1. Radiation dose to eye lens in CT
Kalpana Parajuli
Since its inception in early 1970s ,computed tomography (CT),has undergone paradigm shift in
its technology as a result of which its potential applications have been increasing. According to
united nations scientific committee on effects of atomic radiation (UNSCEAR) 2008 report on
medical exposure ,CT scanning accounts for 43% of the total collective effective dose due to
diagnostic medical radiology1 .Study called population dose from CT scanning 2009, carried out
by health service executive stated that CT contributes 67%of the annual dose from all medical
radiation dose2.annual collective effective dose due to ct is 437500 person –Sv. Estimated
Annual Per-Capita Effective Dose in the World from CT scan during the period 1997-2007, was
0.24 mSv. 3
With the advancement in CT technology, neuroradiologic application of CT is increasing. There
are certain examinations where direct exposure to eye is unavoidable. Lens of eye consist of
very radiosensitive cells that undergo irreversible damage in response to radiation.
Accumulation of damaged cells causes degenerative opacification, leading to radiation induced
cataract. Radiation induced cataract is the well known late effects of radiation, which was first
reported in early 20th century shortly after development of first xray machines.4 Especially
Paediatric age group is at higher risk, cumulative dose of 250 mGy has been documented to
cause radiation induced cataract.5 Although much work has been carried out , the exact
mechanism of radiation cataractogenesis is still debateful. Evidence from various
epidemiological studies previously suggested that it is a tissue reaction (deterministic) that
follows linear threshold relationship. Recently some authors suggest that the dose response may
be more accurately described by a linear no-threshold stochastic (rather than a deterministic)
model.6 A study of the risk of cataract formation after exposure to low doses of radiation in a
cohort of U.S. radiologic technologists suggested a increasing likelihood of cataract formation
with increasing radiation exposure, but with no apparent threshold.7 International commission on
radiologic protection (ICRP), non-governmental organization established in 1928 , reviewed
recent mechanistic and human study and decreased threshold for radiation induced cancer from
2 Gy for acute exposure and 4 Gy for protracted exposure to 0.5 Gy, however still assumes
radiation cataractogenesis as a tissue reaction .8 Also the equivalent dose limit for the lens of the
eye for occupational exposure in planned exposure situations has been reduced from 150 mSv
per year to 20 mSv per year, averaged over defined periods of five years, with no single year
exceeding 50 mSv.8 The lens dose on a multisection scanner is still substantially less than the
threshold dose of 0.5–2.0 Gy, which has been attributed to detectable lens opacities.9 However,
dose reduction is still a priority, given that some patients need many CT scans.
Radiation dose to lens during a cranial CT ranges from 22.4 to 100 mGy, depending on beam
width, reconstruction interval,tube voltage, tube current, angulation, coverage, etc. 5 Radiation
dose to lens during a head CT examination can be reduced by positioning head such that
orbitomeatal line is perpendicular and giving sufficient gantry angulation such that scan plane

2. coincides supraorbitomeatal line (superior orbital margin to the base of skull). Maclennan and
Hadley found that the average orbital dose can be decreased to 18.5 mGy (1.85 rad) by aligning
brain CT scans along the supraorbital meatal baseline. 10 Yeoman et al however, found that only
32% of sites routinely avoided the eye during brain CT. 11 Most users prefer to begin the scan at
or below the level of the foramen magnum and include a portion of the skull base within the
study. Another way is use of commercially available shield, bismuth coated latex. It is useful in
patients who cannot flex their neck and also during the volume acquisition of head and neck
where irradiation of eyes is unavoidable. It is also important when scanners donot have ability to
provide gantry angulation. Hopper et al carried out a study on phantom as well as clinical patient
group and tested the ability of a heavy metal, bismuth, in reducing radiation to the lens of the
eye during routine cranial CT and reported that it is simple in expensive to use and can reduce
dose to eye lens by 50%. No artifacts were observed for any of the shielding thicknesses.
Specifically, no beam-hardening artifact into the deeper orbit, or especially into the cerebrum,
was identified on any case. There was, however, significant artifact projected into the superficial
orbit and the lens.5 Mclaughlin et al .however reported that use of eye shield doesnot reduce
radiation dose to lens to much extent if supraorbitomeatal line is used as a baseline since eye is
not irradiated by primary beam.12
Since there is a direct relationship between tube current time product and radiation dose, dose to
eye during head CT can be minimized by minimizing tube current time product, however a
compromise between dose and image quality is always there. Mullin et al compared image
quality of unenhanced brain CT at 90 and 170 mAs at constant tube voltage of 140 kVp and
reported that 90-mAs images are moderately noisier than 170-mAs image and may be of
acceptable diagnostic quality for some clinical applications but not appropriate for initial
screening examinations in the emergency department.13 Specifically, low-dose head CT scans
might be especially well suited for use in patients with complex disease (eg, malformation,
tumors, trauma, and vascular disease )who need to undergo multiple ct scans. Paediatric patients
need to undergo ct examination frequently for ventricular shunt assessment. Since main aim of
follow up studies is to identify complications and gross morphologic changes and often involves
structures with relatively high contrast-enhancing features (eg, bleeding or ventricular size),
image noise may not be bothersome and alteration in standard scan parameters is thus possible.
Similarly, CT scanners available today has inbuilt features like organ based current modulation
which has been proved to be useful for dose reduction to thyroid breast and lens. In this mode,
tube current is decreased by 75% from the reference scan’s tube current for an angular range of
approximately 120° over the anterior surface of the head, symmetric to the median plane of the
patient .During the remaining 240° of scanning range, tube current is increased by 25% so that
the same total tube current time product is applied over 360°, as used for the reference scan. In
this manner, the same total scanner output is used but allocated more to the lateral and posterior
tube positions than to the anterior tube positions.14
Wang et al compared the dose and image quality of three methods ( bismuth shielding, organ-
based tube current modulation (TCM), and global reduction of the tube current )for reducing the
radiation dose to the eye at head CT and reported 26.4% , 30.4% , and 30.2% reduction in eye
dose with one bismuth shield, organ-based TCM and global reduction in tube current
respectively. A combination of organ-based TCM with one bismuth shield reduced the dose by
47.0%. Organ-based TCM provided superior image quality to that with bismuth shielding while
similarly reducing dose to the eye. Image noise in the brain region was slightly increased for all
dose reduction methods. CT numbers were increased whenever the bismuth shield was used.

3. Increasing the distance between the bismuth shield and the eye lens helped reduce CT number
errors, but the increase in noise remained.14 Tan et al compared the radiation dose delivered to
the eye lens by 16- and 64-section multidetector CT (MDCT) for standard clinical neuroimaging
protocols and found it to be significantly lower, for 64 section CT partly due to improvements in
automatic tube current modulation technology.15
Also the number of CT-guided procedures performed by interventional radiologists have been
increasing , dose to the interventionalist’s eye lens is also a matter of concern . In a study done
by Heush et al for 89 interventions, the median total exposure lens dose was 3.3 µSv. The
author reported that assuming 50-200 cases performed by one radiologist dose will not exceed
20mSv,the maximum dose limit given by ICRP. 16m Several authors have reported that images
of CTA(CT angiography) of brain obtained at 80 kVp show higher contrast and contrast-to-
noise ratio with 40% patient dose reduction compared to that of 120kVp.17 in a study carried
out by Imai et al the lens doses for 100, 120 and 140 kVp at 252 effective mAs, were
estimated to be 28.4, 40.9 and 54.2 mGy, respectively, and the lens dose reduction of 30–
48% was achieved at 100 kVp. The author recommends use of 100 kVp for axial CTA images
and 80 kVP for 3D CTA image acquisition, as increased streak artifacts at 80 kVp donot
affect quality of 3D images like that of axial images.18 Schimmoller et al evaluated the
influence of a new OSDR (organ specific dose reduction ) algorithm on image quality of head
and neck computed tomographic angiography (CTA) in clinical routine and reported that The
novel OSDR algorithm does not compromise image quality of head and neck CTA and can be
recommended for CTA in clinical routine to protect the thyroid gland and ocular lenses from
unnecessary high radiation.19
With the increased z-axis coverage and improved temporal sampling rate of MDCT scanners,
brain perfusion scanning has become a viable tool for evaluating cerebral perfusion defects in
patients with a suspicion of stroke. Along with CTA, CT Perfusion(CTP) also falls under acute
stroke protocol, which requires repeated scanning of same volume. Gantry angulation and
adjustment of lens as far as possible from the scan area are the two techniques to reduce lens
dose from CTP.20 However it should be ensured that the region of interest (mid cerebral area
including the basal ganglia nuclei for suspected stroke patients) is completely within the imaged
volume. Some of the scanners offering whole-brain imaging have introduced methods to assist in
dealing with dose concerns by performing all required imaging functions in a reduced number of
scans, such as an initial unenhanced scan, followed by a second contrast-enhanced scan from
which the arterial, venous, and brain perfusion data are extracted. AAPM recommends
acquisition at 80 kVp for dose management in CTP study.21 Zhang et al calculated peak lens
dose in various scanners at various protocols recommended by AAPM and reported to range
from 81 mGy to 279 mGy.20 Since in brain perfusion study the image quality can afford to be
lower than in routine head examinations. Therefore, it is important for each clinical site to
optimize scanning protocols according to the characteristics of the CT scanner to ensure the
lowest possible radiation dose delivery to patients. For example, lower tube potential should be
used, mAs should be lower than diagnostic head examinations and the number of acquisitions
should be minimized to achieve lower total mAs without sacrificing the sampling rate.
Another most frequent examination where eye is irradiated directly is CT of para nasal sinuses
(PNS). Increased use of functional endoscopic surgery ,minimally invasive way of treating sinus
diseases , requires high quality thin section CT images that acts as a road map by providing
information of the highly variable anatomy of the nasal cavities and paranasal sinuses as well as

4. the relationship of the diseased areas to vital structures such as the optic nerve and internal
carotid artery, and is a well established mandatory preoperative diagnostic tool.22 The total
amount of radiation dose to the lens is about 20mGy from the standard CT of the sinuses . 23 A
persistent high exposure to the lens of the eye from a standard protocol of CT of the sinuses
would lead to cataract .24 Dose to lens during PNS CT can be reduced effectively by reducing
mas without degrading diagnostic quality enabled by high inherent contrast structures in sinus
CT. Shrimpton et al.25, reported dose to lens as 70.3mGy at 475mAs, 17.6mGy at 210mAs and
4.7mGy at 30mAs. Another study by brem et al26 concluded that > 67 effective mAs and > 134
effective mAs are lowest dose that provides diagnostic quality images for osseous structures
and for the optic nerve and the inferior rectus muscle respectively. In A study by Lam et al
55.4% reduction in lens dose was possible by the reduction of mAs from 100 to 40 without
causing any significant effect to the diagnostic image quality and assessment of the anatomical
structures.27 Similarly, according to Moulin et al, an interslice gap of 3–6 mm can also be
utilised during paranasal sinus CT scan to reduce dose to lens28. Increasing slice gap decreases
the number of axial slices and hence reduces dose. Maempel et al reported coronal acquisition
delivers more dose to lens than that of axial . 29 Previously direct axial and direct coronal image
acquisition was necessary, but with the ability to obtain isotropic volume data direct acquisition
in one plane is sufficient and the other is reformatted from the thin section raw data. This has
increased the possibility of reducing dose to the lens during sinus CT study.
Another frequent examination in which lens are encompassed directly by ionizing radiation is
HRCT( High Resolution CT) temporal bone. The American College of Radiology recommends
that the infraorbitomeatal line be used as the baseline. When the infraorbitomeatal line is the
baseline, the lenses are also included in the scanning range and receive direct radiation exposure.
Once the lenses are included in the scanning range, the lens organ dose does not change
substantially with change in scan coverage. Torizuka et al evaluated the radiation dose to the
lens and the visualization of temporal bone structures by scanning along the orbitomeatal line
parallel to the hard palate and found that change of the baseline decreased the lens dose from
12.7 to 0.274 cGy but resulted in difficulty in evaluating the pneumatic space of the temporal
bone and the auditory ossicles.30 Niu et al 31compared lens dose for three different scanning
mode, direct axial (glabellomeatal line as baseline with mandible adduction and gantry tilt in the
cranial direction) and coronal scan (the neck extended and the gantry was angled maximally
(30°) in the caudal direction to obtain images in a plane as perpendicular to the hard palate as
possible) in sequential mode , helical mode without gantry angulation and with the head and
neck in a neutral position (i.e, the orbitomeatal line approximately perpendicular to the table)
and modified helical mode with adducted mandible and acanthiomeatal line as the baseline. Lens
dose for direct coronal scan was 1.73 mGy, the lowest of all scans. For the modified helical scan
it was 10.33 mGy, which was much lower than the 52.69 mGy of the sequential scan and the
40.17 mGy of the routine helical scan. The author strongly recommends modified helical
acquisition with acanthiomeatal line as the baseline. Although produced images are different
from those obtained with glabellomeatal baseline as baseline , they can be readily obtained using
flexible postprocessing tools. Author also suggests The operator to locate the lens 1 cm above the
upper edge of the scanning range to avoid direct radiation because of overranging and reduce
scatter radiation to the lenses. If the lens is the upper boundary of the scanning range, the lens
dose increases greatly. Funama et al 32 investigated lens-dose reduction in pediatric high-
resolution CT temporal bone scanning and reported satisfactory image quality after reducing the

5. tube current to 120 mAs. Additional studies on low-dose pediatric temporal bone CT
recommended further reductions in the scanning technique to 120 kVp , 50–75 mAs , and 0.75
pitch
In summary, radiation dose today is a major public health issue. Radiation dose from CT should
be carefully controlled to minimize patient dose and maximize the benefit-to-risk ratio of the
examination. Clinical institutions should ensure that the protocols that are being operated are
below the limits at which deterministic effects may be seen. Because of growing concern that
there is a stochastic effect for the development of lens opacification , eye lens dose reduction for
operators and patients should be of maximal interest. Dose reduction methods should be applied
when possible, without compromising clinical objective.
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