6 Skeletal Scintigraphy



10.1055/b-0034-77589

6 Skeletal Scintigraphy

Spitz, J.

Three-phase skeletal scintigraphy (TPS) of the hand makes it possible to evaluate the regional circulation and bone metabolism in a single examination. Pathologic factors that can exert an influence are osseous hyperemia, ischemia, callus development following fracture, inflammation of the bones and joints, inactivity osteoporosis, and trophic disorders. Skeletal scintigraphy is primarily employed to exclude pathologic osseous processes, to identify fractures, and to provide further differentiation when radiographic results are inconclusive. In such cases, scintigraphy often plays a decisive role.



Physical-Technical Foundations


In comparison to radiologic procedures, diagnosis in nuclear medicine is more functionally oriented. It is based on the tracer method. A radioactive substance in a pharmacologically ineffective dose is administered to the metabolic system of interest. The spatial distribution of the radiopharmaceutical substance can be measured at the patient’s body surface as a result of its emission of gamma radiation. At present the most commonly used isotope is technetium-99m (99mTc), which, depending on the clinical objective, is linked to various carrier substances. It is obtained from a molybdenum-99 generator in a technically uncomplicated process, so that it is routinely available. Technetium-99m gives off monoenergetic gamma radiation (electromagnetic radiation) with an energy of 140.5 keV. Owing to its brief half-life of 6.03 hours and lack of corpuscular radiation, this isotope has ideal characteristics with regard to radiation exposure as well as imaging quality. The current standard instrument is the digital wide-field gamma camera with an effective field of view of about 40 cm × 55cm.


Three-phase scintigraphy demonstrates arteriovenous circulation, the early distribution phase in tissue, and the late distribution of the tracer in the skeletal system. The standard examination protocol is given in Table 6.1 .


Evaluation and documentation of data can be carried out in different ways ( Fig. 6.1 ). For quantification, the region-of-interest technique (ROI technique) is generally used. Although the ROI areas should be generously allotted for the blood circulation curves (Fig. 6.1b, c), in the late scintigram the area of pathologic uptake should be kept small (Fig. 6.1d) so as not to falsify the quotients by including normal tissue. In a very small ROI (like the carpal bones), it can be useful to set a considerably larger reference ROI, since a low counting rate in few pixels in normal bone can lead to statistical problems.


The reproducibility of results is good; the coefficient of variation is less than 5%. In contrast to radiographic diagnosis, plaster casts up to 1 cm in thickness do not influence optical interpretation and quantitative assessment with scintigraphy.
















Table 6.1 Examination protocol for three-phase scintigraphy

Positioning of the hand:




  • Including the opposite side



  • Below or on the gamma camera


Bolus injection of 99mTc-labeled diphosphonate




  • 7–10 MBq per kg body weight



  • Reduced dose for children as shown in Table 6.6


First phase: Arteriovenous circulation




  • 60 single images lasting 1 second each


Second phase: Early-phase scintigram




  • One or more static scintigrams


Third phase: Late-phase scintigram




  • 2–5 hours after injection of tracer



  • Various single projections

Fig. 6.1a-d Three-phase scintigraphy of a nondisplaced radius fracture of the right side (600 MBq 99mTc-HMDP). a Single films of arteriovenous phase in the first 60 seconds after tracer injection. b Graphs of circulation in the regions of interest (ROI) of the partial image c. c Summation image of the arteriovenous circulation in the first 60 seconds after tracer injection with the framed ROI. Alternatively, the early-phase scintigram (taken 2 minutes after tracer injection) can be used to determine the ROI. d Static scintigram 2 hours after tracer injection with the framed ROI. Obvious rise in the calculated quotient.


Biological Foundations


Knowledge of the local differences in bone perfusion is important for the interpretation of skeletal scintigrams. A well-developed network of vascular collaterals makes the blood supply in periarticular areas considerably higher than that in the shafts of tubular bones. Their blood supply comes mainly from the bone marrow and is much more likely to be disturbed by dislocated fractures.












Table 6.2 Stages of fracture healing (according to Brand)

Stage I: Inflammation




  • A few hours after the trauma



  • Local hyperemia


Stage II: Repair




  • Begins a few hours after the trauma



  • Incorporation of calcium begins on the 3rd-4th day



  • Maximal reaction after 7–12 days (soft-tissue callus)


Stage III: Remodeling




  • Takes months to years



  • Transformation of fibrous into laminar bone



  • With increasing functional use


As a result of the differences in perfusion, 8–10 days after a trauma, for instance, in periarticular areas hyperemia is nearly always present with a relatively higher accumulation of the isotope. A fracture of the shaft also causes reactive hyperemia in the surrounding soft tissues, but not in the affected parts of the bone. Similar mechanisms lead to intense accumulation of the tracer in inflammatory and active degenerative joint diseases.


Furthermore, reactive bone formation after a fracture causes tracer accumulation in scintigrams. Fracture healing can be summarized in three stages ( Table 6.2 ).


According to the transformation law, a reconstruction process also takes place in the final callus.


In the tracer technique, osteotropic radiopharmaceuticals are transported into the osseous metabolism. The scintigraphic image of accumulated tracer in the bone changes according to the different pathophysiologic factors prevailing at different stages of fracture healing.


Finally, the fundamental principles underlying tracer uptake under normal and abnormal conditions of the bone have not been entirely clarified. Arnold’s concept appears suitable to correlate theoretical concepts with empirical clinical data ( Table 6.2 ). A model consisting of two bony compartments, each of which is connected with the blood compartment, offers the following explanation:




  • One compartment represents weak binding sites on the crystalline surface of the quiescent bone. Tracer clearance from the blood onto the weak binding sites correlates with the intensity of blood flow to the bone.



  • The second compartment represents strong binding sites, which are formed by amorphous calcium phosphate in newly formed lamellar bone. Tracer clearance in the strong binding sites correlates with new bone formation. The increased tracer clearance during new bone formation is based on an increase in the extraction efficiency. This increase is proportional to the newly-formed lamellar bone, whether callus formation, an osseous tumor, or osteomyelitis. Differentiation can be achieved only with the aid of other imaging procedures and the clinical findings.


Because of the exposure of the patient to ionizing radiation, skeletal scintigraphy demands an appropriate indication. Table 6.3 shows gonadal exposure in comparison to radiographic diagnostic procedures.


Table 6.4 lists the radiation doses for different organs, demonstrating that the urinary bladder is the critical organ because of elimination of the radiopharmaceutical in the urine. Radiation exposure can be reduced by sufficient diuresis and frequent urination.

Fig. 6.2 Action of 99mTc-labeled phosphate complexes in the capillaries of the terminal bloodstream. (Modified according to Arnold.) The radiopharmaceutical substance (*) carried in the blood diffuses into the extracellular fluid, which washes over the surface of the bones. The radiopharmaceutical substance, which is not primarily bound to the surface or has been released, returns to the efferent branch of the blood vessel.

Skeletal scintigraphy, like other nuclear medicine examinations, is contraindicated during pregnancy and lactation. Lactation constitutes only a relative contraindication, though, since nursing can be resumed without danger two days after such examinations.
























































Table 6.3 Gonadal exposure in scintigraphy with 555 MBq (15 mCi) 99mTc-MDP compared to radiological diagnostic procedures

Examination


Gonadal Exposure in mGy/ 555 MBq (rem/15 mCi)

 

Male

 

Female

 

Skeletal scintigraphy


1.8


(0.18)


2.6


(0.26)


Radiography:

       

Lumbar spine


0.65


(0.065)


0.75


(0.075)


Pelvis


7.7


(0.77)


3.5


(0.35)


Colon


2.85


(0.285)


10.5


(1.05)


Urogram


6.3


(0.63)


6.15


(0.615)




































Table 6.4 Radiation exposure of different organs in skeletal scintigraphy with 99mTc-MDP. Data apply to an adult weighing 7 0 kg exposed to 555 MBq (15 mCi)

Organ


Radiation Exposure in mGy/ 555 MBq 99mTc-MDP (rem/15 Ci)


Whole body


1.0


(0.1)


Skelton


5.3–5.7


(0.53–0.57)


Kidneys


6.0


(0.6)


Wall of urinary bladder


20.0–47.0


(2.0–4.7)


Bone marrow


3.5


(0.35)


Liver


0.8


(0.08)

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Jul 21, 2020 | Posted by in MUSCULOSKELETAL IMAGING | Comments Off on 6 Skeletal Scintigraphy
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