AUA Foundations Introduction to Medical Imaging PDF

Introduction to Medical Imaging David Graham Executive Dean of Preclinical Sciences, AUA College of Medicine, Antigua  Lecture Objectives ANT.4: Understand the science behind common medical imaging methods, their indications, contraindications, and side-effect...

Introduction to Medical Imaging David Graham Executive Dean of Preclinical Sciences, AUA College of Medicine, Antigua  Lecture Objectives ANT.4: Understand the science behind common medical imaging methods, their indications, contraindications, and side-effects. Given a clinical scenario, image, or description, students should be able to: ANT.4.1. Recognize the common imaging methods used to evaluate the anatomy and function of the human body. ANT.4.2. Differentiate the features of a medical image, including how they are generated and described. ANT.4.3. Recognize the indications, contraindications, side-effects and potential complications of medical imaging methods.  What is the aim of medical imaging? To produce a visible image of the anatomy and the function of the human body aquilionvision.com  Why is radiology so important? Seeing internal structures without surgery Can demonstrate function as well as anatomy Pathology is easily demonstrated Monitor response to treatment  Types of Imaging Radiography, Plain films Fluoroscopy CT MRI Ultrasound NM PET Interventional Radiology  Radiography Also known as “x-rays” or “plain films”  Wilhelm Roentgen Earliest radiograph of Current day radiograph hand of Roentgen’s wife of hand Wilhelm Roentgen discovered X rays in 1898 Plain film Radiography Advantages Readily available Relatively low cost Few side-effects Disadvantages Uses ionizing radiation Limited soft tissue contrast How do x-rays produce an image? Electrons produced from a heated filament hit a tungsten target, producing x-rays How do x-rays produce an image?  How do x-rays passing through the body create an image? X-rays that pass through the body to the film render the film dark (black) X-rays that are totally blocked do not reach the film and render the film light (white) Air = low atomic # = x-rays get through = image is dark Metal = high atomic # = x-rays blocked = image is light (white)  5 Radiographic Densities Densities in the Body o Gas o Fat Increasing o Fluid density (whiter) o Bone as atomic density o Metal increases What does this show? www.lbah.com What does this show? www.lbah.com What does this show?  Radiographic Imaging Radiographic imaging is a 2- Dimensional representation of 3-Dimensional structures You need at least two images at 90-degrees to each other Pain in leg after car crash Patient in auto accident Is this single frontal view enough? Tibial fracture Tibial fracture is only seen on one of the two views here With plain films usually need two or more images from different directions  Plain films of Chest Frontal and lateral plain films of chest. Differentiation of individual cardiac chambers is not possible Position of cardiac structures Plain Films You need to remember the 5 different densities to be able to interpret radiographs You need two or more images Soft tissue differentiation is not good Fluoroscopy  Angiography Angiogram = “picture of vessels” Complications: local, hemorrhage, embolus, allergic, death Can be used for therapy as well as diagnosis  Computed Tomography CT = Computerized tomography CAT = Computerized axial tomography windsorstar.com Useless fact of the day! Development of CT scan How does a CT work? CT machine Principle of spiral scan  Cross-sectional imaging Cross sectional imaging is analogous to taking a slice of bread from a loaf and looking at it from the end  Image Orientation Convention is that you are looking at the images from the foot of the patient. The patient’s right is on your left and vice versa. Anterior is the top of the image and posterior is the bottom. Imaging Planes Sagittal Axial or transverse It is essential to understand these different planes of imaging You will hear these terms constantly What is the plane? What is the plane? What is the plane? What is the plane? What is the plane? What is the plane?  CT Terms “Attenuation” o Bone attenuates x-rays most and appears white, o air attenuates least – appears black, o tissue and fluid are in between – varying shades of grey. This allows for characterisation of type of tissue. Attenuation is measured in Hounsfield Units (after inventor of CT)  Computed Tomography Disadvantages Advantages Some patients are Allows cross-sectional claustrophobic imaging Uses ionizing radiation Images can be May need to use reformatted in different intravenous contrast planes Best avoided in Excellent spatial and pregnancy contrast resolution – Avoid in children unless good tissue absolutely necessary differentiation Painless, rapid  CT reconstructions CT 3D reconstruction Image from Coronary CT Angiogram CT scan – what is it? CT scans Much better soft tissue resolution than plain films High radiation dose Can have severe contrast reactions, including death  MRI Uses a very powerful magnetic field e.g. 1.5 Tesla machine is 30,000 times stronger than Earth’s magnetic field Atoms align with the magnetic field Radiopulses ‘flip’ the hydrogen atoms Different tissues take different times to realign with the magnetic field https://www.youtube.com/watch?v=hvXo HU9Cexk  MRI Sequences Different types of MRI scan o T1 (anatomical): fast to acquire, excellent structural detail (e.g. white and gray matter). o T2 (pathological): slower to acquire, therefore usually lower resolution than T1. Excellent for finding lesions.  MRI Planes Coronal Sagittal Axial Comparison to radiographs Femur Tibia (Left) Lateral radiograph and (Right) sagittal MRI of the knee. The MRI gives significantly better visualization of the soft tissues than does radiography.  Complications specific to MRI Ferromagnetic objects Electrical interference with medical implants Artefacts from metal Retroperitoneal fibrosis with gadolinium http://mrimetaldetector.com/blog/ MRI Excellent soft-tissue resolution Not as good as CT for bone No radiation dose Noisy, claustrophobic How to tell whether a CT or MRI o Look for the bone o CT = cortical bone is WHITE o MRI = cortical bone is DARK How to tell whether a CT or MRI How to tell whether a CT or MRI How to tell whether a CT or MRI MRI CT  Ultrasound Uses high-frequency, sound waves Principle is that used by bats and in submarines Pulses of sound pass into the body, are reflected back to the probe and transformed into 2D picture Principle of Ultrasound Ultrasound probe sends out a pulse of high frequency sound (2-20 MHz) Then sits silent waiting for pulse to return As pulse goes through tissues may be reflected, refracted or absorbed If a structure reflects few echoes (e.g. cyst) it appears black, if reflects a lot (e.g. bone) it appears white  Ultrasound Terms Ultrasound, sonogram, echo Echogenic, increased echogenicity (hyperechoic), decreased echogenicity (hypoechoic), anechoic Simple fluid e.g. in a cyst is anechoic (no echoes) Complex fluid e.g. blood, pus has echoes Solid tissue has varying degrees of echogenicity Calcium, bone and air reflect the majority of sound and paper white Common Ultrasound studies OBGYN Gallbladder – RUQ pain, gallstones etc Abdominal – pain, mass, fluid Thyroid – nodules, enlargement Carotid – TIA, stroke Guidance for biopsies Cardiac Ultrasound Ultrasound is a safe, accurate method of imaging cardiac anatomy and function  Doppler Ultrasound Area of plaque in the carotid bulb producing visible narrowing of the vessel caliber This would be associated with an increase in velocity of the blood at that site, which can be quantified with ultrasound Hand-held Ultrasound Philips ultrasound ultrasoundtraining.co.uk Ultrasound No ionizing radiation No known side-effects in levels used User dependent Little value in bone and gas containing organs Very useful for point of care imaging  Nuclear Medicine Agent taken up by organ plus a radioactive tracer Technetium-99m is widely used Emits gamma rays, detected by nuclear camera creative commons Common nuclear medicine procedures Bone scan – for metastatic disease Cardiac scan – myocardial function HIDA scan – for gallbladder function Thyroid scan – thyroid nodules, treatment with radioactive iodine Metastatic Prostatic Cancer A bone scan in a patient with prostate cancer, metastatic to the bones The white areas in the bones represent areas of increased osteoblastic activity secondary to bone metastases  Cardiac scan Myocardial imaging examines the heart at rest and after stress Areas of myocardial ischemia or infarction will not take up tracer normally  PET SCAN Uses a compound metabolized by organ (e.g. glucose in brain) Attaches a tracer which undergoes beta decay metabolic activity are shown as bright or ‘hot’ areas  PET CT Coronal CT PET image Fused PET CT image Nuclear Medicine Uses ionizing radiation Poor spatial resolution Can show function as well as anatomy PET/CT very important in oncology Oncos Greek/Latin = tumor. -ology = study of Contrast radiology Barium studies of the bowel Angiography Arthrography Hysterosalpingography Sonohysterography Pyelography/Urography  Contrast agents Intracavitary o Barium for GI tract – swallowed or rectal administration o Water soluble material e.g. for tube placement o Intrauterine – hysterosalpingography o Intra-articular – arthography o Intrathecal – myelogram Intravenous e.g. for contrast CT, MRI Intra-arterial - angiography  UGI Small bowel series Upper GI study shows contrast in stomach Contrast in small bowel MR and CT Angiography Risks of IV Contrast Material Extravasation, leading to tissue necrosis Nausea/vomiting Local pain Allergic reactions including death Renal failure IV= intravenous  Patient Safety - ALARA ALARA = ‘As low as reasonably achievable’ Optimal protocols for image production are a balance between the image characteristics (e.g. contrast, blurring) and the radiation dose needed to produce the required image quality Radiology Protection  Reducing Radiation Exposure Reduction in unnecessary examinations (e.g. daily ICU films) Dose reduction (CT) – an abdominal CT can be 200-400 times dosage of chest xray Exposure time reduction (fluoroscopy) Use of US and MRI instead of CT or radiographs  Summary A number of imaging methods are available Each has indications, contraindications and side effects which need to be considered when ordering that test Doses of ionizing radiation should be reduced wherever possible travelwithbender.com