining the extent of such cystic formation is much more easily ascertained using transverse images rather than longitudinal images. Transverse images are also very useful in locating micro-pathologies such as covered macular holes, which may be overlooked using conventional longitudinal imaging.

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Figure U.3-9 Correspondence between Reconstructed Transverse and Longitudinal OCT Images

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Figure U.3-10 Reconstructed Transverse and Side Longitudinal Images

In Figure U3.9, the blue green and pink lines show the correspondence of the three images. In Figure U3.10, the Transverse image is highlighted in yellow.