APPLICATION OF THE PIN-HOLE COLLIMATOR IN SMALL ANIMAL NUCLEAR SCINTIGRAPHY: A REVIEW

KAREN YOUNG, BS, GREGORY B. DANIEL, DVM, MS, ANNE BAHR, DVM

The pin-hole collimator is used to improve spatial resolution and magnify areas of interest. The pin-hole collimator has many applications in small animal veterinary scintigraphy. The principles of image formation for the pin-hole and parallel hole collimators are reviewed. The effects of distance on resolution and sensitivity are presented for the pin-hole and parallel hole collimators. Specific appli- cation of the pin-hole collimator in veterinary scintigraphy are discussed. Veterinary Radiology & Ul- trasound, Vol. 38, No. 2 , 1997, p p 83-93.

Key words: pin-hole collimator, nuclear medicine techniques, resolution, veterinary nuclear medicine.

Introduction

MACE CLARITY is an important and often the limiting, I factor in small animal nuclear scintigraphy . Scintigraphic images are typically minified and have poor spatial resolu- tion compared to other imaging modalities. Often the pa- tients or areas of interest are at the resolution limits of the imaging system. Factors that influence spatial resolution include, intrinsic gamma camera resolution, collimator type, radionuclide energy, image count density, patient-to- collimator distance, patient movement, and for digital im- ages, the matrix size.' For small areas of interest or small patient size, it is desirable to enlarge or magnify the image. Most imaging computers can enlarge the image after digi- talization but the image will have poor detail due to the increased pixel size (Fig. 1). More advanced computers can interpolate the digital image into a larger matrix, thus de- creasing the individual pixel size but resolution is limited to the original matrix. Magnification of the image by the col- limator is desirable because it allows the analog image to be enlarged (for microdot imagers) and permits the area of interest to be spread over a larger matrix size during the initial digitalization.

Collimator selection is an important aspect of image res- olution and image size. There are a variety of different collimators available and most veterinary nuclear medicine laboratories have more than one type of collimator. A recent survey was sent to all members of the Society of Veterinary Nuclear Medicine (Table 1) and all of the veterinary nuclear

From the Departments of Small Animal Clinical Sciences (KY, GBD) and Large Animal Clinical Sciences (AB), College of Veterinary Medi- cine, University of Tennessee, Knoxville, Tennessee.

Address correspondence and reprint request to Dr. Daniel, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, P.O. Box 1071, Knoxville, TN 37901.

Received October 9, 1995; Accepted for publication January 16, 1996.

medicine laboratories which responded (n = 18) had more than one type of collimator. The LEAP (low energy, all - purpose) collimator was the most common type, however, 14 of the facilities had a pin-hole collimator and 6 of the 14 use the pin-hole collimator routinely for feline thyroid im- aging.

When imaging small organs or sites, it is advantageous to magnify the area of interest and exclude peripheral regions from the image field. By including only the area of interest in the image field, a greater percentage of the total recorded counts come from the area of interest thus improving count density and image quality. Optimizing spatial resolution is always important when imaging small objects in which the distribution of the activity in the organ is an important cri- teria in image interpretation.

The pin-hole collimator can be used to increase the size of the image and under certain conditions it can produce a better resolution than other collimators.

Properties and Characteristics of Pin-hole Collimator

The pin-hole collimator is a conical shaped lead collima- tor with a small single aperture at the end of the cone2 (Fig. 2). The principle of image formation with the pin-hole col- limator is similar to that of an optical pin-hole camera.' Only those gamma rays that pass through the aperture of the collimator are recorded by the gamma camera. The distri- bution of gamma rays from the patient carry the image information through the collimator aperture and record that information in the gamma camera. The resultant image is upside down and the sides of the image are reversed with respect to the original. ',2

Image size, field of view, resolution, and sensitivity with the pin-hole collimator depend on the length of the cone, cone angle, object to collimator distance, and the diameter of the aperture. l X 2 To acquire an image with no magnifica-

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84 YOUNG ET AL 1997

TABLE 1. Results of a 1994 Survey of the Members of the Society of Veterinary Nuclear Medicine on the Types of Collimators Used in

Their Practice

Collimator Type Number Having

This Type Collimator ..

Pin-hole 1s (79%) LEAP 1s (79%) Low Energy High Resolution 10 (53%) Low Energy High Sensitivity 6 (32%) Medium Encrgy 12 (63%) Converging 5 (26%) Other 1(5%)

LEAP Collimator

tion with the pin-hole collimator, the distance from the ap- erture of the collimator to the imaged organ must be the same as the distance from the collimator aperture to the camera crystal. The closer the imaged organ is to the col- limator aperture, the larger the image recorded by the cam- era will be and the image field will be smaller (Fig. 3).

The designs of the pin-hole collimator vary in length (A) and the size of the aperture (B) (Fig. 2). A longer cone is superior to the shorter cone in sensitivity and resolution. The image field diameter is larger in a short cone compared

FIG. 1. Ventral images of the neck of a hyperthyroid cat acquired 20 minutes after injection of sodium pertechnetate (Na 99’nTc0,). The image at the top was acquired with a LEAP collimator. The image in the lower left is a digital enlargement of the thyroid region from the LEAP collimator acquired image. Note the boxy appearance due to the enlarged pixel size. The image in the lower right is of the same cat acquired with a pin-hole collimator.

VOL. 3 8 , No. 2 APPLICATION OF THE PIN-HOLE COLLIMATOR

FIG. 2. Line drawing of a pin-hole collimator. The length of the cone (A) and the aperture diameter of the collimator (B) effect the sensitivity and resolution of the collimator.

to a long cone collimator thus there is greater magnification of the image with a shorter cone collimator.' The diameter of the aperture affects resolution and sensitivity. As the aperture diameter decreases, number of gamma rays that

Contact

3 mm

6 mm

Distance 2.54 cm

85

P a t i e n t Camera

FIG. 3. Line drawing showing the effect of patient-to-collimator dis- tance on imagc size. As the distancc from the collimator aperture to patient increases, the image size recorded by the crystal will decrease.

pass through the collimator decreases, thus decreasing the sensitivity. Resolution, on the other hand, increases with a decrease of aperture diameter234 (Figs. 4 & 5 ) . Spatial res- olution is limited to the diameter of the aperture opening. For example, if the diameter of the pin-hole aperture is 3

from Collimator 7.62 c m 12.70 cm

4.2 mm Bar Phantom FIG. 4. Images of a bar phantom acquired with a 3 mm and 6 mm aperture pin-hole collimator at varying distances from the bar phantom. The images

were produced by placing a flood phantom containing 3 mCi of Na 99mTc0, immediately behind the bar phantom.

86 YOUNG ET AL 1997

1s - 16 - 14 - 12 - 10 -

S - 6 - 4 - 2 -

~

-

-

...

- 3 mm Pin-hole

0 ' 1 I I i I 1 I I - 0.6 3.8 7.0 10.2 13.3 16.5 19.7 22.9

Distance from Collimator (cm) FIG. 5 . Measured resolution, in terms of full width half maximum (FWHM), as a function of distance for 3 mm and 6 mm aperture pin-holc collimators.

mm then the spatial resolution will not be less than 3 mm. ',4.*

Comparison of Pin-hole to LEAP Collimator The LEAP is a multi-hole collimator which permits all

gamma rays traveling parallel to the collimator holes to be recorded by the gamma camera. The sensitivity of the LEAP collimator is not a function of distance from the patient to the collimator face2 (Fig. 6) . At increasing dis- tances, the incident angle of the gamma rays hitting the collimator surface decreases (ie, gamma rays which hit the collimator are more parallel) which allows a greater per- centage of those gamma rays to pass through the collimator openings and be recorded by the gamma camera. This off- sets the principle of the inverse square law and thus results in similar sensitivity over a wide range of collimator-to- patient distances.

The pin-hole collimator is a single hole collimator. The sensitivity of the pin-hole collimator dramatically decreases with increasing distance2 (Fig. 6). Under certain circum-

*Quantitatively, resolution limit was defined as full-width at half max- imum (FWHM) of the line-spread function. See Appendix 1,

stances, the pin-hole collimator will have greater sensitivity than the LEAP collimator. If a point source of radioactivity is centered at a distance 8 cm or less from the collimator surface, a pin-hole collimator with a 6 mm diameter aper- ture has greater sensitivity than a LEAP collimator (Fig. 6). However in practice, the radioactivity is distributed in the body over a larger area and in this situation the LEAP collimator will have greater sensitivity than a pin-hole col- limator (Fig. 7).

Spatial resolution for the LEAP and pin-hole collimators is best with short patient-to-collimator distances. Spatial resolution decreases with increasing patient-to-collimator distances' (Figs. 4 & 8). By increasing distance from the LEAP collimator to the patient, a greater percentage of emitted photons pass through the collimator openings be- cause of a lower incident angle of the emitted photons rel- ative to the collimator openings, thus decreasing the spatial resolution. The resolution of the pin-hole will be superior to the LEAP collimator when the pin-hole aperture is 4 mm or less' (Fig. 9). If a large diameter aperture is used, such as a 6 mm, the pin-hole collimator has essentially no advan- tage in resolution to the LEAP collimator.2

In many applications the pin-hole collimator is preferred to the LEAP collimator because it magnifys the images. However, if the distance from the patient is equal to cone

VOL. 38, NO. 2 APPLICATION OF THE PIN-HOLE COLLIMATOR 87

-x* LEAP -.- 3 mm Pin-hole + 5 mm Pin-hole * 6 mm Pin-hole

x a 10 - a

=

I I I I I 2.54 5.08 7.62 10.16 12.70 O+

Distance from the Source (cm) 0

FIG. 6. Sensitivity of a function of distance from the source for 3 mm? 5 mm, and 6 mm aperture pin-hole collimators and LEAP collimator. A radioactive point source of Na yy"TcO, was centered at varying distances from the collimator surfaces. Numbers were decay corrected and converted to counts per minute (CPM)

6

5 E p l r 4 u

0 0 2.54 5.08 7.62 10.16 12.70 25.40 38.10 50.80

Distance from Source (cm) FIG. 7 . Sensitivity as a function of distance from the collimator for 3 mm, 5 mm, and 6 mm aperture pin-hole collimators and a LEAP collimator. A

radioactive flood phantom source of Na 99mTc04 was centered at varying distances from the collimator surfaces. Numbers were decay corrected and converted to counts per minute (CPM) .

88 YOUNG ET AL 1997

length of the pin-hole collimator (i.e. no magnification), the LEAP is preferred because of better resolution and sensi- tivity. The field-of-view of the LEAP collimator will be constant over varying patient-to-collimator distances but the size of the image field will vary with the pin-hole collima- tor, depending on cone length and patient-to-collimator dis- tance. The image field decreases when magnifying an im- age.'

Thyroid Scintigraphy

The feline thyroid gland is difficult to evaluate using the LEAP collimator. The functional status of the gland is ob- vious but the distribution of uptake within the lobes may be impossible to discern due to the small gland size (Fig. 10). Because most cameras in use today are large-field-of-view (LFOV) cameras, the image acquired with the LEAP col- limator will include the entire head, neck, thorax and cra- nial abdomen of the cat. The area of interest (thyroid gland) occupies only a small percentage of the total image area. The thyroid lobes may overlap making it difficult to distin- F ~ ~ . 8. images a bar phantom acquired with LEAP collimator at

varying distances from the bar phantom. Thc images were produced by placing a flood phantom containing 3 mCi of Na yy"TcO, immediatcly behind the bar phantom. Noted the decrease in resolution as thc distance of the bar phantom from the collimator increases.

guish between unilateral and bilateral disease, an important criterion if surgery is considered ( ~ i ~ , 11). ~h~ distribution of uptake within the thyroid gland and the margination of the gland, which are important factors in differentiating between malignant verses benign disease, are difficult to

FIG. 9. Images of a bar phantom acquired with 3 mm aperture pin-hole and LEAP collimator. The images were produced by placing a flood phan- tom containing 3 mCi of Na 99'"Tc04 immediately behind the bar phantom. Both collimators were in contact with the bar phantom during acquisition. Note the superior resolution with the pin-hole collimator images. Individ- ual bars can be seen down to 2.1 mm using the pin-hole collimator. Resolution was defined as a drop of at least 50% of the peak values over the lead bars and as such the resolution limit of this pin-hole was calculated to be closer to 3 mm' (See Fig. 5 ) .

FIG. 10. Ventral views of the neck of two hyperthyroid cats (A & B) acquired 20 minutes after injection of sodium pertechnetate (Na ""'TcO,). The images on the left were acquired with a LEAP collimator and the images on the right with a 3 mm aperture pin-hole collimator. Note the superior detail of the thyroid gland in the pin-hole acquired images.

VOL. 38, No. 2 APPLTCATION OF THE PIN-HOLE COLLIMATOR 89

FIG. 11. Ventral views of the neck of a hyperthyroid cat acquired 20 minutes after injection of sodium pertechnetate (Na 99””Tc0,). The images on the left were acquired with a LEAP collimator and the images on the right with a 3 mm aperture pin-hole collimator. Note the thyroid lobes overlap on the LEAP collimator acquired image and appear as a single foci of uptake but the lobes are visible as separate structures on the pin-hole collimator image.

discern using the LEAP collimator5 (Fig. 12). A high res- olution collimator will improve spatial resolution but will not magnify the image. The pin-hole collimator will mag- nify the image to include only the area of the thyroid gland. If a small aperture pin-hole collimator is used, the spatial resolution will be maximized. The distribution of uptake within the gland and the margination of the gland are easier to evaluate from a pin-hole collimator acquired image. By increasing the collimator to patient distance, the image can be minified to include the salivary gland thus allowing as- sessment of the functional status. We routinely acquire ven- tral and, if necessary, ventral oblique views of the thyroid gland using the pin-hole collimator and lateral views of the

neck and ventral and lateral views of the thorax with the LEAP collimator. To avoid having to change collimators during the procedure we use two cameras, one with the pin-hole collimator and the other with the LEAP collimator. In a facility with only one camera, collimators could be switched between images or the pin-hole collimator used to acquire both the cervical and thoracic images. By changing the collimator-to-patient distance, the degree of magnifica- tion will vary to include the desired areas of interest such as the entire thorax. At collimator-to-patient distances of less than or equal to 8 cm, the spatial resolution and image quality of the 3 4 mm aperture pin-hole collimator will be comparable to the LEAP collimator in contact with the pa-

90 YOUNG ET AL I997

FEG. 12. Ventral views of the neck of two hypeithyroid cats (A & B) acquired 20 minutes after injection of sodium pertechnetate (Na yymTc04). Both cats have thyroid carcinoma. The images on the left were acquired with a LEAP collimator and the images on the right with a 3 mm aperture pin-hole collimator. Note the superior detail of the thyroid gland in the pin-hole acquired images.

VOL. 38, No. 2 APPLICATION OF THE PIN-HOLE COLLIMATOR 91

Feline MUGA LEAP Collimator Pin-Hole Collimator

FIG. 13. Left lateral views of the thorax of a cat ac- quired 20 minutes following injection of Na 9y’”Tc04. The cat had been given stannous pyrophosphate 20 minutes prior to the pertechnetate injection for an in-vivo red blood cell label. The image on the left was acquired using a LEAP collimator and the image on the right with a 3 mm aperture pin-hole collimator positioned 5 cm from the thorax. Note the superior visualization of the interventric- ular septum on the pin-hole acquired image.

tient. By increasing the distance from the pin-hole collima- tor to the patient, the field-of-view can be made to include the entire thorax or head and neck region but at the cost of decreased sensitivity.

Nuclear Cardiology

In small dogs and cats it is difficult to evaluate individual cardiac chambers using the LEAP collimator. It is necessary to visually separate the left and right ventricle in both first pass and equilibrium blood pool studies of the heart. The appearance of radioactivity in the right ventricle during the levophase of a first pass study is an important finding in diagnosis of ventricular septa1 defects. If the right ventricle cannot be resolved then the type of left-to-right shunt cannot be determined. Analysis of the gated equilibrium pool im- age requires the placement of regions of interests around the left and right ventricles for the creation of time activity curves. These time activity curves represent ventricular vol- ume curves from which the indices of ventricular function are derived. If the margins of the left and right ventricle cannot be resolved, then manual and even computer assisted methods of ROI placement will not be reliable. The pin- hole collimator will magnify the image as needed. By vary- ing the patient-to-collimator distance, the operator can

change the field to view to include just the heart and sur- rounding thorax and cranial abdomen (Fig. 13). By magni- fying the area of interests and maximizing spatial resolution using a small aperture pin-hole collimator, functional im- ages of the heart can be obtained and indices of ventricular function determined in small dogs and cats.

Exotic Animal Images The increasing popularity of avian and reptilian species

as pet animals has resulted in a greater demand for advanced diagnostic test in the evaluation of these patients. The pin- hole collimator is very useful in obtaining the optimal qual- ity images of these animals. Resolution is often the limiting factor in the ability to acquire diagnostic images and the pin-hole collimator is a valuable tool.

Conclusion The pin-hole collimator is useful in small animal nuclear

scintigraphy to maximize spatial resolution and to enlarge the area of interest. We routinely use the pin-hole collimator for feline thyroid, feline cardiac, and exotic animal scintig- raphy . Other potential applications include bone scintigra- phy of the coxofemoral joint in patients with ischemic ne- crosis of the femoral head or establishing bone viability or sequestration.

REFERENCES

1 . Erickson J . Imaging Systems. In: Harbert J , Da Rocha AFG. Text- hook of Nuclear Medicine, Volume I 2nd ed. Lea and Febiger, Philadel- phia, 1984: 105-1 59.

the American College of Veterinary Radiology. Chicago, IL Dec 1-4, 1993.

Appendix 1: Method for Measuring the Spatial Resolution of an Imaging System

2. Tsui BMW. Collimator Design, Properties, and Characteristics. In: Simmons GH. The Scintillation Camera. The Society of Nuclear Medi- cine, 1988: 17-45,

3 . Tsui BMW, Gunter DL, Beck RN. Physics of Collimator Design. In: Gottschalk A, Hoffer PB, Potchen EJ. Diagnostic Nuclear Medicine. Bal- timore: Williams and Wilkins, 1988:42-54.

~ ~ l l width Haif ~~i~~~ (FWHM) 4. Wilson RJ. Collimator Technology and Advancements. Journal of A discrete line or point of radioactivity recorded by a

The spatial resolution can be determined from the width of

Nuclear Medicine Technology 1988;4: 198-203.

graphic appearance of feline thyroid carcinoma. Proc Annual Meeting of 5. Daniel GB, Poteet B, Peterson M, Adams W, Arrington K . Scinti- gamma appear wider than line Or point‘

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YOUNG ET A L

FWHM 50%

1997

1OOVC

50%

0%

A

100%

50 YG

0%

1 50% 50%

0%

1 I 0%

B C

100 %

50 o/c

100%

50%

2.5mm 7.1 mm FIG. 14. Ventral images of the neck of a hyperthyroid cat acquired 20 minutes after injection of sodium pertechnetate (Na '"'"TcO,). The image at the

top was acquircd with a LEAP collimator. Thc image in the lower left is a digital enlargement of the thyroid region from the LEAP collimator acquired image. Note the boxy appearance due to thc enlarged pixel size. The image in the lower right is of the bame cat acquired with a pin-hole collimator.

the line or point on the recorded image. A pixel intensity profile can be made through the radioactive source and dis- played as a graph. The x-axis represents distance and the y-axis the counts per pixel. In an ideal system, the pixel intensity profile would appear as a single spike (Fig. 14A ).

In actuality, the pixel intensity profile will appear as a peak shaped curve. The higher the resolution the narrower the base of the peak shaped curve (Fig. 14B). As resolution decreased the peak will have a broader base (Fig. 14C). Full width half maximum (FWHM) is the width of the curve

A B

FIG. 15. Line drawing of a pin-hole collimator. The length of the cone (A) and the aperture diameter of the collimator (B) effect the sensitivity and resolution of the collimator.

VOL. 38, No. 2 APPLICATION OF THE PIN-HOLE COLLIMATOR 93

(usually measured in mm) at the points of SO% maximum y value. A high resolution system will have a small FWHM whereas a poor resolution system will have a large FWHM. Spatial resolution describes the ability of the imaging sys- tem to see two objects as being separate. The if objects are separated by a distance less than the FWHM, they with blur together. Figure 1SA shows two discrete lines imaged with a poor resolution system. As the objects become closer than the FWHM it becomes hard to separate them as separate objects. Figure 15B shows two discrete lines imaged with a

high resolution system, note the line can be closer together than in Figure 1SA and still be seen as separate.

All resolution measurements made in this paper were made from a piece of 2-0 silk suture material soaked in pertechnetate as the radioactive line source. An image of the suture material was acquired with the various collimators at various distances. The images were stored in a 512 X 512 X 16 matrix. Line profiles were made through the string and the width of the line profile at 50% of it maximum y value were used as FWHM.