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Introduction of Nuclear Medicine

In nuclear medicine studies, the radiologist administers a radioactive atom, either alone or coupled to a molecule, that is known to target a certain organ or organs.
Its distribution is then examined to determine any pathologic condition in that particular organ.

  1. Nuclear medicine scans are accurate, sensitive, and reliable but are costly, require greater time to generate data, and utilize immobile equipment.

Photon-Emitting Radionuclides For Imaging:

  1. Technetium-99m         
  2. Molybdenum-99           
  3. Iodine-123           
  4. Iodine-131
  5. Xenon-133                  
  6. Gallium-67                   
  7. Indium-III           
  8. Indium-113m
  9. Thallium-20 I                       
  10. Krypton-81m
  1. Technetium-99m
    1. Technetium-99m fulfills many of the criteria of an ideal radio nuclideQ and is used in more than 70% of nuclear imaging procedures in the United States.
    2. It has no particulate emission, a 6-hour half-lifeQ, and a predominant (98 %) 140-keV photon with only a small amount (10%) of internal conversion.
    3. Technetium-99m is obtained by separating it from the parent 99Mo (67-hour half-life) in a gen­erator system. Q
    4. In the alumina generator system, the molybde­num activity is absorbed on an alumina column. By passing physiologic saline over the column, 99rnTc is eluted or washed off as sodium pertechnetate.
    5. It rapidly concentrates in the salivary glands, choroid plexus, thyroid gland, gastric mucosa, and functioning breast tissue; during pregnancy, it crosses the placenta.
    6. Q The colon is the critical organ and receives I to 2 rad (0.02 mGy/MBq) of 99mTc pertechnetate administered. The principal emission (140-ke V photon) of 99mTc has a half~value layer (HVL) of 0.028 cm in lead.
  • Gamma Scintillation Camera
    1. The most widely used imaging devices in nuclear medicine are the simple gamma scintillation (Anger) camera, single-photon emission (SPECT)­capable gamma cameras, and positron emission (PET) scanners. Several other instruments are com­monly used in the nuclear medicine laboratory, in­cluding the dose calibrator, well counter, and thyroid probe.
    2. A gamma camera converts photons emitted by the radionuclide in the patient into a light pulse and subsequently into a voltage signal. This signal is used to form an image of the distribution of the radionuclide.
    3. The basic components of a gamma camera system are:
      1. The collimator,
      2. he scintillation crystal,
      3. An array of photomultiplier tubes (PMTS),
      4. Preamplifiers,
      5. A pulse-height ana¬lyzer (PHA),
      6. Digital correction circuitry,
      7. A cathode ray tube (CRT),
      8. The control console. 
      9. A com¬puter and picture archiving systems (PACs) are also integral parts of the system.
Extra Edge

Nuclear medicine is unique in that its strength lies in portraying the functional status of an organ rather than producing images that are predominantly anatomic in content.

  1. Radionuclide Scans
    1. MDP Bone scan
      1. Radionuclide bone scans are used to evaluate cancer spread to osseous sites. This test is sensitive but relatively nonspecific because areas of increased uptake are not always related to metastatic disease. Healing fractures, arthritis, Paget’s disease, and other conditions will also cause abnormal uptake. True-positive bone scans are rare if the PSA is <8 ng/mL and uncommon when the PSA is <10 ng/mL unless the tumor is high-grade.
      2. Three-phase bone scan (99mTc-MDP): Characteristic finding in osteomyelitis: increased uptake in all three phases of scan. Highly sensitive in acute infection; somewhat less sensitive if blood flow to bone is poor. Specificity moderate if plain films are normal, but poor in presence of neuropathic arthropathy, fractures, tumor, infarction.
    2. GI nuclear imaging:
      Scintigraphy both evaluates structural abnormalities and quantifies luminal transit. 99mtc RBC scintigraphy localize bleeding sites in patients with brisk hemorrhage so that therapy with endoscopy, angiography, or surgery may be directed. it can be used to pick up intermittent bleeding also. THE sensitivity is more than catheter angiograpy with bleeding rate as low as 0.05 ml/min can be picked up. Radiolabeled leukocyte scans can search for intraabdominal abscesses not visualized on CT. Biliary scintigraphy is complementary to ultrasound in the assessment of cholecystitis. Scintigraphy to quantify esophageal and gastric emptying are well established, while techniques to measure small-intestinal or colonic transit are less widely used. 
    3. Meckel’s scan
      99mTc-pertechnetate scintigraphy for diagnosis of Meckel’s diverticulum should be done, especially in the evaluation of young patients.
    4. LGIB
      99mTc-labeled red cell scan allows repeated imaging for up to 24 h and may identify the general location of bleeding.
      Nuclear medicine scintigraphy after ingestion of a radiolabeled meal is the best study to document delayed gastric emptying, but may not correlate well with symptoms. .Gastric emptying scintigraphy is considered to exclude gastroparesis in patients whose dyspeptic symptoms resemble postprandial distress when drug treatment fails. Gastric scintigraphy also assesses for gastroparesis in patients with GERD, especially if surgical intervention is being considered.
    5.  ​EAP imaging
      Tumors can also be localized using radioactive tracers including 131I- or 123I-metaiodobenzylguanidine (MIBG), 111In-somatostatin analogues, or 18F-dopa (or dopamine) positron-emission tomography (PET). Because these agents exhibit selective uptake in paragangliomas, nuclear imaging is particularly useful in the hereditary syndromes.
    6. Scanning for infection/inflammation
      1. Radionuclide scanning procedures using technetium (Tc) 99m sulfur colloid, gallium (Ga) 67 citrate, or indium (In) 111–labeled leukocytes may be useful in identifying and/or localizing inflammatory processes.
      2. Ga scintigraphy might actually be used before other imaging techniques if no specific organ is suspected of being abnormal.
      3. It is likely that PET scanning, which provides quicker results (hours vs days), will prove even more sensitive and specific than 67Ga scanning in FUO. 
      4. 99mTc bone scan should be undertaken to look for osteomyelitis or bony metastases; 67Ga scan may be used to identify sarcoidosis or Pneumocystis infection in the lungs or Crohn’s disease in the abdomen.
      5. 111In-labeled white blood cell (WBC) scan may be used to locate abscesses. With these scans, false-positive and false-negative findings are common.
      6. Fluorodeoxyglucose F18 (FDG) PET scanning appears to be superior to other forms of nuclear imaging.
    7. V-P (Ventilation-perfusion) scan
      Radioisotope scan in the form of V-P (Ventilation-perfusion) scan is the 2nd line investigation  for the diagnosis of pulmonary thromboembolism as it has 84% overall accuracy rate with 100% sensitivity due to the occurrence of multiple emboli (usually > 6–8) at least one of which causes a perfusion defect!
To classify a ventilation-perfusion scan as a high probability for pulmonary embolism (PE), the scan must have the equivalent of two or more large segmental perfusion defects (75-100% involvement of the segment) that are not matched by ventilatory abnormalities. Four or more moderately sized perfusion defects (25-75% involvement of the segment) would also represent a high probability for PE. The implication of a high probability scan suggests a greater than 80% chance of having a PE.

QThe most specific finding of a pulmonary embolus is a partial or complete intraluminal filling defect in a pulmonary artery. On CT angiography (CTA), the filling defect should be present on at least two contiguous sections. Abrupt cut-off of the artery also indicates a pulmonary embolus.

  1. Renal Scan

    1. I-131 0IH (Orthoiodohippurate ) - Largely replaced by Tc –99m MAG3 used for evaluation of Renal tubular function / effective renal plasma flow.
    2. QTc –99m DTPA ( Diethlene triamine Pentaacetic acid ) is agent of choice for assessment of ;
      1. Perfusion                                              
      2. GFR
      3. Obstructive uropathy                            
      4. Vesicouretral reflux
    3. Tc –99m DMSA( dimercoptosuccinic acid ) is suitable for imaging of ; functioning cortical mass pseudotumor versus the lesion .
    4. Tc – 99m Mercaptoacetyltriglycine ( MAG3) is a better agent used. Q
    5. True renal plasma flow detected ACE inhibitor scintigraphy is for screen of Reno vascular hypertension.
  2. Heart imaging
    Equilibrium radionuclide angiography, also known as multiple-gated blood pool imaging, involves the imaging of 99mTc-labeled albumin or red cells that are uniformly distributed throughout the blood volume. Resting images of the blood pool of isotopes within the cardiac chambers are obtained by electrocardiographic gating through multiple cycles, so that sufficient counts can be detected to obtain an image. This requires that the heart rate be reasonably constant. It provides an ac curate, reproducible method for assessment of LV function. It is most commonly used when echocardiography is technically difficult or when poor LV function requires accurate quantitation.

MRI is the most accurate method for evaluation of ventricular function.
Gated single-photon emission computed tomography (SPECT) is the nuclear cardiology technique that is most commonly utilized to assess ejection fraction and regional wall motion. This is usually performed post-stress by gating the acquisition of SPECT myocardial perfusion images using 99mTc- labeled compounds (see below). An automated technique determines the endocardial borders of the LV cavity, and a geometric model is used to calculate the ejection fraction.

Myocardial scan

  1. Myocardial Ischemia & viability can be studied :-
    1. Directly with myocardial perfusion imaging by
      1. Thallium – 201 chloride SPECT imaging
      2. Tc-99m sestamibi / tetrofosmin SPECT imaging
      3. PET
    2. Indirectly with ventricular function imaging by
      1. Multigated acquisition scan (MUGA)
      2. First pass radionuclide coronary Agniography
    3. Simultaneous assessment of myocardial perfusion + ventricular function by
      1. First pass radionuclide angiography + gated SPECT perfusion imaging
  2. Thus direct measure in the form of Thallium scan will best tell about reversibility of myocardial ischemia & viability.
  3. In multiple gated acquisition (MUGA) for cardiac imaging, gated equilibrium images depict average cardiac contraction by summation over several minutes. Q
  4. Recording of ejection fraction of left ventricle before & after exercise, regional wall motion of ventricular chambers & Regurgitant index is done.
  5. Advantages of MUGA scan are : -
    1. Higher information density than 1st pass method
    2. Assessment of pharmacological effect possible
    3. “Bad beat” rejection possible
  6. Disadvantages of MUGA scan are: -
    1. Significant background activity
    2. Inability to monitor individual chambers
    3. Plane of AV valve difficult to identify
  7. Relative advantages of thallium 201 and technetium 99m
    1. Thallium
      1. Lower radiopharmaceutical cost
      2. Measurement of increased pulmonary uptake
      3. Less hepatobiliary and bowel uptake
      4. Detection of resting ischemia (hibernating myocardium)
    2. Technetium
      1. Better image quality (particularly in obese patients)
      2. Ventricular function assessment (gated SPECT)
      3. Shorter imaging time
      4. Shorter imaging protocols (patient/scheduling convenience)
      5. Acute imaging in myocardial infarction and unstable angina
      6. Superior quantification
  1. Biliary Scintigraphic Scan
    Hepatic Iminodiacetic acid scan (HIDA Scan) is a biliary scintigraphic scan. Q
    Tc-99m acetamilide iminodiacetic acid analogs are HIDA agents & depending on their liphophility, there is a trade-off between renal excretion & hepatic uptake (HIDA is least lipophilic).
Applications & indications include: Q
  1. Acute cholecystitis (investigation of choice)
  2. Congenital biliary atresia
  3. Biliary leak evaluation
  4. Biliary – enteric fistula
  5. Chronic GB dysfunction
  1. Thyroid Scan
  1. Hot nodule → Adenoma & thyroid Carcinoma (extremely rare)
  2. Cold thyroid nodule  → Inflammatory mass, Benign tumor & Malignant tumor
Younger male patients with cold thyroid nodules are more likely to have cancer than are older female patients   with similar findings. Exposure of the neck to radiation is also an important risk factor for cancer in a cold nodule. Finally, US findings of mixed cystic and solid components within a cold nodule are also more suggestive of thyroid cancer. Cold nodules in the setting of a multinodular goiter are substantially less likely to be cancer than other cold nodules. 
  1. Parathyroid scans:
    Radionuclide scintigraphy with 201 T1 (Technetium thallium) or 99m Tc- MIBI (Technetium –99m sestamibi) Q, using a subtraction technique whereby the image of thyroid gland as shown by 99m Tc or 123 I is removed, is thus a noninvasive & other best technique for tumor & other lesions localization of parathyroid gland. 
  2. Brain scan
    1. Brain Tumors. Both primary and metastatic brain lesions present on SPECT brain perfusion imaging as localized defects that correspond to the mass lesions. This technique alone is of limited value in the primary diagnosis or evaluation of intracranial mass lesions. In conjunction with Thallium-201, however, SPECT brain perfusion imaging may be valuable in distinguishing between radia­tion necrosis and tumor recurrence in patients with malignant gliomas treated with high-dose radia­tion. The study may also localize suspected recur­rences for biopsy.
    2. In the differentiation of recurrent malignant glioma from radiation necrosis, 99mTc-HMPAO imagesQ generally show a focal defect in the region of abnormality, whether containing necrotic tissue, recurrent tumor, or both. Thallium-20 I activity, however, is a marker of viability, localizing in living tumor cells but not in nonviable tumor cells or necrotic tissue.
    3. Epilepsy. Patients with partial (focal) epilepsy refractory to therapy may benefit from surgical ablation of the seizure focus. The most common pathology at these foci is mesial temporal sclerosis (gliotic temporal scarring) Q. The value of SPECT and PET imaging in this setting is well established.
    4. PET imaging using FDG is the method of choice for evaluating metabolism, whereas SPECT imaging with 99mTc perfusion agents, such as HMPAO or ECD, appears to be the method of choice for evaluation of perfusion status. Q
    5. Ictal SPECT Imaging. By using 99mTc-HMPAO or 99mTc-ECD, which do not significantly redistribute, patients can be injected during the seizure or within 30 seconds after its completion. To obtain ictal studies, the patient may be hospitalized and moni­tored with electroencephalography. The radiophar­maceutical is kept at the bedside until a seizure occurs, at which time it is injected.
    6. Interictal SPECT Imaging. Because interictal SPECT perfusion studies are performed between seizures, blood flow to epileptic foci is normal or reduced. To be detected on SPECT imaging, these must be seen as areas of decreased activity (hypoperfusion).
    7. The common indications for brain imaging are perfusion abnormalities (stroke), dementia (Alzheimer's or multi-infarct), epilepsy, brain death, and distinguishing recurrent tumor from radiation necrosis.
    8. The radiopharmaceuticals 99"Tc-HMPAO (SPECT), 99"Tc-ECD (SPECT), and nitrogen-13 C3N)-ammonia (PET) are perfusion agents.

The radiopharmaceuticals 2o1TI (SPECT) and 18FDG (PET) are metabolic agents that show activity in viable recurrent or persistent tumors but not in areas of radiation necrosis.

  1. Lymphoscintigraphy :
    1. Rarely indicated, but either can be used to confirm the diagnosis or to differentiate primary from secondary lymphedema.
    2. Lymphoscintigraphy involves the injection of radioac- tively labeled technetium-containing colloid into the distal subcutane- ous tissue of the affected extremity.
    3. In primary lymphedema, lymphatic channels are ab- sent, hypoplastic, or ectatic. In secondary lymphedema, lymphatic channels are usually dilated, and it may be possible to determine the level of obstruction.
  2. Positron Emission Tomography (PET) / 18-F DG PET SCAN
    1. Positron is a positively charged electron.
    2. Positron emission is a type of beta-decay of an unstable isotope. In this unstable isotope, a proton undergoes spontaneous decay into a neutron, a neutrino, and a β+ particle (positron). The spontaneous release of positrons from these unstable nuclei leads to their interaction with electrons, which cause the release of gamma- radiation (photons) upon collision with tissue. It is this gamma emission that is detected by the gamma camera in the PET scanner.
    3. Basic principle: Coincidence detection of paired high energy (511KeV) annihilation gamma photons from positron emitting radionuclides like carbon-11, nitrogen-13, oxygen-15 and fluorine-18. Q
    4. Agent used: Fluorine-18 (F-18) is most commonly preferred. To make the agent to go specifically to the site of interest, F-18 is coupled with deoxyglucose which gets more concentrated in malignant cells as they have much more metabolic demand compared to normal cells.
    5. PET camera: Routine gamma camera is not used for PET imaging. PET detectors are made up of special material, called BGO (Bismuth germinate) crystals, which sensitively and specifically detects the high energy (511 kev) gamma photons produced after electron and emitted positron combine with each other.
      1. Advantages of PET:
        1. The kind of radiopharmaceutical that can be used most physiological molecules in body are made of carbon, nitrogen and Oxygen, enabling them to be labeled with “ 11C, 13N and 15O and 18F” which are position emitters.
        2. It is a ‘unique tool’ to study and quantify physiological and pathological function of human tissues and organs.
        3. Imaging modality that permits noninvasive in vivo examination of metabolism (biochemical imaging), blood flow, electrical activity and neurochemistry:
        4. Most accurate non-invasive method of detecting and evaluating most cancers.
      2. Clinical Applications of PET:
        1. Oncology                  
        2. Neurology:                      
        3. Cardiology

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