Ralph A. A. Khan

Fluid Space in Bone

This paper reviews the extraction of solutes through capillaries in bone, by means of outflow dilution techniques in the canine tibia. Solutes leave capillaries in bone by passive diffusion dependent on molecular size. The fluid space in bone is also examined by means of analyzing washout curves of radioactive substances. It is proposed that there are 4 definable compartments in bone; a vascular space, a perivascular fluid space a bone fluid space and a bone space. The relevance of these spaces is in bone mineral short and long term ion exchange activity.

The uptake by the canine tibia of the bone-scanning agent 99mTc-MDP before and after an osteotomy

The residue and extraction of technetium-labelled methylene diphosphonate (99mTc-MDP), a substance used in bone scanning, was examined in the canine tibia and found to be low. Examination of washout curves suggested that there were four compartments in cortical bone, a vascular, a perivascular, a bone fluid and a bone compartment. After an osteotomy in the canine tibia the residue of 99mTc-MDP increased. This was believed to be due to an increase in the blood supply to the bone and to an associated increase in new bone available for exchange. Bone scanning in a fracture is therefore a reflection of the vascular status of the bone being examined and of the uptake by bone. This is dependent on there being an adequate blood supply to the bone and an increased number of mineral-binding sites.

Autoradiography of technetium-labelled diphosphonate in rat bone

The passage of technetium-labelled methylene diphosphonate (99mTc-MDP) across rat bone was examined by autoradiography. The autoradiographs showed that shortly after an injection of the bone-seeking agent there was activity outside the bone, within the bone marrow and also adjacent to the highly vascular epiphysial plate; the distribution of the isotope in the incubated bone appeared to be non-uniform, a high concentration being seen adjacent to the epiphysial plate and also on the surfaces of the bone. The evidence suggested that a two-fold mechanism resulted in the uneven distribution of 99mTc-MDP. The first factor probably represented the regional distribution of blood flow with a transcapillary movement of the tracer from the capillary bed to the extravascular space; the subsequent incorporation of the tracer into bone appeared to depend on the nature of the bone matrix.

Factors influencing the distribution of 99m-technetium methylene diphosphonate in bone and soft tissues

The influences of dilution, storage and pH on the distribution of 99m-technetium methylene diphosphonate (99m-Tc-MDP) in bone and various soft tissues have been investigated in the rat. The results show a marked departure from normality when large dilution factors and very acidic or basic preparations are used.

Use of stannous methylene diphosphonate for labelling red blood cells with99mTc

A technique is described for in vitro labelling of red blood cells with99mTc using stannous methylene diphosphonate as the reducing agent. The labelling yield has been found to be >95% after three washes with normal saline.

An anatomical study of the microcirculation in the rabbit femur.

A technique is described which will allow a study of the microcirculation to a long bone such as the femur. It involves use of a silicone rubber compound (Microfil) in combination with the Spalteholz clearing technique. Preliminary results in the normal rabbit skeleton suggest that this technique can be used to study the changes in vascularity which occur under various pathological conditions.

A rabbit model to study the biokinetics of radiopharmaceuticals in bone.

A technique is described using the rabbit as the ‘animal model’ to measure the rate of extraction of the bone-seeking agent, technetium labelled ethane-1-hydroxy-1, 1-diphosphonate (99Tcm-EHDP) following constant-rate infusion into the femoral nutrient artery. The extraction is shown to be approximately linear for a period of 60 min. Washout of the tracer from bone was followed for a further 60 min period and graphical analysis shows that each curve could be resolved into two distinct exponential functions. The 2-h residue of 99Tcm-EHDP in bone has been examined and is found to be dependent upon its route of administration. A high residue (43.7±5.1%) results when the infusate is delivered directly into the nutrient artery and the single muscular branch is tied off. Retrograde infusion via the femoral artery results in a lower residue (7.8±2.3%). The close correlation of these results in the rabbit with our previous findings in the dog suggests that the rabbit model described can be used to study the kinetics of other radio-pharmaceuticals during their passage through bone.

EVALUATION OF A 99m Tc‐PYRIDOXYLIDENEGLUTAMATE KIT: A HEPATOBILIARY RADIOPHARMACEUTICAL

Pyridoxylideneglutamate has been prepared in kit form for complexing with technetium‐99m and used as a hepatobiliary scanning agent. The active complex was formed as a result of autoclaving the mixture for 30 mins at a temperature of 121oC. A series of sequential studies in rabbits revealed rapid clearance from the blood by the liver followed by a high excretion into the gall‐bladder and finally clearing via the bile duct into the small intestine. The optimum period for visualization of the gall‐bladder was found to be in the region of 10–15 mins. The tissue distribution of this agent has been studied in mice, and results obtained at 5, 10, 15, 30 and 60 mins following injection are presented.

USE OF 131 I-19-IODOCHOLESTEROL FOR LOCALIZATION OF THE ADRENAL GLANDS: A FEASIBILITY STUDY

A tracer dose of 131I‐19‐iodocholesterol was given intravenously to a group of rats in order to assess its clinical usefulness as a localizing agent for the adrenal glands. The clearance from blood was monitored as a function of time, and tissue sampling carried out to measure the level of radioactivity which was associated with the thyroid, liver, and kidney. The results show that 131I‐19‐iodo‐cholesterol rapidly clears from the blood during the first 3–4 days after injection, and is selectively taken up by the adrenal glands. It also appears to have a high affinity for thyroidal tissue, and this probably results from the rapid in vivo deiodination which occurs in the liver and kidneys.

Preparation of a stannous methylene diphosphonate kit and its biological distribution in animals

A method is described for the preparation of a freeze‐dried stannous methylene diphosphonate kit* (Sn‐MDP) suitable for use in bone scanning after complexing with 99mTc. Several combinations of the reagents were used, and these resulted in different levels of bone and soft tissue localization. Two of such combinations produced a kit which localized predominantly in bone, and of these only one showed consistent batch to batch reproducibility. The biological distribution of the labelled compound has been tested in animals, and the results obtained at 2 h post‐injection are presented. The shelf‐life has been found to be in excess of 3 months, and quality control using thin layer chromatography has shown a labelling yield >95%.