Advantages of Molecular Imaging

• Molecular imaging studies permit the evaluation of biological pathways.
• Molecular imaging permits both the temporal and the spatial biodistribution of a molecular probe and related biological processes to be determined in a more meaningful manner throughout an intact living subject.
• it eliminates the need to kill small animals as part of their phenotype determination
• by repetitive imaging it is possible to investigate mutants that are otherwise difficult to interpret with data taken at a single time point
• it allows concomitant visual and analytical biological phenotyping of animals; and
• it allows the researcher to exercise options of multiple imaging strategies (e.g., by using different imaging probes or modalities)

Which are/are Not Examples of Molecular Imaging?

• Tc-MAA for pulmonary perfusion studies
• Tc-RBCs for Blood Pool Imaging
• Tc-MDP for Bone Scan
• Tc-Mebrofenin for Hepatobiliary Scan • Tc-Sulfur Colloid for Live Scan
• I¹²³ mIBG for Neuroendocrine Tumor Imaging
• Rb⁸² Ion for Myocardial Perfusion Study
• I¹³¹ NaI for Therapy of Hyperthyroidism
• I¹³¹ Bexxar for Therapy of NHL
• In¹¹¹ Octreoscan for Imaging Carcinoid

Red shows the correct answer.

• Tc-MAA for pulmonary perfusion studies
• Tc-RBCs for Blood Pool Imaging
• Tc-MDP for Bone Scan
• Tc-Mebrofenin for Hepatobiliary Scan
• Tc-Sulfur Colloid for Live Scan
• I¹²³ mIBG for Neuroendocrine Tumor Imaging
• Rb⁸² Ion for Myocardial Perfusion Study
• I¹³¹ NaI for Therapy of Hyperthyroidism
• I¹³¹ Bexxar for Therapy of NHL
• In¹¹¹ Octreoscan for Imaging Carcinoid

How Do You Know That The Following Are NOT Examples of Molecular Imaging?
Tc-MAA for pulmonary perfusion studies

Why Tc-MAA is NOT an Example of Molecular Imaging:
When one uses Tc-MAA for pulmonary perfusion studies, the chemical formula of the molecule is irrelevant. Any compound in the physical form of macro-aggregates (large particles in the range of 10 – 90 μm) will occlude capillaries (example: Tc-macroaggregates of oatmeal). We are therefore not working on a molecular level with a mechanism of localization that requires a particular molecule for a particular receptor site, but rather one that requires a particular particle size range.

Tc-Sulfur Colloid for Liver/Spleen Scan

Why Tc-SC is NOT an Example of Molecular Imaging
Similar to the use of Tc-MAA for lung imaging, when one injects Tc-SC for RES imaging studies, the chemical formula of the molecule is irrelevant. Any compound in the physical form of microaggregates (small particles in the range of 0.1 – 2.0 μm) will be phagocytized by Kupffer cells in the reticuloendothelial system (example: Tc- microaggregates of oatmeal). As before, we are not working on a molecular level with a mechanism of localization that requires a particular molecule for a particular receptor site, but rather one that requires a particular particle size range.

Tc-RBCs for Blood Pool Imaging

Why Tc-RBCs is NOT an Example of Molecular Imaging The use of Tc- RBC for visualization of the blood pool is technically not molecular imaging because RBCs are not molecules, but rather cells. While one could make the argument that we are forming the molecule Tc-hemoglobin within the red cells, thereby radiolabeling a part of the body, the imaging was not performed following the injection of the molecule Tc-hemoglobin.

Functional Studies:
The Bottom Line Uptake, distribution, and clearance of a radioactive drug are based upon organ function rather than tissue density. Working on a molecular level results in earlier detection of certain diseases than with other imaging modalities.

Examples of Earlier Detection of Disease

• Bone scan detects metastases earlier than X-rays: more sensitive, but less specific
• I¹²³ mIBG detects pheochromocytoma metastatic to lymph nodes that can not be visualized on a CT scan

Examples of Molecular Imaging:

Imaging of neuroendocrine tumors with I¹²³ mIBG identifies lymph nodes containing metastatic pheochromocytoma long before they appear on CT scans This patient had only 1 tumor (arrow) evident on a CT scan.

Imaging of hyperthyroidism (Graves Disease) using I¹²³ NaI. Note uniformity of uptake with no hot spots of cold spots

Imaging of hot nodules using I¹²³ NaI. Note suppression of uptake in the remainder of the thyroid gland.

Skeletal imaging with Tc-99m MDP or another labeled phosphate analog provides planar and/or tomographic images of bony metastases months or years before they can be identified by any other imaging modality. It traces phosphate uptake by hydroxyapatite molecules in bone tissue.

In-¹¹¹ Octreoscan Study positive for liver metastases. The tumor has somatostatin receptor sites which readily take up this somatostatin analog.

Following injection of F¹⁸ FDG, a sugar analog, a PET scan can reveal the presence of breast cancer cells in specific areas of the body, in particular, lymph nodes that have not yet changed size or shape. These tumors are frequently missed by other imaging modalities since molecular changes predate structural changes. This is a breast cancer patient with a normal CT scan.

Prostate Cancer imaging with In¹¹¹ ProstaScint involves a radiolabeled antibody specific for a tumor membrane surface antigen on prostate cancer cells. The resulting antigen-antibody complex is precipitated at the tumor site and permits external detection of both primary and metastatic disease. Although disliked by patients, this is considered to be the best diagnostic procedure available for detecting recurrent prostate cancer. This is a Whole Body ProstaScint Scan of a Patient with Rising PSA while undergoing Hormonal Therapy. ProstaScint activity in a left supraclavicular lymph node and in many central abdominal lymph nodes (arrows) indicates a high likelihood of metastatic disease and hormone resistant.

Bone Imaging with F-18 NaF provides excellent quality images of bony metastases by tracing the absorption of fluoride ion by bone tissue, with preferential uptake in metastases. F-18 NaF PET scans have improved anatomic detail compared with conventional gamma camera systems, with higher accuracy in detecting both osteolytic and osteoblastic metastases. They have a greater differentiation rate of benign versus malignant lesions and improved ability to identify the extent of bone metastases.