Experimental validation of dose calculation algorithms for the GliaSite RTS, a novel 125I liquid-filled balloon brachytherapy applicator.

This paper compares experimentally measured and calculated dose-rate distributions for a novel 125I liquid-filled brachytherapy balloon applicator (the GliaSite RTS), designed for the treatment of malignant brain-tumor resection-cavity margins. This work is intended to comply with the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee's recommendations [Med. Phys. 25, 2269-2270 (1998)] for dosimetric characterization of low-energy photon interstitial brachytherapy sources. Absolute low dose-rate radiochromic film (RCF) dosimetry measurements were performed in coronal planes about the applicator. The applicator was placed in a solid water phantom, machined to conform to the inflated applicator's surface. The results were used to validate the accuracy of Monte Carlo photon transport (MCPT) simulations and a point-source dose-kernel algorithm in predicting dose to water. The absolute activity of the 125I solution was determined by intercomparing a National Institute of Standards and Technology (NIST) 125I standard with a known mass of radiotherapy solution (Iotrex) in an identical vial and geometry. For the two films not in contact with applicator, the average agreement between RCF and MCPT (specified as the mean absolute deviation in successive 4 mm rings) was found to be within +/-5% at distances 0.2-25 mm from the film centers. For the two films touching the catheter, the mean agreement was +/-14.5% and 7.5% near the balloon surface but improving to 7.5% and 6% by 3.5 mm from the surface. These errors, as large as 20% in isolated pixels, are likely due to trim damage, 125I contamination, and poor conformance with the balloon. At larger distances where the radiation doses were very low, the observed discrepancies were significantly larger than expected. We hypothesize that they are due to a dose-rate dependence of the RCF response. A 1%-10% average difference between a simple one-dimensional path-length semiempirical dose-kernel model and the MCPT calculations was observed over clinically relevant distances.

Biodistribution and dosimetry of an aqueous solution containing sodium 3-(125I)iodo-4-hydroxybenzenesulfonate (Iotrex) for brachytherapy of resected malignant brain tumors.

UNLABELLED: Iotrex is an aqueous radiotherapy solution containing sodium 3-(125I)iodo-4-hydroxybenzenesulfonate (125I-HBS), which is used as the radiation source for the brachytherapy of resected of brain tumor cavity margins with the GliaSite catheter. During routine clinical use of this brachytherapy applicator and radiation source, approximately 0.1% of the afterloaded Iotrex will diffuse through the GliaSite balloon. Our purpose was to assess the radiation doses to normal organs under routine clinical use of the GliaSite. METHODS: Five groups of rats received intracerebral injections of an 131I-HBS solution (131I used as a surrogate for 125I in the synthesis of 125I-HBS) with one group sacrificed at 15 minutes, 30 minutes, 1 hour, 2 hours and 4 hours post-administration. Urine was collected and activity retention in numerous organs was measured. The biodistribution data were used to estimate radiation doses to normal organs of the Reference Adult Male and Female phantoms. RESULTS: Radioactivity was rapidly and completely cleared from the brain (98% cleared by 2 hours) and total body (urinary clearance; 93%@2 hours). No organ retained > 0.7% of the radioactivity at 4 hours. For 100% loss of the radiotherapy solution from the balloon catheter (device failure), all organs would receive less than 100 mGy (10 rad), except the bladder wall (2800 mGy, 280 rad), uterus (130 mGy, 13 rad) and distal colon (270 mGy, 27 rad). Under normal conditions, all organ doses are 1000-fold lower (< 3 mGy or 0.3 rad). CONCLUSIONS: Under routine clinical conditions, the radiation doses to normal organs are inconsequential. Should the maximum clinical load of Iotrex (16.7 GBq of 125I) be released intracerebrally, the radiation doses to all organs would be below the thresholds for deterministic effects.

Radiation dosimetry of a 99mTc-labeled IgM murine antibody to CD15 antigens on human granulocytes.

UNLABELLED: 99mTc-labeled anti-stage specific embryonic antigen-1 (anti-SSEA-1) is an injectable IgM antibody derived from mice. It binds to CD15 antigens on some granulocytic subpopulations of human white blood cells in vivo after systemic administration. The purpose of this study was to measure biodistribution of 99mTc-labeled anti-SSEA-1 and perform radiation dosimetry in 10 healthy human volunteers. METHODS: Transmission scans and whole-body images were acquired sequentially on a dual-head camera for 32 h after the intravenous administration of about 370 MBq (10.0 mCi) of the radiopharmaceutical. Renal excretion fractions were measured from 10 to 14 discrete urine specimens voided over 27.9 +/- 2.0 h. Multiexponential functions were fit iteratively to the time-activity curves for 17 regions of interest using a nonlinear least squares regression algorithm. The curves were integrated numerically to yield source organ residence times. Gender-specific radiation doses were then estimated individually for each subject, using the MIRD technique, before any results were averaged. RESULTS: Quantification showed that the kidneys excreted 39.5% +/- 6.5% of the administered dose during the first 24 h after administration. Image analysis showed that 10%-14% of the radioactivity went to the spleen, while more than 40% went to the liver. Residence times were longest in the liver (3.37 h), followed by the bone marrow (1.09 h), kidneys (0.84 h) and the spleen (0.65 h). The dose-limiting organ in both men and women was the spleen, which received an average of 0.062 mGy/MBq (0.23 rad/mCi, range 0.08-0.30 rad/mCi), followed by the kidneys (0.051 mGy/MBq), liver (0.048 mGy/MBq) and urinary bladder (0.032 mGy/MBq). The effective dose equivalent was 0.018 mSv/MBq (0.068 rem/mCi). CONCLUSION: The findings suggest that the radiation dosimetry profile for this new infection imaging agent is highly favorable.

MIRD pamphlet no. 16: Techniques for quantitative radiopharmaceutical biodistribution data acquisition and analysis for use in human radiation dose estimates.

This report describes recommended techniques for radiopharmaceutical biodistribution data acquisition and analysis in human subjects to estimate radiation absorbed dose using the Medical Internal Radiation Dose (MIRD) schema. The document has been prepared in a format to address two audiences: individuals with a primary interest in designing clinical trials who are not experts in dosimetry and individuals with extensive experience with dosimetry-based protocols and calculational methodology. For the first group, the general concepts involved in biodistribution data acquisition are presented, with guidance provided for the number of measurements (data points) required. For those with expertise in dosimetry, highlighted sections, examples and appendices have been included to provide calculational details, as well as references, for the techniques involved. This document is intended also to serve as a guide for the investigator in choosing the appropriate methodologies when acquiring and preparing product data for review by national regulatory agencies. The emphasis is on planar imaging techniques commonly available in most nuclear medicine departments and laboratories. The measurement of the biodistribution of radiopharmaceuticals is an important aspect in calculating absorbed dose from internally deposited radionuclides. Three phases are presented: data collection, data analysis and data processing. In the first phase, data collection, the identification of source regions, the determination of their appropriate temporal sampling and the acquisition of data are discussed. In the second phase, quantitative measurement techniques involving imaging by planar scintillation camera, SPECT and PET for the calculation of activity in source regions as a function of time are discussed. In addition, nonimaging measurement techniques, including external radiation monitoring, tissue-sample counting (blood and biopsy) and excreta counting are also considered. The third phase, data processing, involves curve-fitting techniques to integrate the source time-activity curves (determining the area under these curves). For some applications, compartmental modeling procedures may be used. Last, appendices are included that provide a table of symbols and definitions, a checklist for study protocol design, example formats for quantitative imaging protocols, temporal sampling error analysis techniques and selected calculational examples. The utilization of the presented approach should aid in the standardization of protocol design for collecting kinetic data and in the calculation of absorbed dose estimates.

Biodistribution and dosimetry of TRODAT-1: a technetium-99m tropane for imaging dopamine transporters.

UNLABELLED: Technetium-99m TRODAT-1 is an analog of cocaine that selectively binds the presynaptic dopamine transporters. The primary purpose of this study was to measure its whole-body biokinetics and radiation dosimetry in healthy human volunteers. The study was conducted within a regulatory framework that required its pharmacological safety to be assessed simultaneously. METHODS: The sample included 4 men and 6 women ranging in age from 22-54 yr. An average of 20 whole-body scans were acquired sequentially on a dual-head camera for up to 46 hr after the intravenous administration of 370+/-16 MBq (10.0+/-0.42 mCi) 99mTc TRODAT. The renal excretion fractions were measured from 12-24 discrete urine specimens. The fraction of the administered dose in 17 regions of interest and each urine specimen was quantified from the attenuation and background corrected geometric mean counts in conjugate views. Multiexponential functions were iteratively fit to each time-activity curve using a nonlinear, least squares regression algorithm. These curves were numerically integrated to yield source organ residence times. Gender-specific radiation doses were then estimated with the Medical Internal Radiation Dose technique for each subject individually before any results were averaged. RESULTS: There were no pharmacological effects of the radiotracer on any of the subjects. The early planar images showed differentially increased activity in the nose, pudendum and stomach. SPECT images demonstrated that the radiopharmaceutical localized in the basal ganglia in a distribution that was consistent with selective transporter binding. Image analysis showed that the kidneys excreted between 20% and 32% of the injected dose during the first 22-28 hr postadministration, after which no more activity could be recovered in the urine. The dose limiting organ in both men and women was the liver, which received an average of 0.046 mGy/MBq (0.17 rads/mCi, range 0.14-0.22 rad/mCi). In the worst case, which was clearly an over-estimation, it would have taken 22.7 mCi to deliver 5 rad to the liver. CONCLUSION: TRODAT may be a safe and effective radiotracer for imaging dopamine transporters in the brain and the body.

Radiation absorbed doses to the walls of hollow organs.

UNLABELLED: Many radiopharmaceuticals are excreted from the body through the gastrointestinal (GI) tract. The doses to the walls of the organs involved often are very significant. As significant fractions of the administered activity pass through them, these organs may receive the highest doses in the body for many radiopharmaceuticals. The absorbed dose to these walled organs, from activity in their contents, is typically calculated as 50% of the average absorbed dose to the contents, for nonpenetrating emissions. The internal surface of the GI tract, and to a certain extent the urinary bladder, is lined with a variable thickness of mucus. In addition, the radiosensitive cell populations (crypt or stem cells) are located at some depth into the mucosa. These two factors suggest that the surface dose, often used to characterize the clinically relevant absorbed doses for walled organs, may represent an overestimate in some cases. METHODS: In this study, the radiation transport code MCNP was used to simulate the deposition of energy from nonpenetrating emissions of several radionuclides of interest: 90Y, 99mTc,123I and 131I. Absorbed doses as a function of distance from the wall-contents interface were calculated for three geometric shapes representing different organs along the routes of excretion. RESULTS: The absorbed dose from nonpenetrating emissions to the sensitive cell populations was consistently lower than estimated by the standard model assumption. The simulated absorbed doses to radiosensitive cells in the GI tract for 99mTc and 123I are tenfold lower; those for 131I are fivefold lower and those for 90Y are 20% lower. CONCLUSION: This study demonstrates that the normally reported dose to the walls of hollow organs probably should be modified to account for the attenuation of these nonpenetrating emissions in the linings of the walls. This study also demonstrates that Monte Carlo codes continue to be useful in the evaluation of the dose to sensitive cells in walled organs.

Dosimetric properties of a novel brachytherapy balloon applicator for the treatment of malignant brain-tumor resection-cavity margins.

PURPOSE: This paper characterizes the dosimetric properties of a novel balloon brachytherapy applicator for the treatment of the tissue surrounding the resection cavity of a malignant brain tumor. METHODS AND MATERIALS: The applicator consists of an inflatable silicone balloon reservoir attached to a positionable catheter that is intraoperatively implanted into the resection cavity and postoperatively filled with a liquid radionuclide solution. A simple dosimetric model, valid in homogeneous media and based on results from Monte Carlo photon-transport simulations, was used to determine the dosimetric characteristics of spherical geometry balloons filled with photon-emitting radionuclide solutions. Fractional depth-dose (FDD) profiles, along with activity densities, and total activities needed to achieve specified dose rates were studied as a function of photon energy and source-containment geometry. Dose-volume histograms (DVHs) were calculated to compare idealized balloon-applicator treatments to conventional 125I seed volume implants. RESULTS: For achievable activity densities and total activities, classical low dose rate (LDR) treatments of residual disease at distances of up to 1 cm from the resection cavity wall are possible with balloon applicators having radii between 0.5 cm and 2.5 cm. The dose penetration of these applicators increases approximately linearly with balloon radius. The FDD profile can be made significantly more or less penetrating by combining selection of radionuclide with source-geometry manipulation. Comparisons with 125I seed-implant DVHs show that the applicator can provide a more conformal therapy with no target tissue underdosing, less target tissue overdosing, and no healthy tissue "hot spots;" however, more healthy tissue volume receives a dose of the prescribed dosage or less. CONCLUSIONS: This device, when filled with 125I solution, is suitable for classical LDR treatments and may be preferable to 125I interstitial-seed implants in several respects. Manipulation of the dosimetric properties of the device can improve its characteristics for brain tumor treatment and may make it suitable for boosting the lumpectomy margins in conservative breast cancer treatment.

Review of the fetal radiation doses received from 59Fe kinetic studies at Vanderbilt University in the 1940's.

Fetal radiation dose estimates were calculated for women who received oral administrations of 59Fe at Vanderbilt University in the 1940's. A similar dosimetry analysis was performed by Dyer and Brill in 1972; the availability of recently developed physical models of the pregnant female at different stages of gestation and of a new kinetic model for iron metabolism afforded an opportunity to re-evaluate these doses. Radiation dose estimates for the fetus (and fetal liver in three cases) were calculated for a number of oral and intravenous studies using these new models and the available data about the original experiments. The total fetal doses do not appear to have exceeded a few hundred microgray (a few tens of millirad) for the majority of the study subjects. Considerable uncertainty exists, however, in the amount of activity administered to these subjects.

Dosimetry of an iodine-123-labeled tropane to image dopamine transporters.

UNLABELLED: N-(3-Iodopropen-2-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl)tropane (IPT) is an analog of cocaine that selectively binds the presynaptic dopamine transporter. The present study sought to measure the radiation dosimetry of IPT in seven healthy human volunteers. METHODS: Dynamic renal scans were acquired immediately after the intravenous administration of 165 +/- 16 MBq (4.45 +/- 0.42 mCi) of [123I]IPT. Between 7 and 12 sets of whole-body scans were acquired over the next 24 hr. The 24-hr renal excretion fractions were measured from conjugate emission scans of 7-11 discreet voided urine specimens. The fraction of the administered dose in 11 organs and each urine specimen was quantified from the attenuation-corrected geometric mean counts in opposing views. Subject-specific residence times were calculated for each subject independently by fitting the time-activity curves to a multicompartmental model. The radiation doses were estimated with the MIRD technique from the residence times for each subject individually before any results were averaged. RESULTS: The findings showed that IPT was excreted rapidly by the renal system. There were no reservoirs of retained activity outside the basal ganglia, where SPECT images in these subjects showed that the mean ratio of caudate to calcarine cortex averaged 25:1 at 3 hr after injection (range 19.6-32 hr). The basal ganglia received a radiation dose of 0.028 mGy/MBq (0.10 rad/mCi). The dose-limiting organ in men was the stomach, which received an estimated 0.11 mGy/MBq (0.37 rad/mCi). In women, the critical organ was the urinary bladder at 0.14 mGy/MBq (0.51 rad/mCi). CONCLUSION: Relatively high-contrast images of the presynaptic dopamine transporters in the basal ganglia can be acquired with 185 MBq (5 mCi) of [123I]IPT. The radiation exposure that results is significantly less than the maximum allowed by current safety guidelines for research volunteers.

Human biodistribution and dosimetry of the SPECT D2 dopamine receptor radioligand [123I]IBF.

The research discussed in this article aimed to characterize better the biodistribution, excretion and radiation dosimetry of the single photon emission computed tomography (SPECT) D2 Dopamine receptor radioligand [123I]IBF. Following administration of 111 +/- 12 MBq [123I]IBF, seven healthy human subjects were scanned serially with a whole body imager over a 48-h period. Transmission images were obtained with a scanning line source for attenuation correction of the emission images. Urine was collected for 48 h to measure the fraction of activity voided by the renal system. Radiation absorbed dose estimates were performed using biokinetic modeling and the Medical Internal Radiation Dose (MIRD) schema. Highest absorbed doses were to the kidney (0.13 +/- 0.02 mGy/MBq) and urinary bladder wall (0.11 +/- 0.01 mGy/MBq). The effective dose equivalent was 0.041 +/- 0.005 mSv/MBq. Peak brain uptake represented 8% of the injected activity. Rapid urinary excretion minimized the absorbed dose to most tissues. The mean cumulative urinary excretion fraction was 69%. Thus [123I]IBF is a promising SPECT agent for imaging the D2 dopamine receptor in humans with high brain uptake and favorable dosimetry.

Dosimetry of a D2/D3 dopamine receptor antagonist that can be used with PET or SPECT.

UNLABELLED: FIDA-2 (R-(+)-2,3-dimethoxy-5-iodo-N-[(1-4'-fluorobenzyl)-2-pyrrolidinyl) methyl] benzamide) is a simultaneously fluorinated and iodinated D2/D3 dopamine receptor antagonist. The purpose of this study was to measure its biodistribution and radiation dosimetry in humans. METHODS: Whole-body emission scans were sequentially acquired in eight healthy volunteers 24-43 hr after the intravenous administration of 101-150 MBq 123I FIDA-2. Regions of interest (ROIs) were placed on the initial set of conjugate emission images and transposed as a single set onto all the other scans without manipulating any of the regions for solid organs independently. The counts in each ROI were corrected for attenuation with transmission scans and compared to the net counts in images of the injection syringe containing the administered dose. The radiation doses were estimated with the MIRD formalism from the residence times for both the 18F- and 123I-labeled ligands. RESULTS: There were no subjective or objective pharmacological effects of the tracer on any of the subjects. The findings showed that the dose-limiting organ for the 123I-labeled product was the thyroid gland in this sample. If the 18F-labeled product had been used, then the urinary bladder would have received 0.086 mGy/MBq (0.32 rads/mCi) and become the dose-limiting organ. The effective dose equivalents were 0.025 mSv/MBq (0.092 rem/mCi) for both the 123I- and the 18F-labeled versions of the tracer. CONCLUSION: The data suggest that FIDA-2 can be used to produce relatively high contrast images of the D2/D3 dopaminergic system with substantially less than the maximum allowable radiation dose for research volunteers.

Specific absorbed fractions of energy from internal photon sources in brain tumor and cerebrospinal fluid.

Transferrin, when injected intracranially into glioblastoma multiforme lesions, acts as a cytotoxic substance. Transferrin, radiolabeled with In-111, can be coinjected and subsequent scintigraphic imaging can demonstrate the biokinetics of the cytotoxic transferrin. The administration of 111In transferrin into a brain tumor results in distribution of radioactivity in the brain, brain tumor, and the cerebrospinal fluid (CSF). Information about absorbed radiation doses to these regions, as well as other nearby tissues and organs, is important for evaluating radiation-related risks from this procedure. The radiation dose is usually estimated for a mathematical representation of the human body. We have included source/target regions for the eye, lens of the eye, spinal column, spinal CSF, cranial CSF, and a 100-g tumor within the brain of an adult male phantom developed by Cristy and Eckerman. The mathematical models of the spinal column, spinal CSF, and the eyes were developed previously, however, these source/targets have not been routinely included in photon transport simulations. Specific absorbed fractions (SAFs) as a function of photon energy were calculated using the ALGAMP computer code, which utilizes Monte Carlo techniques for simulating photon transport. The ALGAMP code was run three times, with the source activity distributed uniformly within the tumor, cranial CSF, and the spinal CSF volumes. These SAFs, which were generated for 12 discrete photon energies ranging from 0.01 to 4.0 MeV, were used with decay scheme data to calculate S-values needed for estimating absorbed doses. S-values for 111In are given for three source regions (brain tumor, cranial CSF, and spinal CSF) and all standard target regions/organs, the eye and lens, as well as to tissues within these source regions. S-values for the skeletal regions containing active marrow are estimated. These results are useful in evaluating the radiation doses from intracranial administration of 111In transferrin. The SAFs are also generally useful for calculation of absorbed dose from any radionuclide in these source regions.

Human biodistribution and dosimetry of the SPECT benzodiazepine receptor radioligand iodine-123-iomazenil.

UNLABELLED: SPECT imaging of the brain with [123I]iomazenil has shown avid uptake of the radioligand in a distribution consistent with benzodiazepine receptor binding. The purposes of this study were to measure the whole-body distribution of activity following i.v. administration of [123I]iomazenil and to evaluate the resulting organ radiation burdens. METHODS: Serial total body scans were obtained in healthy volunteers after thyroid blockade and demonstrated avid brain uptake of radioligand. RESULTS: Abdominal imaging showed significant activity retention within the urinary and gastrointestinal tracts consistent with excretion via these routes. Absorbed dose to the urinary bladder was calculated to be 0.19 mGy/MBq, to the lower large intestine 0.079 mGy/MBq, to the upper large intestine 0.066 mGy/MBq, and to the thyroid 0.063 mGy/MBq. CONCLUSION: Thyroid uptake may in part have represented binding to benzodiazepine receptors, since radioligand binding to tissue homogenates prepared from human thyroid showed the presence of benzodiazepine binding sites.

Biodistribution and dosimetry of iodine-123-IBF: a potent radioligand for imaging the D2 dopamine receptor.

Iodine-123-labeled iodo-benzofuran (IBF) is a potent D2 dopamine receptor antagonist that has been developed as a potential SPECT imaging agent. This report documents its biodistribution and radiation dosimetry in seven healthy humans. Approximately 100 MBq of IBF were administered to each volunteer. Urine was collected to measure the fraction of the activity that was voided by the renal system. Conjugate images were serially acquired over 24 hr to determine the fraction of activity in the other organs. Standard image analysis techniques were used to measure the geometric mean count rates in the brain, GI tract, heart, liver, lungs and thyroid at each time point. Corrections for attenuation were made with 123I transmission scans. Multicompartmental modeling was used to stimulate and predict the biokinetic behavior of 123I-IBF in the rest of the body. The absorbed doses for 24 organs were then estimated with the MIRD formalism. Rapid biological washout minimized the absorbed dose to most tissues. The excretory organs were exposed to the most radiation. The lower large intestine received about 0.13 mGy/MBq (0.48 rad/mCi), and the urinary bladder received 0.11 mGy/MBq. This low radiation burden will allow more than 370 MBq (10 mCi) to be administered to healthy research subjects during each study of the D2 receptor. Since high quality images of the brain can be obtained with half this amount, the findings suggest that 123I-labeled IBF has a large margin of radiation safety in humans. Its stability in vivo and its high target-to-background contrast ratio in the human brain may make it a useful SPECT imaging agent.

Dosimetry of iodine-123 iomazenil in humans.

The distribution of the central benzodiazepine receptor specific ligand iodine-123 iomazenil was investigated in seven human adults from whole-body scans, blood samples and urine collected up to 24 h after injection. Using 12 source organs, the MIRD method was applied to calculate the absorbed radiation dose of the radioligand in various organs. The urinary bladder wall (0.15 mGy/MBq), lower large intestinal wall (0.071 mGy/MBq) testes (0.044 mGy/MBq) and upper large intestined wall (0.038 mGy/MBq) received the highest absorbed doses. The average effective dose equivalent of 123I-IBZM for adults was estimated to be 0.033 mSv/MBq.

Radiation dose estimates for the carbon-14-labeled urea breath test.

The use of the 14C-urea breath test for diagnosis of Helicobacter pylori infection in gastric mucosa has gained widespread acceptance and utilization. In order to obtain regulatory approval for this procedure, new dose estimates were required. Previous radiation dose equivalent estimates for males only were based upon data published in 1975 for bicarbonate metabolism. Since that time, calculational techniques for dose estimation have been significantly improved and the organ masses of Reference Man updated. We have calculated dose estimates for males and females who test positive (HP+) and negative (HP-) for gastric H. pylori infection. Our results indicate that the urinary bladder wall receives the highest absorbed dose in all four of the above subject populations (HP- males = 0.14 mGy/MBq; HP- females = 0.19 mGy/MBq; HP+ males = 0.10 mGy/MBq; HP+ females = 0.14 mGy/MBq). Gonadal absorbed doses were similar to those previously estimated (testes < 0.065 mGy/MBq and ovaries < 0.084 mGy/MBq, respectively).

A noninvasive scintigraphic assessment of the colonic transit of nondigestible solids in man.

A noninvasive, scintigraphic technique for quantifying large intestinal transit time that provides low radiation doses was developed. The scintigraphic large intestinal transit (SLIT) method uses a total of 100 microCi of 111In encapsulated in ten 2-cm nondigestible capsules, which are ingested after a 6-hr fast. Two hundred fifty microcuries of 99mTc-sulfur colloid were given to outline the gastrointestinal tract. Images were acquired at 4-hr intervals until all capsules were excreted. Normal volunteers (n = 10) consumed a standardized diet 2 days prior and during imaging. Segmental transit times were measured in the following: ascending, transverse, descending, recto-sigmoid colons; hepatic and splenic flexures. The radiation absorbed dose to the large intestine for the SLIT technique is less than half of that associated with other radiographic methods of colonic transit time measurement.