Technical report: Digital subtraction dacryocystography

ClinicalRadiology(1990) 42, 351-353

Technical Report: Digital Subtraction Dacryocystography S. J. K I N G and S. F. H A I G H

pepartment of Diagnostic Radiology, King's College Hospital, London Twenty-one patients presenting clinically with obstructive epiphora were investigated by digital subtraction dacryocystography (DSDCG). This technique is quick and simple to perform, with good patient acceptance. It produces diagnostic images on a single radiographic projection. The proximal lacrimal apparatus is well demonstrated. Patient radiation dose to the orbit is apparently reduced in DSDCG without the use of lead eye shields. The technique of DSDCG was readily acquired by junior radiologists. Reliable and accurate demonstrations of lacrimal duct obstructions were produced in a general radiological department where dacryocystography is an occasional examination. We advocate the use of digital techniques in dacryocystography when the equipment is available. King, S.J. & Haigh, S.F. (1990). ClinicalRadiology 42, 351-353. Technical Report: Digital Subtraction Dacryocystography

I)acryocystography has long been used by ophthalmologists to confirm or refute the clinical suspicion o f lacrimal duct obstruction. Radiological assessment of the lacrimal duct system has progressed from conventional dacryocystography (Ewing, 1909), through macro-dacryocystograpby (Campbell, 1964), photographic subtraction (Lloyd and Welham, 1974) and isotope scintigraphy (Rose and Clayton, 1985). : The choice of surgical procedure for an obstructed lacrimal system may be influenced by the site of the obstruction (Lloyd and Welham, 1974). Improvements in dacryocystography have achieved improved visualization of the proximal lacrimal apparatus in particular. Better detail of the canalicular system is of importance when deciding the appropriate surgical treatment (Lloyd, 1983). Digital subtraction angiography is well described and many centres now have digital radiography units available. The application of digital subtraction imaging to other duct systems has been described in the United States for sialography and a single case of lacrimal duct obstruction (Lightfoote et al., 1985). In addition, a digital subtraction technique has been used specifically to study the lacrimal system in a series of six patients (Galloway et al., 1984). At the moment digital subtraction imaging of non-vascular structures is not widely performed in the United Kingdom. We describe a technique of dacryocystography employing digital subtraction, highlighting its merits over conventional imaging.

MATERIALS AND M E T H O D S The 21 patients (9 male, 12 female, aged 20-83) were referred for imaging by the Department of Ophthalmic Surgery, King's College Hospital, following outpatient assessment for epiphora. The patients were being assessed for potential surgical c o r r e c t i o n of a lacrimal duct Occlusion. An IGE D F 5000 digital angiography system With a carbon fibre 'floating' table, 0.6 mm focal spot and an image intensifier screen of 9 inches was used. No Patient preparation was required. Patients who had a history of cutaneous iodine sensitivity or severe local Correspondence to: Dr S. J. King, Department of Diagnostic Radiology,King's CollegeHospital, Denmark Hill, London SE5 9RS.

sepsis were excluded. The procedure was explained to the patient and verbal consent obtained. The patient was positioned supine and the head immobilized with 'micropore' sticking plaster or Velcro straps. Local anaesthetic (amethocaine 0.5% 1 drop or oxybuprocaine 0.4% 1 drop) was instilled into the conjunctival sac of the eye under investigation. Our current practice is to use oxybuprocaine which has a much shorter length of action than amethocaine. Formal protection of the eye post-procedure is therefore unnecessary and bilateral D S D C G may be performed at the same attendance. The inferior canaliculus was cannulated using either a fine metal-tipped sialography catheter or a 'Portex' intravenous catheter after dilatation o f the canaliculus and lacrimal sac massage. The superior canaliculus was used if the inferior could not be dilated. If there was a severe stenosis of the origin of the canaliculus, a Rycroft lacrimal needle was employed. The catheter was flushed with Iohexol 240, connected to an extension tube and 2 ml syringe all pre-filled with contrast medium. The catheter was stabilized by taping to the patient's forehead. (Usually only 0.5 ml of contrast medium is needed for each run.) The patient was positioned under fluoroscopic control for a coned occipitomental view centred in the midline at the infraorbital margin. Test exposures were performed to achieve the optimal exposure factors. The radiologist, standing behind a lead glass protective screen, injected 0.5-1.0 ml of contrast medium whilst viewing the real time images taken at a rate of 1 image every 2 seconds. One to six images were required for as soon as patency or obstruction was demonstrated the run was stopped. If on review the images were inadequate, an immediate repeat run could be performed. A repeat 'mask' ensured subtraction of residual contrast medium. With more experience it was clear that the number of images could be reduced to one or two. On completion of the examination the patient was counselled and if amethocaine had been used, the eye was shielded. The images were then digitally manipulated to achieve the most useful/aesthetic result and hard copy images produced. Further permanent storage of the raw or processed data, e.g. on optical disc is also possible.

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CLINICAL RADIOLOGY

Thermoluminescent dosemeters (TLDs) were used to measure the radiation dose to the lens of the eye of 18 patients in the study. They were placed on the nasion during fluoroscopic positioning, test exposures and imaging. TLDs were used to compare the radiation dose between D S D C G and conventional dacryocystography (DCG). Occipitomental and lateral or occipitomental and 20 degree oblique lateral views were performed on a phantom skull using a Siemens Orbix isocentric skull unit with a 0.6 mm focal spot and 1.5 magnification. Fig. 2 - Right common canalicular duct obstruction.

RESULTS On the basis of the D S D C G findings the results were divided into four categories dependent on patency or the level of duct obstruction. These were patent duct (Fig. 1), common canalicular block (Fig. 2), lacrimal sac block and lacrimal duct block (Fig. 3). Following clinical review by ophthalmic surgeons, eight symptomatic patients with obstruction on D S D C G who were fit for surgery were referred for dacryocystorhinostomy (DCR). The surgical findings in this small group have so far agreed with the pre-operative assessment by D S D C G (see Table 1). Individual radiation doses for the 18 patients investigated by D S D C G (1 to 3 runs per patient) and for 12 sets of radiographs (occipitomental and lateral or occipitomental and2_0 degree oblique lateral) performed on a skull phantom were measured by TLDs (mSv) (see Table 2). The dose range varied for each patient depending on the amount of screening during fluoroscopic positioning, the number of images taken per run as well as the number of runs performed. The need for several runs in certain cases probably relates to operator inexperience. A statistical analysis of the results reveals that the

Fig. 3 - Right distal nasolacrimal duct obstruction with dilatation of the upper duct.

Table I No, o f patients

DSDCG findings

Management DCR not indicated clinically

7 11

2 1

Patent * Common canalicular block Lacrimal sac block Lacrimal duct block

21

DCR awaited

DCR performed

-

-

2

6

0

1

1

0

0

I

i0

3

8

7

3

* Includes seven patients with distal common canalicular block and four patients with proximal common canalicular block.

Table 2 - Measured dose at nasion by TLD (mSv) for DSDCG and conventional dacryocystography (DCG) DSDCG

No. of patients Dose range

Fig. 1

Right patent nasolacrimal duct.

DCG

(2films)

1 run

2 runs

3 runs

12 * <0.2-1.6

5 1.1-2.0

1 1.7

12 1.4--1.8

Mean dose/run D S D C G 0.68. Mean dose per examination D C G 1.53. * Dose levels below 0.2 mSv could not be accurately recorded.

DIGITAL SUBTRACTION DACRYOCYSTOGRAPHY

difference between the mean dose of the conventional technique (1.53 mSv) and the mean dose of D S D C G (0.68 ~Sv per run) is significant even at the 99.9% confidence level. The results indicate therefore that D S D C G (with up to two runs) provides a dose reduction over conventional dacryocystography.

DISCUSSION In general radiology departments where dacryocystography is infrequently performed the conventional technique m a y produce a sub-optimal examination of the lacrimal system. Dedicated equipment m a y not be available, e.g. a skull unit with serial changer (Lloyd and Ardagh, 1972). Inadequate positioning of the patient and incorrect radiographic exposure can contribute to a reduction of image quality and increase the patient radiation dose through repetition of views. This may in part be due to inexperience of the technique within the radiology department. In our hospital we may perform only one or two dacryocystograms per month. D S D C G allows positioning of the patient under fluoroscopic control. Correct radiographic exposures are assured by the test exposures taken before the run. Reflux of contrast medium into the eye during the injection can obscure canalicular detail in conventional dacryocystography. Under fluoroscopic control sequential images are taken during a single dynamic injection and therefore early images demonstrating canalicular detail are obtained. This is also possible in conventional DCG if a serial film changer is available. The D S D C G can be terminated if duct patency is demonstrated thus minimizing patient radiation exposure. The images can be reviewed immediately and further views may be taken, for example lateral views but these have not been found necessary in our series. Conventional photographic subtraction is time consuming. D S D C G simplifies subtraction and allows manipulation of the image. Resolution in conventional and digital radiography differs. The former has superior spatial resolution but inferior contrast resolution (Harrison and Isherwood, 1984). The digital method produces images with sufficient spatial information to diagnose lacrimal duct obstruction or stenosis at various levels with greatly enhanced contrast resolution. Immediate display and review of images reduces the total length of the examination benefiting the patient and radiology department. Various bard copy formats for display and storage are available. 0Dtical disc technology permits the permanent storage and rapid retrieval of images. Conventional dacryocystography usually employs oilbased contrast media with the small attendant risk of tissue extravasation. I f water soluble contrast media are Used in conventional dacryocystography, high iodine content is necessary to provide adequate contrast. BSDCG allows the use of small vOlumes of low iodine Content water-soluble contrast media. This reduces the Cost of the contrast media required for the investigation. The physical characteristics of modern contrast media

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m a y also be considered to resemble those of tears more closely than oil based contrast media. Radiation dose to the eye and the lens in particular is an important concern in dacryocystography. Attempts have been made to reduce lens dosage in conventional dacryocystography (Jackson et al., 1989) by the use of lead eye protection and 30 degree oblique lateral projection in bilateral D C G . In our small study, the lens dose measured by thermoluminescent dosimetry in those patients investigated by D S D C G showed a statistically significant radiation dose reduction compared to conventional dacryocystograms. The doses recorded were of the order of doses documented in a recent study of conventional D C G with added lead eye protection (Jackson et al., 1989). At least three radiographs are usually required in conventional dacryocystography. The dose reduction in D S D C G m a y therefore be more marked. The advantage is naturally reduced by increasing the number of images in D S D C G . However, up to two runs m a y still achieve a dose comparable to conventional D C G . Some of the recorded dose reduction in D S D C G may be due to cranial shielding of the lens due to the occipitomental position. I f a mento-occipital position is employed in conventional dacryocystography it may lead to an increase in patient lens dosages. The radiation dose to the radiologist in D S D C G may be minimized by standing behind a lead glass screen and using extension tubing whilst injecting, as in conventional DCG. We conclude that D S D C G has advantages over D C G and advocate its use where digital equipment is available. Acknowledgements. The authors would like to thank Mr R. Coakes and other Consultant Ophthalmic Surgeons at King's CollegeHospital for referral of their patients and comments, Ruth Reilly and Elizabeth Crouch for their help with the radiography and Chris Lawinskifor help with the statistics. REFERENCES

Campbell, W (1964). The radiology of the lacrimal system. British Journal of Radiology, 37, 1-26. Ewing, AE (1909). Roentgen ray demonstration of the lacrimal abscess cavity. American Journal of Ophthalmology, 26, i-4. Galloway, JE, Kavic, TA & Reflo, GT (1984). Digital subtraction macrodacrocystography.A new method of lacrimal systemimaging. Ophthalmology, 91, 956-962. Harrison, RM & Isherwood, I Eds (1984). Digital Radiology: Physical and ClinicalAspects (IPSM 1). The Hospital PhysicistsAssociation, London. Jackson, A, Hardcastle, MP, Shaw, A & Gibbon, WW (1989). Reduction of ocular lens dosage in dacryocystography. Clinical Radiology, 40, 615-618. Lightfoote, JB, Friedenberg, RM & Smolin, MF (1985). Digital subtraction ductography. American Journal of Roentgenology, 144, 635-638. Lloyd, GAS (1983). The Orbit. In Techniques in Diagnostic-Radiology, Whitehouse, GH & Worthington, BS Eds, pp. 301-302. Blackwell Scientific Publications, Oxford. Lloyd, GAS & Ardagh, J (1972). Adaptation of a standard skull table for enlargementangiography and macrodacryocystography.X-Ray Focus, 12, 16-18. Lloyd, GAS & Welham, RAN (1974). Subtraction macrodacryocystography. British Journal of Radiology, 47, 379-382. Rose, JDG & Clayton, CB (1985). Scintigraphy and contrast radiography for epiphora. British Journal of Radiology, 58, 1183-I 186.