The added value of SPECT/CT in dacryoscintigraphy for the diagnosis of nasolacrimal duct obstruction

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Learning objectives
Epiphora, an abnormal overflow of tears, is commonly caused by tear drainage system anomalies including nasolacrimal duct obstruction. To evaluate the function of the system, dacryoscintigraphy is a commonly available non-invasive method under a nuclear medicine dynamic acquisition. We recommend the use of SPECT/CT imaging after the dynamic acquisition to localize the obstruction site and to distinguish from any radioactive tracer spillage as sometimes encountered in dacryoscintigraphy. A clinical case is presented to support the use of SPECT/CT in the protocol.

Background
Dacryicintigraphy is a useful method to assess the patency of nasolacrimal drainage pattern and to evaluate the success of dacryocystorhinostomy treatment to the obstruction [1][2][3]. Because of the obstruction, small amount of radioactive tracer would overflow and drain along anterior skin surface to mask the nasolacrimal duct. Fused images after SPECT/CT imaging can localize the site of obstruction and of radiation contamination due to overflow of radioactive tracer.

Findings and procedure details
A 66 year-old male patient with the history of bilateral epiphora was referred to our department for dacryoscintigraphy imaging. Following instillation of one drop of technetium pertechnetate ( 99m TcO 4 ) tracer of concentration about 4 MBq/ml in the inferior fornix of the eyes (outer canthus of both eyes), dynamic imaging over the anterior nasalorbit region with the patient in a sitting position was performed (1 minute/frame for 15 minutes) with the use of low energy high resolution collimator mounted onto a Symbia Siemens gamma camera. These dynamic images showed accumulation of the tracer in the lacrimal sacs bilaterally without any progression into the lacrimal ducts (Figure 1), compatible with bilateral lacrimal apparatus obstruction at the sac-duct level.
Delayed dynamic imaging showed similar finding but abrupt tracer draining inferiorly and laterally was observed from the right eye after 25 minutes ( Figure 2). As this image track masked part of the lacrimal duct, the degree of obstruction at the delayed dynamic images could not be concluded.
The patient was then placed under SPECT/CT imaging (GE Infinia-4; CT imaging: 120 kVp, 2.5 mA). Fused images of SPECT/CT clearly demonstrated the tracer was located on the superficial skin surface due to the spillage of radioactive tracer from the obstructed right eye (Figure 3).

Conclusion
Dacryoscintigraphy is a simple but yet an underutilized test [4] providing high sensitivity in epiphora detection in patients before and after dacryocystorhinotomy. The trend to use hybrid system SPECT/CT in dacryoscintigraphy is increasing because SPECT/CT can help localize the obstruction site [5]. We suggest to firstly perform dynamic imaging because radioactive tracer can drain along the nascolacrimal duct by gravity while patient is in an erect position. SPECT/CT has its value to demonstrate the blockage location and importantly to differentiate the actual blockage from possible radiation contamination, the latter of which is sometimes encountered in dacryoscintigraphy procedure. It was reported that radiation dose to the eye may be 0.14 mSv/eye for an unobstructed eye and 4 mSv/eye in the presence of an obstruction due to the radioactive tracer of 99m TcO 4 [6].
We measured the CT radiation dose to each eye due to the CT component of SPECT/ CT system using mosfet detectors attached to the eyes of a head phantom. The average CT dose to each eye was measured with the use of GE infinia-4 SPECT/CT as 2 mSv, which is equivalent to an annual background radiation dose. Therefore even with the use of SPECT/CT in dacryoscintigraphy, the extra CT radiation dose to each eye is indeed minimal.