Lecture 8

Lecture 8 - Radiological Health Science Careers Medical...

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Unformatted text preview: Radiological Health Science Careers: Medical Imaging Career Opportunities: Radiologist, M.D. Medical Physicist, M.S./Ph.D. Radiobiologist, Ph.D. Medical Health Physicist, B.S./M.S. Technologists, B.S. (X-ray, nuclear medicine, ultrasound, CT, MRI, PET,Spect) Nuclear Pharmacist, Ph.D./D.Pharm. Nurse, BSN Keywords for this lecture: imaging modalities, reconstruction, physical principles, uses of imaging in medicine Common Imaging Modalities Common X-ray Computed Tomography (CT) Magnetic Resonance Imaging (MRI) Ultrasound (US) Imaging Positron Emission Tomography (PET) Imaging SPECT and Planar Imaging What use is medical imaging? What Homer’s Brain Information provided by imaging: 1). Anatomic, structural and geometric 2). Functional data on organ and tissues 3). Signals may be related to physical properties of tissue such as temperature and density. 4). Temporal changes in organs/ tissue 5) Real-time precision guidance of therapy or intervention. 6). Biochemistry using spectrosopy Basic Principles of X-ray Basic ray Computed Tomography (CT) Computed Greek word composite Tomos = To section or cut Graphein = To write or Draw CAT = Computed Axial Tomography X-ray Computed Tomography (CT) Basic Principle 5 4 2 45 54 245 The internal structure of an object can be reconstructed from multiple projections of the object. Backprojection 10 9 7 9 10 9 8 689 7 6 46 7 Summation Relative units are Hounsfield units 9 8 689 10 9 7 9 10 Computed Tomography Computed Geometry Geometry collimated “fan” beam linear detector array Linear Attenuation Coefficient Linear X-ray CT is an imaging modality based on tissue density contrast. Measurements are made of the attenuation of pencil beams of x-rays through the tissue. X-ray beam attenuation depends on ray – – – the x-ray photon’s energy (E) and energy the subject’s physical density (ρ) and atomic composition (Z) Shaded Surface Display of Hip Shaded Images of Multi-slice Spiral CT Images 3D shaded display of CT angiography (Imaged by Marconi MX8000 CT system) 3D Volume rendering of the heart Basic Principles of MRI Basic •signal for MR images –proton density –T1 relaxation –T2 relaxation T1 and T2 Relaxation T1 relaxation is the release of energy during a relaxation transition from an excited state to a lower energy state energy nuclear protons absorb and release nuclear energy in MRI energy Proton Density:Why are Proton protons important? protons positively charged positively spin about a central axis a moving (spinning) charge creates moving a magnetic field magnetic the straight arrow (vector) indicates the the direction of the magnetic field the Proton nuclei have the largest Proton magnetic moment and are the most abundant in the human body by far abundant 21 21 MR Image MR In a typical voxel there are ~10 protons net magnetization 15 from ~10 15 at 1T single voxel detail fat and water protons Fully Relaxed State Fully no magnetic field external magnetic field B0 Single voxel Single voxel Single voxel Single voxel M0=0 Β0 random alignment, net magnetization=0 M0=+ slightly lower energy state when aligned with main magnetic field,net magnetization M0 =+ 0 Proton Density, T1, T2 and T2* weighted Imaging weighted Radiofrequency pulse sequences effect Radiofrequency the magnetization of tissue and determine the MRI signal weighting. the Examples of RF pulse sequences are Examples gradient recalled echo, inversion recovery and spin echo. and Spin Echo Contrast Spin PD weighted T2 weighted Basic Principles of Ultrasound Ultrasound propagation properties properties Periodic motion causes pressure waves Velocity of sound in “soft tissue” is nearly constant = 1500 m/sec. Velocity of sound in bone and air differ greatly from soft tissue. Velocity = Frequency x Wavelength “Ultra”sound implies f > 1 MHz Wavelength = Velocity/Frequency Wavelength < 1.5 mm Ultrasound reflection properties Ultrasound Acoustic energy is reflected at interfaces Acoustic between tissues with differing acoustic impedances (Z). impedances For soft-tissue/air, soft-tissue/bone and tissue/bone bone/air interfaces, almost total reflection occurs. occurs. Pulse-echo principle Pulse Pressure pulse is “launched” into tissue. Pressure Acoustic energy is reflected at Acoustic boundaries separating regions of differing acoustic impedances. differing Fraction of sonic energy returns to Fraction transducer. transducer. Overall delay time is proportional to Overall distance to boundary. distance Ultrasonic Display Ultrasonic The ultrasonic image is an electronic representation of data generated from returning echoes and displayed on a TV monitor. The image is assembled from returning echos, one by one. A-mode: static one dimensional (amplitude) TM mode: A-mode +motion correction B-mode: two dimensional (brightness) B-mode scan target B-mode scan At each lateral position of the transducer the At echo signal as a function of time is recorded. echo Transducer is moved laterally to new position Transducer or steered in a different direction and a new pulse-echo sequence is acquired. pulse Two-dimensional image is assembled one dimensional line at a time. line Lateral resolution is dependent on beam Lateral width width Patient Set-Up Patient Gel used on skin for good acoustic contact to the transducer Ultrasound Images Ultrasound B-mode scans Doppler kidney Doppler Effect: Is the change in signal frequency of sound due to motion - blood flow Liver Fetal 3-D Ultrasound Images Basic Principles of PET Basic Positron Emission Tomography 511 keV Photon e+ e 11C 11B 511 keV Photon Collinearity: Annihilation photons are generated simultaneously and back to back (nearly). Imaging Principle: Coincidence detection and line Registration. PET: Produce an in vivo quantitative estimate of the distribution of positron emitting radionuclides in the body. The uptake of these radionuclides are closely related to the metabolic processes in human physiology. PET Goal: To Visualize Physiologic Function or Functional Metabolism Detectors are arranged in a circle about the patient and only record an event when the diametrically opposed detectors trigger simultaneously. Siemens 951/31R PET Scanner Coronal PET Scan of the Breast and Lymphoma and Radiological Health Careers Radiological “Radiation Therapy” Keywords: radiation, cancer treatment, teletherapy, brachytherapy, Keywords: teletherapy brachytherapy treatment planning, dose conformation, multileaf collimators, multileaf collimators, Fletcher-Suit implant, radioactive sealed sources, cesium-137 Fletcher Radiological Health Science Radiological Three Tracks at the Undergraduate Level •Radiological Health Science (Pre-medicine) •Radiological Health Science (Honors) •Radiological Health Science (Health Physics) Honors track: Designed to lead to a 4+1 MS degree in 1). Nuclear Engineering and Radiological Health 2). Medical Physics Radiation Therapy Radiation This is the treatment of cancer and other diseases using radiation Radiation Oncologist (MD) ~$250,000 Medical Physicist (MS/PhD) ~$130,000 Medical Health Physicist (BS/MS)~$100,000 Dosimetrist (BS)~$70,000 Radiation Therapist (BS)~$60,000 Nurse (BSN)~$50,000 Treatment: Radiation Oncology Treatment: X-rays (most common treatment) Electrons Protons Heavy charged particles Neutrons How does ionizing radiation kill cancer? kill X-ray photons, neutrons and charged particles interact with tissue atoms to create fast charged electrons. Fast electrons entering or being formed in the tissue cell cytoplasm create highly reactive chemical species (e.g. free radicals) that diffuse through the cell nuclear membrane and lethally attack the DNA. Fast electrons directly ionize DNA causing lethal damage and strand breaks Radiation Therapy Process Radiation Step 1: Diagnosis Step Determine patient has cancer Discuss treatment choices Step 2: Consultation Step First trip to the radiation oncologist Stage the cancer Set Treatment Goals (cure or palliation) Step 3: Treatment Selection Step • Two categories of Radiation Therapy Teletherapy (Tele- at a distance) External beam radiation from a machine Brachytherapy (Bra- at arms length) Radiation from radioactive sources put inside or close to the patient Step 4: Treatment Planning Step Construction of immobilization devices Simulation of treatment Locate target, select beam sizes, beam orientations, blocking/shielding Patient placed in position Computer simulation and dose computation Immobilization Immobilization Simulation Simulation Computer simulation and dose Computer computation Step 5: Therapy Step Teletherapy Teletherapy Linear accelerators, cobalt machines, or orthovoltage x-ray machines Ionizing radiation: x-rays, gamma rays, or electron beams High radiation doses cover the tumor and margin for geometric error and microscopic spread Teletherapy Teletherapy Teletherapy Teletherapy Teletherapy Teletherapy Usually given 5 times a week (2 Gy daily) Only a few minutes for actual treatment Typical course of treatment lasts 6 weeks (60 Gy) Fractionated treatment allows normal tissue to repair (tumor not as efficient at repair) Teletherapy Teletherapy Most people can continue with normal daily activities After course of treatment patient is followed up to assess treatment outcome Gamma-knife: Cobalt-60 therapy Gamma Precision Treatment of Small Brain Lesions Brachytherapy Brachytherapy Puts radiation source within or close to tumor Wires, seeds, or needles Cesium-137, iridium-192, iodine-125 etc. Brachytherapy Brachytherapy Common cancers treated with brachytherapy: Cervix, uterus, vagina, rectum, eye, and certain head and neck cancers Others: Breast, brain, skin, anal, esophageal, lung, bladder and prostate 2 Types of Brachytherapy Types Intracavitary treatment Containers that hold radioactive sources are put near the tumor by way of body cavity (unsealed sources) Interstitial treatment Radioactive sources are put into tumor Brachytherapy Brachytherapy Sources of radiation may be in for short periods (hours or days) or permanently May be done in conjunction with external beam therapy Brachytherapy Brachytherapy Teletherapy plus brachytherapy brachytherapy Teletherapy destroys cancerous cells in a large area surrounding the tumor Brachytherapy delivers a higher dose of radiation to destroy the main mass of tumor cells Step 6: Evaluation Step Patients followed up to evaluate success of treatment If tumor has gone away, cancer has “completely responded to treatment” Cancer isn’t considered “cured” for several years, usually 5 years, after treatment ends If the therapy was palliative, therapy is considered successful if symptoms are relieved Protection and Safety Protection Each patient’s treatment is tailored to be safe and effective Machines double-check to make sure the proper treatment is given (interlocks) State and federal regulations and standards Cancer specialty organizations (e.g. RTOG) have treatment protocols Acute Side Effects Acute May begin by middle or end of treatment program (adema, erythema, necrosis) Last a short time after the final radiation treatment Temporary and easily treated Most patients can continue with daily activities It is rare that a serious chronic side effect occurs Fatigue is a common side effect Side Effects Side Fatigue is a common side effect Patients who have had cancer are at increased risk of another cancer no matter how it was treated Benefits outweigh any small risks Teletherapy Teletherapy ...
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This note was uploaded on 10/24/2011 for the course HSCI 101 taught by Professor Staff during the Spring '08 term at Purdue.

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