Introduction to Imaging Modalities and Main Utilities

Introduction to imaging modalities and main utilities.

Aim and Objectives

At the end of the lecture the student should be able to:

n Define indications for radiological imaging

n Know different modalities

n Know the principles of each modality

n Know the utility of each modality

MRI

· It uses powerful magnetic field, radio frequency pulses and a computer to produce detailed images of organs, soft tissues, bone and virtually all other internal body structures.

· MRI does not use ionizing radiation (x-rays).

· It provides much greater contrast between different soft tissues of the body than computed tomography (CT)

Principles

· In the magnet, radio waves redirect the axes of spinning protons (nuclei of hydrogen atoms in the body) in a strong magnetic field.

· Magnetic field is produced by passing an electric current through wire coils in most MRI units.

· Coils, located in the machine and placed around the part of the body being imaged, send and receive radio waves, producing signals that are detected by the coils.

· Computer then processes the signals and generates a series of images each of which shows a thin slice of the body.

Imaging

· schemes have been devised for combining field gradients and radio frequency excitation to create an image:

o 2D or 3D reconstruction from projections

o Building the image point-by-point or line-by-line

o Gradients in the RF field rather than the static field.

Image contrast

· Created by differences in the strength of the NMR signal recovered from different locations within the sample

· Depends upon the relative density of excited nuclei (usually water protons), on differences in relaxation (T1, T2, and T*2) of those nuclei after the pulse sequence

· Contrast in most images is a mixture of all effects, but careful design of the imaging pulse sequence allows one contrast mechanism to be emphasized while the others are minimized

· The ability to choose different contrast mechanisms gives MRI tremendous flexibility. In the brain, T1-weighting causes the nerve connections of white matter to appear white, and the congregations of neurons of gray matter to appear gray, while cerebrospinal fluid (CSF) appears dark. The contrast of white matter, gray matter and cerebrospinal fluid is reversed using T2 or T*2 imaging,

Contrast enhancement

· contrast agent used when not able to generate enough image contrast to adequately show the anatomy or pathology requierd

o water taken orally, for imaging the stomach and small bowel.

o Paramagnetic contrast agent, gadolinium compound. Gadolinium-enhanced tissues and fluids appear extremely bright on T1-weighted images. Provides high sensitivity for detection of vascular tissues (e.g., tumors) and permits assessment of brain perfusion (e.g., in stroke).

Equipment

· MRI unit is a large cylinder-shaped tube surrounded by a circular magnet.

· A moveable examination table that slides into the center of the magnet.

Procedure

· Patient placed on moveable examination table.

· Straps and bolsters may be used to help you stay still and maintain the correct position during imaging.

· Sedative might be used in claustrophobic patients

· If contrast used, it is injected at this stage.

· Examination table moved inside the unit.

· Series of images taken

· Entire exam is usually completed in 15 to 45 minutes.

Precautions

· No metal item should be worn

· Safe for patients with metal implants, except for

o internal (implanted) defibrillator or pacemaker

o Cochlear (ear) implant

o Some types of clips used on brain aneurysms

· Medical or electronic devices in body may interfere with the exam or potentially pose a risk, depending on their nature and the strength of the MRI magnet

o artificial heart valves

o Implanted drug infusion ports

o Implanted electronic device, including a cardiac pacemaker

o Artificial limbs or metallic joint prostheses

o Implanted nerve stimulators

o Metal pins, screws, plates, stents or surgical staples

Utility

· Tumors of the chest, abdomen or pelvis.

· Blockages or enlargements of blood vessels,

· Diseases of liver, bile ducts, gallbladder, and pancreatic ducts.

· Cysts and solid tumors in the kidneys and other parts of the urinary tract.

· Tumors and other abnormalities of the reproductive organs (e.g., uterus, ovaries, testicles, prostate).

· Gynecological problems eg fibroids, endometriosis and adenomyosis.

· Breast cancer and implants.

Benefit

· Noninvasive, no exposure to ionizing radiation.

· Better resolution for the soft-tissue structures of the body eg heart, liver for early diagnosis and evaluation of many focal lesions and tumors.

· Enables abnormalities obscured by bone to be detected that are not with other imaging methods.

· Assess the biliary system noninvasively and without contrast injection.

Risks

· Has almost no risk to the average patient when appropriate safety guidelines are followed.

· If sedation is used there are risks of excessive sedation.

· Implanted medical devices that contain metal may malfunction or cause problems during an MRI exam.

· There is a very slight risk of an allergic reaction if contrast material is injected.

Ultrasound Scan

· Uses high-frequency sound waves to produce images

· Do not use ionizing radiation

· 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.

Equipment

· Consist of a console containing a computer and electronics, a video display screen and a transducer

· Transducer is a small hand-held device attached to the scanner by a cord. It sends out high frequency sound waves into the body and then listens for returning echoes from the tissues

· Image is immediately visible on screen

Procedure

· Patient is positioned lying face-up on an examination table that can be tilted or moved.

· A clear water-based gel is applied to the body to help transducer make secure contact with body and eliminate air pockets between the transducer and the skin.

· The transducer is firmly pressed against the skin and swept over the area of interest.

Utility

· Examine most internal organs

· Guide procedures such as needle biopsies,

· Image the breasts and to guide biopsy of breast

· Diagnose a variety of heart conditions and to assess damage after MI or diagnose valvular heart disease.

Benefits

· Noninvasive and is usually painless.

· Gives clear picture of soft tissues not visualized on x-ray images.

· Causes no health problems and may be repeated as often as is necessary.

· Preferred imaging modality for diagnosis and monitoring of pregnant women and fetus

· Provides real-time imaging, for guiding minimally invasive procedures such as needle biopsies and needle aspiration.

Risks

· Mostly extremely safe

· Two potential physiological effects

o It enhances inflammatory response

o Can heat soft tissue

Doppler ultrasound

· It measures velocity of blood cells as they move through vessels.

· The movement causes a change in pitch of the reflected sound waves (called the Doppler effect) that is visualized.

Utility

· Blockages to blood flow (such as clots).

· Narrowing of vessels

· Tumors and congenital malformation.

· Determine patient suitability for procedures like angioplasty

Types:

· Color doppler

o Uses a computer to convert doppler measurements into an array of colors to visualize the velocity of blood flow through vessel.

· Power doppler

o Greater detail of blood flow, especially when blood flow is little or minimal.

o Does not determine the direction of blood flow, which may be important in some situations.

· Spectral doppler

o Displays blood flow measurements graphically, in terms of the distance traveled per unit of time.

Other Types

Transducer is attached to a probe and inserted inside an orifice

§ Transesophageal echocardiogram. Transducer is inserted into oesophagus to visualise heart.

§ Transrectal ultrasound. Transducer is inserted into rectum to view the prostate.

§ Transvaginal ultrasound. Transducer is inserted into vagina to view the uterus and ovaries.

Radionuclide Scan

A way of imaging bones, organs and parts of body by using a small dose of a radioactive chemical. There are different types of radionuclide chemical. The one used depends on which organ or part of the body is to be scanned.

Principle

· Radionuclide (radioisotop/isotope) is a chemical which emits gamma rays.

· Tiny amount of radionuclide is injected into a vein, breathed in, or swallowed

· Different types of radionuclides used for different area of the body

· Cells most 'active' in the target tissue or organ will have higher uptake.

· Gamma rays are detected by gamma camera.

· Converted into an electrical signal, and sent to a computer.

· The computer builds a picture by converting the differing intensities of radioactivity emitted into different colours or shades of grey.

· Areas emitting lots of gamma rays may be shown as red spots ('hot spots'). Areas emitting low levels may be shown as blue ('cold spots').

Equipment

· Performed using gamma camera, encased in metal that is capable of detecting radiation and taking images from different angles.

· It may be suspended over the examination table or it may be beneath the table.

· Gamma cameras are dual-headed with one camera above and one camera beneath the table.

· The camera could also be located within a large, doughnut-shaped scanner

· Computer creates images from the data obtained by the camera or scanner.

· A probe is a small hand-held device resembling a microphone that can detect and measure the amount of the radiotracer in a small area of the body.

Procedure

· Radionuclide is administered.

· There are different timings for uptake of different radionuclide in their target tissue.

· Gamma camera detects the gamma rays from the body

· Computer turns the information into an image.

· Number of pictures taken, and the time interval between each picture, varies depending on what is being scanned.

Utility

· Renal function analyses

· Myocardial perfusion scan

· Ventilation perfusion scans of the lung.

· Bone evaluation for fractures, infection, arthritis and tumors.

· Detect cancer and its spread in various parts of the body.

· Identify bleeding into the bowel.

· Gall bladder inflammation

· Locating infection.

· Measure thyroid function for hyper or hypothyriod

· Brain abnormalities, such as seizures, memory loss and abnormalities in blood flow.

· Lymph nodes detection before surgery for breast cancer or melanoma.

Benefits

· Information provided is unique and often unattainable from other imaging procedures.

· Provides information about most diseases to diagnose and plan treatment and management

· Less expensive and provides more precise information than exploratory surgery.

Risks

· Low risk of radiation exposure due to radioactive substance administered but radiation risk is very low compared with the potential benefits.

· No known long-term adverse effects from such low-dose exposure.

· Allergic reactions to radiopharmaceuticals may occur but are extremely rare and are usually mild.

· Injection of the radiotracer may cause slight pain and erythema

· Precautions for pregnant and breastfeeding women

Radionuclide therapies

· Radioactive iodine (i-131) therapy used to treat hyperthyroidism and thyroid cancer.

· Radioactive antibodies used to treat certain forms of lymphoma

· Radioactive materials used to treat painful tumor metastases to the bones.