• Basic concept: using radioactive (element that remits radiation as it decays) substances coupled with biologically active chemicals to visualize structures (some organs love certain substances- like thyroid and iodine)

• Important physics concepts to know:
  • Isotope = element on the periodic table with different # of neutrons but same atomic # 
    • Radioisotopes: technetium-99m, iodine-131

• How nuclear medicine actually works: patient is made radioactive, they emit gamma rays, a gamma camera has a detector made of crystal that scintillates in response, computer creates an image, multiple types of scans that utilize this principles

• Types of scans:
  • Positron emission tomography (PET) = uses radioisotope that produces positively charged electrons (positrons) that are attached to pharmaceuticals (glucose analog fluorodeoxyglucose [FDG] for example) to image based on metabolic activity 
  • Single photon emission computed tomography (SPECT) = uses gamma camera to take 2D pictures circling around the patient to create a 3D projection

• Bone scans:
  • Screening for metastatic disease, diagnosing early fractures 
  • Tracer = Technetium-99m (Tc99m) methylene diphosphonate (MDP)
  • Deposits best where there is bone turnover
  • Metastases, superscan, triple-phase

• Ventilation/perfusion (V/Q) scans:
  • Used to diagnose pulmonary embolism when patients cannot undergo CT angiography 
  • Tracer = Tc99m macroaggregated albumin (MAA)
  • Pulmonary embolism = segmental mismatch with normal ventilation but abnormal perfusion scans; probability can be categorized into normal, low, intermediate, high

• Cardiac scans:
  • Heart cells with decreased perfusion or viability will take up less tracer
  • Usually perform a resting and stress (adenosine, treadmill) scan
  • Decreased uptake on stress that corrects on rest suggests ischemia rather than infarct (reversible vs irreversible)

• GI bleed scans:
  • Couple Tech-99m to RBCs and scan abdomen
  • Bleeds show up as increased uptake of radiotracer in bowel lumen that increases in amount and moves through the bowel over time

• Thyroid scintigraphy:
  • Used to assess nodules, Grave’s disease, cancer
  • Hyperyhyroid patients will show increased uptake if there is truly an increased amount of thyroid hormone being actively produced 
  • Performed using radioactive iodine or T99m pertechnetate which both go to the thyroid 
  • 95% of hot nodules are benign, cold nodules are more concerning for malignancy

• Biliary scans:
  • Hepatobiliary iminodiacetic acid (HIDA) scan = couples Tc99m to iminodiacetic acid to assess the hepatobiliary system
  • Often used to diagnose acute cholecystitis (very sensitive and specific) or biliary leaks after surgery 
  • Lack of filling of the gallbladder (photopenic area) suggests obstruction of cystic duct and can diagnose acute cholecystitis 
  • Can scan the abdomen to see if tracer is outside of biliary system to suggest leak

References: Herring's Learning Radiology, Nuclear Medicine: The Requisites, Radiopaedia, Mandell's CORE Radiology

• Understanding normal anatomy is key before being able to understand what is abnormal

• Anatomy review
  • Visceral pleura hugs the lungs (forms fissures), parietal pleura lines chest wall
  • Lobes of the lung: right lung has upper, middle, and lower lobes, left lung has upper and lower lobes
  • Fissures: right lung has horizontal/minor fissure and oblique major fissure, left lung has one huge oblique major fissure 
    •  Invisible or fine white lines, if thickened may represent excess fluid from a process such as congestive heart failure
  • Trachea -> carina -> main bronchus -> lobar (secondary) bronchi -> segmental (tertiary) bronchi -> subsegmental bronchi -> bronchioles -> secondary lobules (centrilobular artery and bronchus) -> respiratory bronchioles -> alveoli 
    • Cannot usually see bronchi on chest x-ray
    • Conduction zone = trachea, bronchi, bronchioles, terminal bronchioles
    • Respiratory zone = respiratory bronchioles, alveolar ducts, alveoli 
    • Lung parenchyma = alveoli, ducts, and respiratory bronchioles
    • Type 1 pneumocytes do gas exchange, type 2 pneumocytes produce surfactant, alveolar macrophages ingest and process debris
  • Hemidiaphragms: left is obscured by heart, on lateral radiograph can follow the right hemi all the way across
  • Left main pulmonary artery arches over the left main bronchus 
    • The right pulmonary artery will be anterior and inferior to the bronchus/left pulmonary artery on lateral radiograph
    • Each pulmonary artery may appear as slightly opaque compared to the lumen of the bronchus 
    • Pulmonary vessel markings taper peripherally and will show up as white lines thicker at the base of the lung 
  • Lungs have supply from pulmonary arteries and bronchial arteries (from aorta)
  • Pulmonary veins drain into left atrium
  • Innervation: parasympathetic from vagus, sympathetic from thoracic ganglia 
  • Lymph drainage goes to hilum 
  • Physiology: parasympathetic causes vasoconstriction, bronchoconstriction, gland secretion, sympathetic does opposite and opens things up 

• Lateral radiograph is important to view areas of the lung that may not be obviously abnormal on frontal view 
  • Normal features of lateral radiograph:
    • Space behind the sternum (lack of could point to mediastinal masses)
    • Absence of a major shadow from the hila (presence could point to sarcoidosis)
    • Consistent height of vertebrae
    • Sharp posterior (requires less fluid to visualize) costophrenic angles (blunted/opacity filled could point to pleural effusion)
    • Continuous right hemidiaphragm (and slightly higher)

• As with all studies/indications, when looking at an image, do not forget to look at all of the structures in field of view; even if you are reading a chest x-ray to rule out pneumonia/pneumothorax, remember to view all of the other structures, such as the thoracic vertebrae!

References: Herring's Learning Radiology, Radiopaedia, Mandell's CORE Radiology 

• Indications: broad (respiratory or cardiac disease, tube positioning, trauma) 

• Characteristics of a good chest x-ray (PIER): 
  • Projection (AP, PA, lateral, lateral decubitus): heart will appear bigger on AP,  but not by much, AP is better for intubated/sick patients, two views is KEY
  • Inspiration and ribs: do you see at least 8-9 posterior ribs (if too little inspiration, things can crowd and mimic abnormalities)
  • Exposure: can you see the spine through the heart (too much penetration makes things dark, too little makes things bright and fuzzy)
  • Rotation and clavicles: what is the relationship between the clavicles and thoracic spinous processes (patient rotated to their right will have their left clavicle appear closer to the spinous process)
  • Angle of patient: should be perpendicular, but x-ray beams may be angled upward (apical lordotic), which can make anterior structures look more superior (clavicles above first rib)

• Approach
  • Start every time with verifying patient information and imaging quality (PIER)/information 
  • Then execute your systematic approach for consistency 
  • Common approach is the tubes + ABCDEFGHI approach
  • First looks at tubes, lines, drains
  • A = airway, B= bones, C = cardiac, D = diaphragm, E = effusions/extra-thoracic tissues, F = fields, fissures, foreign bodies, G = great vessels, gastric bubble, H = hilum and mediastinum, I = impression
  • A/airway = follow the trachea down, is it midline
  • B/bones = follow outline of bones to look for fractures
  • C/cardiac = heart should be around or less than 50% diameter of chest
  • D/diaphragm = right hemi is slightly higher due to liver, are they flattened
  • E/effusions and extra-thoracic tissues = check costophrenic angles, lateral films, look for swelling, subcutaneous air
  • F/fields, fissures, and foreign bodies = check lung fields for opacities, masses, pneumothorax, vessel markings, look at major and minor fissures, assess any foreign bodies (wires)
  • G/great vessels and gastric bubble = follow path of aorta, pulmonary arteries and veins, gastric bubble under left hemidiaphragm
  • H/hilum and mediastinum = look for prominence (sarcoid), lymphadenopathy, masses, check for mediastinal widening (thymus can be normal in kids)
  • I/impression = overall conclusion or what is going on considering your findings 

References: Herring's Learning Radiology, Radiopaedia, Mandell's CORE Radiology 

• If this episode doesn't fascinate you, I don't know what will!!

• Radiographs
  • Ionizing radiation hits a photosensitive film to produce an image
  • Digital radiography: photosensitive plate processed by electronic reader to be stored digitally
  • PACS system: picture archiving, communications, and storage system
  • Plain films = X-ray = radiograph = without contrast material
  • Advantages: availability, less expensive
  • Disadvantages: limited detail, uses radiation (caution in pregnancy)
  • Uses: chest x-ray, abdomen x-ray, bone visualization

• Densities
  • Air is black (absorbs the least), bone is white (calcium), metal appears whitest, fat and soft tissues appear in between as gray 

• CT scanners
  • Uses x-ray machine that rotates around patient to create different planes and produce a large series of 2D images slices
  • Important to understand positioning of patient from image (you're look up from the bottom of their feet, with patient's right on the left of the screen)
  • Hounsfield units: -1000 to +1000, water is zero, air is -1000, bone is +400-600, fat is -40 to -100, soft tissues is 20-100
  • Windowing: range of densities to most optimally view certain structures
  • Advantages: greater detail
  • Disadvantages: more radiation, more expensive 
  • Uses: non-contrast head CT for stroke, traumas, 3D reformats 

• Ultrasound
  • Uses high frequency sound waves emitted from a probe
  • Advantages: less expensive, availability, no ionizing radiation
  • Disadvantages: operator dependent, low resolution 
  • Uses: great for pregnancy, gallstones, breast masses, thyroid nodules
  • Doppler flow: red towards probe, blue away from probe

• MRI
  • Uses magnetic fields and radio waves to affect hydrogen items 
  • Advantages: great resolution, no radiation
  • Disadvantages: expensive, time-consuming, special precautions (pacemakers)
  • Uses: brain imaging (MS), soft tissues like muscles, tendons, ligaments, herniated discs, spinal cord pathologies 

• Fluoroscopy
  • Uses ionizing radiation in real-time 
  • Can give barium to a patient which will show up black
  • Uses: esophagrams, voiding cystourethrograms, interventional radiology (angiography)
  • Advantages: mobile, procedure guidance, dynamic
  • Disadvantages: higher dose of radiation

• Nuclear medicine
  • Uses radioactive substances (elements that emit radiation as they decay) and couples them with drugs that will accumulate in certain tissues
  • Different types of scans: positron emission tomography (PET) scans use radioactive glucose (fluorodeoxyglucose, FDG), single photon emission computed tomography (SPECT) uses a gamma camera to acquire images from many angles to create a 3D map
  • Different organs use different substances; the brain loves glucose and the thyroid loves iodine 
  • Uses: cancer imaging, assessing for metastases
  • Less radiation than CT scans, but must use caution with radiation exposure via shielding and appropriate timing 

References: Herring's Learning Radiology, Radiopaedia, Mandell's CORE Radiology 

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