For a skillful evaluation of an SAP scintigraph some knowledge of the normal metabolism of SAP and iodine is necessary. After intravenous administration SAP (a molecule with a high molecular weight) is rapidly distributed throughout the plasma volume. Therefore in the normal situation the blood pool inside the heart, large blood vessels, and well-circulated organs (such as liver, spleen, and kidneys) can be seen. Despite this normal uptake, an increased uptake (e.g. in the liver) can be recognized because of a relatively higher uptake compared to body background (blood pool) than expected. Rapid disappearance of radioactivity from the circulation to the extravascular body compartment is one of the characteristics of extensive deposition of amyloid (1-3).
After about 6 hours another phenomenon can be observed, i.e. the normal clearance of (radioactive) iodine disconnected from the iodine SAP complex by normal body catabolism. Despite blocking the thyroid with cold iodine, some uptake in the thyroid remains. The salivary glands, oral cavity, nose, stomach, (kidneys vaguely), and the bladder all can show some uptake of radioactivity before it leaves the body via the urine. A slow appearance in the urine of only a small percentage of the dose administered is a result of a strong binding of the radioactivity to the extravascular body compartment and is therefore also a characteristic feature of massive deposition of amyloid or almost complete renal failure (1-3). The excretion of radioactive iodine in the stomach may hinder proper judgment of splenic uptake, especially in case of a large stomach caused by gastric paresis or dysmotility.
|
|
||
| Front | Back | |
An example of a normal scintigraph after 24 hours. Some blood pool activity is still visible inside the heart and large blood vessels (and the scrotum). Some free radioactive iodine can be seen in the thyroid, salivary glands, nose, oral cavity, stomach, and bladder. The uptake in liver, spleen, and kidneys is as intensive as expected in the normal situation. Some activity of the stomach can be seen vaguely in the region of the spleen.
|
|
||
| 1. Front | 1. Back | |
SAP scintigraph with clearly increased uptake in the spleen. The background activity is lower than in the normal situation. Some activity is vaguely visible in the stomach and also in the bladder. Myocardial tissue (no matter how pathologically thickened by amyloid) almost never shows increased uptake compared to the remaining blood pool activity inside the heart. One explanation might be the difficulty of the large SAP molecule to pass through the basement membrane of the the blood vessel wall before it can enter the myocardial tissue itself within the limited time period.
|
|
|
||
| 2. Front | 2. Back | 3. Front | 3. Back |
Figure 2 shows minimally increased uptake in the spleen and kidneys. The activity in the stomach present on the front view is also seen on the back view in the upper region of the spleen, creating an inhomogeneous aspect of the spleen. Thyroid uptake may be normal, although uptake by amyloid can not be excluded.
Figure 3 shows high uptake in the spleen, and also uptake in the kidneys. The uptake in the right adrenal gland is noticeable (especially the back view, but even somewhat visible in the front view). Possible uptake in the left adrenal gland will go unnoticed, because of the uptake of the spleen in that region. Like figure 2 no definitive judgment can be made about thyroid uptake. The uptake may be normal, although uptake by amyloid in the thyroid can not be excluded.
|
|
|
|
|||
| 4. Front | 4. Back | 5. Front | 5. Back | 6. Front | 6. Back |
Figure 4 clearly shows increased uptake in liver and spleen. However, some background activity is still present in blood vessels and bladder. In figure 5 the contrast between the uptake in the liver and blood pool activity is higher than in figure 4. In figure 6 almost all radioactivity is concentrated in an enormously enlarged liver.
Measurement of the disappearance of radioactivity from the blood and of its appearance in the urine may be helpful to assess the severity of deposition of amyloid within the same person at different moments. In short: the extravascular retention of radioactivity in the body after 24 or 48 hours (as percentage of the dose administered) is correlated to the amyloid load. This may be helpful in the individual patient to assess progression, stabilization, and regression during therapy. However, this quantitative method (especially its accuracy) is not satisfactorily evaluated yet. Thus for the moment, visual assessment of the scans is a good and reproducible method to assess progression or regression in an individual patient.
