Half-Fanbeam Collimators Combined with Scanning Point Sources fo...

Half-Fanbeam Collimators Combined with Scanning

Point Sources for Simultaneous Emission-

Transmission Imaging

Freek J. Beekman, Chris Kamphuis, Brian F. Hutton and Peter P. van Rijk

Department of Nuclear Medicine, Image Sciences Institute, University Hospital Utrecht, Utrecht, The Netherlands; and

Department of Medical Physics, Westmead Hospital, Sydney, Australia

One type of SPECT system often used for simultaneous emission-

transmission tomography is equipped with parallel-hole collimators,

moving line sources (MLS) and electronic windows that move in

were replaced by two 153Gd point sources of only 750 MBq each

and positioned to move along the focal lines of two half-fanbeam

collimators. A suitable acquisition protocol for a moving point source

(MPS) system was selected by considering the results of a simula

tion study. With this protocol, physical phantom experiments were

conducted. Results: Simulations showed that by using two half-

fanbeam collimators, a gantry rotation of 90°,such as used for 180°

acquisition with parallel-beam collimators for cardiac imaging, was

insufficient. A gantry rotation of 180°resulted in attenuation maps

where only an area to the posterior of a 400-mm wide thorax

electronic window (100 keV) was reduced by a factor of 1.81.

Transmission images of a rod phantom with segments containing

acrylic rods of different diameters showed an improvement of

resolution in favor of the MPS system from about 11 mm to about 6

mm (five instead of two segments of rods were clearly visible). In

addition, the noise level in the MPS thorax transmission images was

significantly lower. Conclusion: The MPS system has important

advantages when compared with the MLS system. The use of

low-activity point sources is economically beneficial when com

pared with line sources and reduces radiation exposure to staff and

Accurate correction for attenuation can be performed when the

density distribution of the patient is known [for example from

transmission CT (TCT) images]. Attenuation correction using

separate transmission CT is difficult, since the three-dimen

sional radiograph image needs to be registered accurately to the

SPECT scan. Much effort has been expended during the last

decade in developing SPECT systems that acquire TCT data

simultaneously with the emission data [emission CT (ECT)-

Received Aug. 5, 1997; revistan accepted Feb. 18, 1998.

For correspondence or reprints contact: Freek J. Beekman PhD, University Hospital,

infectious foci), for registration of images from other imaging

modalities, for dose calculations (2-4) and attenuation, map-

based, scatter correction (5-11). A large variety of hardware

configurations and reconstruction algorithms for combined

ECT-TCT imaging are summarized in (12,13).

One class of ECT-TCT systems uses parallel-hole collimators

combined with sheet sources (14-19) (Figure 1A). To reduce

both radiation dose to the patient and scatter and to improve

resolution, the sheet source may be collimated (14,20). ECT-

TCT systems equipped with parallel-hole collimators have been

improved significantly by using high-intensity, moving line

a low number of transmission counts is obtained and a high

amount of downscatter from the emission distribution is de

tected. When the transmission source is a concentrated moving

line source, the same amount of transmission counts are

acquired in the moving electronic window (Fig. IB), but the

downscatter contribution is lower since downscatter now is

acquired only during a fraction of the total acquisition time, as

defined by the ratio of the electronic window width to the field

size. Therefore, the moving line source (MLS) system permits

improved separation of ECT and TCT data and possible use of

subject in projection images is avoided in nearly all cases

provided the camera field size is sufficiently large. The disad

vantage is that the required activity of a fresh 153Gdline source

is relatively high (typically 100-250 mCi). Even with this

source activity, very few TCT counts are acquired in some

pixels when large patients are scanned. This leads to problems

in reconstruction, especially when these low-count pixels have

to be corrected for downscatter from 99mTc. Regular replace

ment of 153Gd line sources (half-life 241.6 days) is expensive,

and difficulties have been experienced by some departments in

obtaining licenses for the use of these potent sources. The