Medical Imaging Lecture 3

Ultrasound Imaging Ultrasound imaging, also called sonography, involves exposing part of the body to high- frequency sound waves to produce pictures of the inside of the body. Ultrasound examinations do not use ionizing radiation (as used in x-rays). Because ultrasound images are captured in real- time, they can show the structure and movement of the body's internal organs, as well as blood flowing through blood vessels.

How Ultrasound Works?? The ultrasound machine transmits high-frequency (1 to 18 megahertz) sound pulses into your body using a probe. The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected. The reflected waves are picked up by the probe and relayed to the machine. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (5,005 ft/s or1,540 m/s) and the time of the each echo's return (usually on the order of millionths of a second). The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.

Creation of an image from sound The creation of an image from sound is done in three steps – producing a sound wave, receiving echoes, and interpreting those echoes. Producing a sound wave A sound wave is typically produced by a piezoelectric transducer encased in a plastic housing. Strong, short electrical pulses from the ultrasound machine drive the transducer at the desired frequency. The frequencies can be anywhere between 1 and 18 MHz. The sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner (Beamforming). This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth. The sound wave is partially reflected from the layers between different tissues or scattered from smaller structures.

Creation of an image from sound (…) Receiving the echoes The return of the sound wave to the transducer results in the same process as sending the sound wave, except in reverse. The returned sound wave vibrates the transducer and the transducer turns the vibrations into electrical pulses that travel to the ultrasonic scanner where they are processed and transformed into a digital image.

Creation of an image from sound (…) Forming the image: To make an image, the ultrasound scanner must determine two things from each received echo: How long it took the echo to be received from when the sound was transmitted. How strong the echo was. Once the ultrasonic scanner determines these two things, it can locate which pixel in the image to light up and to what intensity. Transforming the received signal into a digital image may be explained by using a blank spreadsheet as an analogy. First picture a long, flat transducer at the top of the sheet. Send pulses down the 'columns' of the spreadsheet (A, B, C, etc.). Listen at each column for any return echoes. When an echo is heard, note how long it took for the echo to return. The longer the wait, the deeper the row (1,2,3, etc.). The strength of the echo determines the brightness setting for that cell (white for a strong echo, black for a weak echo, and varying shades of grey for everything in between.) When all the echoes are recorded on the sheet, we have a greyscale image.

Creation of an image from sound (…) Displaying the image Images from the ultrasound scanner are transferred and displayed using the DICOM standard. Normally, very little post processing is applied to ultrasound images.

The Ultrasound Machine Transducer probe - probe that sends and receives the sound waves Central processing unit (CPU) - computer that does all of the calculations and contains the electrical power supplies for itself and the transducer probe Transducer pulse controls - changes the amplitude, frequency and duration of the pulses emitted from the transducer probe Display - displays the image from the ultrasound data processed by the CPU Keyboard/cursor - inputs data and takes measurements from the display Disk storage device (hard, floppy, CD) - stores the acquired images Printer - prints the image from the displayed data

The Ultrasound Machine (Cont..)

Doppler ultrasonography Anyone who has heard a police or ambulance siren speed past will be familiar with the influence of a moving object on sound waves, known as the Doppler effect. An object travelling towards the listener causes sound waves to be compressed giving a higher frequency; an object travelling away from the listener gives a lower frequency. The Doppler effect has been applied to US imaging. Flowing blood causes an alteration to the frequency of sound waves returning to the US probe. This frequency change or shift is calculated allowing quantization of blood flow. The combination of conventional two- dimensional US imaging with Doppler US is known as Duplex US. Colour Doppler is an extension of these principles, with blood flowing towards the transducer coloured red, and blood flowing away from the transducer coloured blue. Colour Doppler is used in many areas of US including echocardiography and vascular US. Colour Doppler is also used to confirm blood flow within organs (e.g. testis to exclude torsion) and to assess the vascularity of tumours.

Doppler ultrasonography (Cont..)

Contrast ultrasonography The accuracy of US in certain applications may be enhanced by the use of intravenously injected microbubble contrast agents. Microbubbles measure 3–5 μm diameter and consist of spheres of gas (e.g. perfluorocarbon) stabilized by a thin biocompatible shell. Microbubbles are caused to rapidly oscillate by the US beam and, in this way, microbubble contrast agents increase the echogenicity of blood for up to 5 minutes following intravenous injection. Beyond this time, the biocompatible shell is metabolized and the gas diffused into the blood. Microbubble contrast agents are very safe, with a reported incidence of anaphylactoid reaction of around 0.014 per cent.

Contrast ultrasonography (Cont..)

Strengths It images muscle, soft tissue, and bone surfaces very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces. It renders "live" images, where the operator can dynamically select the most useful section for diagnosing and documenting changes, often enabling rapid diagnoses. It shows the structure of organs. It has no known long-term side effects and rarely causes any discomfort to the patient. Equipment is widely available and comparatively flexible. Small, easily carried scanners are available; examinations can be performed at the bedside. Relatively inexpensive compared to other modes of investigation, such as computed X-ray tomography, DEXA or magnetic resonance imaging. Spatial resolution is better in high frequency ultrasound transducers than it is in most other imaging modalities.

Weaknesses Sonographic devices have trouble penetrating bone. For example, sonography of the adult brain is very limited though improvements are being made in transcranial ultrasonography. Sonography performs very poorly when there is a gas between the transducer and the organ of interest, due to the extreme differences in acoustic impedance. Even in the absence of bone or air, the depth penetration of ultrasound may be limited depending on the frequency of imaging. Consequently, there might be difficulties imaging structures deep in the body, especially in obese patients. Physique has a large influence on image quality. Image quality and accuracy of diagnosis is limited with obese patients, overlying subcutaneous fat attenuates the sound beam and a lower frequency transducer is required (with lower resolution) The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses. There is no scout image as there is with CT and MRI. Once an image has been acquired there is no exact way to tell which part of the body was imaged.

Major Uses of Ultrasound Obstetrics and Gynecology measuring the size of the fetus to determine the due date checking the position, sex of the baby. seeing the number of fetuses in the uterus checking the fetus's growth rate by making many measurements over time detecting ectopic pregnancy, the life-threatening situation in which the baby is implanted in the mother's Fallopian tubes instead of in the uterus determining whether there is an appropriate amount of amniotic fluid cushioning the baby Cardiology seeing the inside of the heart to identify abnormal structures or functions measuring blood flow through the heart and major blood vessels Urology measuring blood flow through the kidney seeing kidney stones detecting prostate cancer early

What is nuclear medicine? Nuclear medicine is a medical specialty that uses radioactive tracers (radiopharmaceuticals) to assess bodily functions and to diagnose and treat disease. Specially designed cameras allow doctors to track the path of these radioactive tracers. Single Photon Emission Computed Tomography or SPECT and Positron Emission Tomography or PET scans are the two most common imaging modalities in nuclear medicine.

What are radioactive tracers? Radioactive tracers are made up of carrier molecules that are bonded tightly to a radioactive atom. These carrier molecules vary greatly depending on the purpose of the scan. Some tracers employ molecules that interact with a specific protein or sugar in the body and can even employ the patient’s own cells. For most diagnostic studies in nuclear medicine, the radioactive tracer is administered to a patient by intravenous injection. Approved tracers are called radiopharmaceuticals since they must meet FDA’s exacting standards for safety and appropriate performance for the approved clinical use. The nuclear medicine physician will select the tracer that will provide the most specific and reliable information for a patient’s particular problem. The tracer that is used determines whether the patient receives a SPECT or PET scan.

How do radiopharmaceuticals work? Radiopharmaceuticals are introduced into the patient's body by injection, swallowing, or inhalation. The amount given is very small. The pharmaceutical part of the radiopharmaceutical is designed to go to a specific place in the body where there could be disease or an abnormality. The radioactive part of the radiopharmaceutical that emits radiation, known as gamma rays (similar to X-rays), is then detected using a special camera called gamma camera. This type of camera allows nuclear medicine physician to see what is happening inside the body. During this imaging procedure, the patient is asked to lie down on a bed and then the gamma camera is placed a few inches over the patient's body. Pictures are taken over the next few minutes. These images allow expert nuclear medicine physicians to diagnose the patient's disease.

How do radiopharmaceuticals work? (…)

What is a Gamma Camera? A gamma camera is a machine that is able to detect and make images from the very small amounts of ionising radiation emitted from patients having a nuclear medicine study. The gamma camera usually has a table, often narrow, on which the patient lies. The images are taken using the camera ‘head’. A camera might have one, two or occasionally three heads, with one or more being used to obtain the images. Each camera head has a flat surface that has to be very close to the patient. The camera heads might be supported in a number of different ways using strong metal arms or a gantry. There are no unusual sensations associated with having images taken with a gamma camera and the machine makes no noise.

How is Nuclear Medicine different from normal X-ray and CT examinations? During a normal X-ray or CT examination, an image is formed from the ‘shadow’ created by the body as it is positioned between the X- ray machine (source of the X-ray beam) and the X-ray detector. In nuclear medicine studies, the radiopharmaceutical given to the patient makes them, and the organ system or body part being studied, radioactive for a short time. This ionising radiation (usually a gamma ray) is emitted or released from the body, and can be detected and measured using a nuclear medicine gamma camera. An X-ray or CT image is formed from ionising radiation (X-rays) that passes through the body, but does not arise from the body; whereas a nuclear medicine image is formed from the ionising radiation (usually gamma rays) emitted from within the body.

What is Single Photon Emission Computed Tomography (SPECT)? SPECT imaging instruments provide 3 dimensional (tomographic) images of the distribution of radioactive tracer molecules that have been introduced into the patient’s body. The 3D images are computer generated from a large number of projection images of the body recorded at different angles. SPECT imagers have gamma camera detectors that can detect the gamma ray emissions from the tracers that have been injected into the patient. Gamma rays are a form of light that moves at a different wavelength than visible light. The cameras are mounted on a rotating gantry that allows the detectors to be moved in a tight circle around a patient who is lying motionless on a pallet.

What is Positron Emission Tomography (PET)? PET scans also use radiopharmaceuticals to create 3 dimensional images. The main difference between SPECT and PET scans is the type of radiotracers used. While SPECT scans measure gamma rays, the decay of the radiotracers used with PET scans produce small particles called positrons. A positron is a particle with roughly the same mass as an electron but oppositely charged. These react with electrons in the body and when these two particles combine they annihilate each other. This annihilation produces a small amount of energy in the form of two photons that shoot off in opposite directions. The detectors in the PET scanner measure these photons and use this information to create images of internal organs.

What are nuclear medicine scans used for? SPECT scans are primarily used to diagnose and track the progression of heart disease, such as blocked coronary arteries. There are also radiotracers to detect disorders in bone, gall bladder disease and intestinal bleeding. SPECT agents have recently become available for aiding in the diagnosis of Parkinson's disease in the brain, and distinguishing this malady from other anatomically-related movement disorders and dementias. Recently, a PET probe was approved by the FDA to aid in the accurate diagnosis of Alzheimer's disease, which previously could be diagnosed with accuracy only after a patient's death. The major purpose of PET scans is to detect cancer and monitor its progression, response to treatment, and to detect metastases. A combination instrument that produces both PET and CT scans of the same body regions in one examination (PET/CT scanner) has become the primary imaging tool for the staging of most cancers worldwide.

What are the risks of a Nuclear Medicine Allergic reactions, but are very rare and almost always minor. If you have ever had an allergic reaction to a medication, you should tell the technologist, nurse or doctor supervising your study before you have the radiopharmaceutical. In most cases, there will be no reason to cancel the study, but you might be observed more closely during the test to ensure any reaction is treated appropriately. Radiation risk

Corresponding References Ultrasound Related Link: http://www.physics.utoronto.ca/~jharlow/teaching/phy138_0708/lec04/ultrasoundx.htm https://en.wikipedia.org/wiki/Medical_ultrasound#Diagnostic_applications Nuclear Medicine Related Link: http://interactive.snm.org/docs/whatisnucmed2.pdf https://en.wikipedia.org/wiki/Nuclear_medicine http://www.insideradiology.com.au/pages/view.php?T_id=66#.VmFWULh97IU