Diagnostic Imaging Notes

Computed Tomography (CT)

  • Technique developed by Sir Godfrey Hounsfield in 1972.
  • X-ray slices are taken and converted into cross-sectional images.
  • Used in diagnostic studies of internal bodily structures to assist in the detection of pathology.

General Components of CT Scanner

  • CT Gantry
  • Patient Table
  • Contrast Injector
  • Control Panel

CT Scanning

  • Allows scanning from multiple directions to form a 3D image using mathematical algorithms adapted for computer processing.

X-Ray vs CT Images

  • X-ray images are 2D and composed of pixels.

  • CT images are 3D and composed of voxels.

    • Pixel: 2D
    • Voxel: 3D

CT Slice

  • A thin transverse slice of the body (1 to 10 mm thick) is selected for imaging.
  • The slice is x-rayed from many different directions by rotating the tube around the body.

CT Coordinates

  • X, Y, Z coordinates are used to communicate the three spatial dimensions.

    • Y-axis: Vertical - Table height
    • Z-axis: Axial - Table movement in/out of gantry
    • X-axis: Horizontal - Table Movement side to side

Image Formation

  • X-rays are recorded by multiple detectors.
  • The output of the detectors is fed to a computer, which forms a digital cross-sectional image from the data.
  • Resulting images arise from differential x-ray absorption of tissue.

First Clinical Scan

  • Atkinson Morley's Hospital, October 1971

Helical “spiral” CT – 6th Generation

  • Major advance used in all modern CT machines.
  • Previously only axial scanning was possible (CAT).
  • X-ray beam traces a helical path.
  • Results in a three-dimensional data set (raw data) which can be reconstructed into sequential images.
  • Allows quicker scans (a scan to be performed in a single breath hold).

Transverse Axial Image

  • Computed tomography can provide a transverse axial image at 90 degrees to the long axis of the body, i.e., a cross-sectional image of the body structures.

Multiplanar Imaging

  • Axial
  • Sagittal
  • Coronal
  • 3D bony reconstruction

Paediatric CT

  • Dose considerations (children are not small adults).

CT (Hounsfield) Numbers

  • The numbers in the image matrix are called CT numbers.
  • Each pixel of the image matrix has a number representing the x-ray attenuation, displayed on the monitor as a level of brightness in gray scale (differential absorption).
  • Range from -1000 (air) to +1000.
  • CT number of water is 0 HU.
  • CT number of water =0 HUCT \text{ number of water } = 0 \text{ HU}

CT Windowing

  • Once a CT image is reconstructed, CT windowing allows manipulation of how it is displayed.
  • Window width (image contrast) and window level (image brightness) determine the shades of gray displayed.

Window Level (Image Brightness)

  • Decrease brightness (WL High)
  • Increase brightness (WL Low)

Window Width (Image Contrast)

  • Increase contrast (Narrow WW)
  • Decrease contrast (Wide WW)

CT Advantages

  • Easily accessible in most areas.
  • Fast scan times.
  • High resolution.
  • Excellent bone detail.
  • Reduced contrast media required.
  • Reduced breath holds.
  • Multiplanar imaging capability.
  • Post-processing Capability.
  • Non-invasive CT Angiogram / CT colonoscopy capability.

Non-invasive CT Angiogram

  • Ability to perform CTA which does not require arterial catheterization, only an IV injection of contrast.

CT Disadvantages

  • Dose: CT constitutes only about 4% of examinations but is responsible for 20% of medical imaging dose (technology now for “Low Dose CT scans”).
  • Motion, beam hardening, or streak artifacts can degrade images.
  • Weight limitations of bed.
  • Poor tissue contrast compared to MRI; may need IV contrast for soft tissue or vascular delineation.
  • High cost of equipment ($1 million+) and scans.
  • Usually not portable.
  • Adverse reaction to contrast (if required).

Clinical Indications

  • Extent of disease staging
  • Demonstration of tumors, masses, enlarged organs, or lymph nodes
  • Stroke Management
  • Nerve pain injections. Drainages. Biopsies.
  • Monitoring fractures
  • Trauma Management
  • Plan medical

Contraindications

  • Pregnancy
  • Contrast allergy (if required)
  • Claustrophobia
  • Physical limitations (patient unable to lie flat, keep still, patient refusal)

Safety Considerations

  • Radiation Safety

    • ALARA – Justification
    • Optimisation
    • Limitation
    • Use of Radiation reduction techniques
    • Children are not young adults – adapt protocols accordingly

  • Contrast Media Reactions and Precautions

    • Screening Questionnaire
    • Monitoring and knowledge of departmental protocol in case of a reaction
    • Clear explanation of risks and expectation – e.g., metallic taste

  • DRL Departmental levels

  • Monitoring and benchmarking

Magnetic Resonance Imaging (MRI)

MRI Components

  • Hardware components of MRI

    • Gantry

    • Superconducting magnet

      • Typically 3 Tesla
      • (1.5 – 9.4)

    • Containing the magnetic gradient coils

      • X, y, and z axes

    • RF coils surround the patient

      • Transmit and receive signals

Different Coils for Different Body Parts

  • Head Coil
  • Neck Coil
  • Shoulder Coil
  • Knee Coil
  • Wrist Coil
  • Medium Body Coil
  • Large Body Coil
  • Head & Neck Coil
  • Kinematic Joint Analysis Coil
  • Small Body Coil
  • Spine Coil
  • TMJ Coil
  • 6-inch Flex Coil
  • 9-inch Flex Coil
  • Breast Coil

MRI Scanner

  • Scanners come in different sizes and strengths, with open scanners becoming more accessible.

MRI and Hydrogen Atoms

  • Our bodies are made up mostly of water molecules (that’s two parts hydrogen and one part oxygen) H2OH_2O
  • It is the behaviour of these hydrogen atoms when in a magnetic field that is recorded as an image.

MRI Procedure (and Physics)

  • MRI utilises Hydrogen atoms because:

    • its nucleus has a single proton
    • It is abundant in the body (water and fat)
    • When the body is placed in a strong magnetic field, the proton axes all line up.
    • This uniform alignment creates a magnetic vector orientated along the axis of the MRI scanner
    • The hydrogen proton spins on its axis with a north/south pole

MRI Physics

  • All nuclei ‘precess’ (or spin)
  • The protons are first "excited" and then "relaxed," by loud radiofrequency pulses (specific to Hydrogen) and the resultant radio signals emitted by the nuclei in response to this are computer-processed to form an image
  • Different types of tissue can be distinguished due to relaxation times.
  • Each sequence has its own degree of contrast and shows a cross-section of the body in one of several planes

MRI Sequences

  • By changing exam parameters, the MRI system can cause tissues in the body to take on different appearances.

    • T1: - Anatomically detailed with high resolution. Fat is bright, fluid and calcium are black
    • T2 :- Fluid is bright, muscles are dark
    • STIR :– Suitable for highlighting fluid (e.g. bone bruising, effusions).

  • Intravenous gadolinium Gd-DTPA (Gadopentetate dimeglumine) is a clear injectable contrast agent used in MRI.

MRI Brain Imaging

  • Head coil
  • T1/T2 sequences
  • Gd-DTPA if indicated
  • Patient supine, headfirst
  • Sagittal, T1
  • Coronal, T2
  • Axial, T2

MRI Advantages

  • No ionising radiation is used
  • Excellent soft tissue contrast
  • Multiplanar imaging capability
  • Can tailor examination parameters to answer specific medical questions
  • Post processing Capability
  • Non-invasive Angiography capability without an injection of contrast
  • If contrast (Gadolinium) is required, reaction rates are very small

MRI Disadvantages

  • Requires stringent Safety considerations – 4 zones due to magnetic strength
  • 5% of patients are claustrophobic
  • Patients with metallic/surgical implants may be contraindicated
  • Can not scan very large patients
  • Long (and loud) scans
  • Motion artifacts (respiratory, cardiac, patient)
  • Other artifacts - metal etc
  • Very expensive equipment (1 to 2 million dollars) and high running costs – not available everywhere
  • Installation restrictions
  • Each examination is expensive ($400 - $1000)
  • Availability of Helium Gas (MRI coolant)

MRI Indications

  • Soft tissue pathology

    • Infections
    • Ligament damage
    • Tendonitis
    • Masses, cysts or tumors
    • Musculoskeletal trauma

  • Bone pathology

    • Tumours
    • Bruising
    • Stress and insufficiency fractures

  • Neurological system

MRI Contraindications

  • Metallic fragments in the eye
  • Pacemakers
  • Aneurysm clips
  • Orthopaedic and dental hardware
  • Patient too large
  • Claustrophobic
  • Cochlear implants
  • Spinal stimulators
  • Inserted pumps
  • Some artificial valves
  • Some tattoos

MRI Safety Considerations

  • Magnetic Strength

  • Safety Zones

  • Safety protocols and practice e.g., handheld metal detectors / Annual education programs and training of all staff

  • Patient Protection

    • Screening Questionnaire
    • Metal implant assessment -non-ferrous or non-ferromagnetic metals are safe in MRI e.g., titanium
    • Ear protection from loud MRI scanner noise

MRI Magnetic Strength

  • The magnets in MRI scanners can be up to 40000 times the strength of the earth’s magnetic field
  • Items such as paperclips, pens, keys, scissors, coins, jewellery, watches, and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet at very high speeds, posing a threat to everyone in the room
  • Credit cards, bank cards and anything else with magnetic encoding will be erased by most MRI systems

MRI Safety Zones

  • Zone I: Reception/waiting room
  • Zone II: Patient preparation and holding
  • Zone III: Control room computer rooms
  • Zone IV: MRI Scanner room (Magnet)

MRI Safety Gone Wrong

  • Metal equipment such as drip poles, crash trolleys, O2 tanks, SATS machines, mop buckets and stethoscopes have all been pulled into the bore of magnets.

Ultrasound

  • Ultrasound imaging utilises sound waves - a form of energy consisting of mechanically produced waves. Non ionising radiation

  • The frequencies are above the range of human hearing (human hearing 20 - 20,000 Hz)

  • Diagnostic ultrasound uses frequencies in the range of 1 to 20 MHz (commonly 3 to 17 MHz)

  • Different frequencies used for different parts

    • Higher frequencies – better spatial resolution but less tissue depth (foot - superficial musculoskeletal structures)
    • Lower frequencies – better penetration, but less resolution and clarity

Ultrasound Interactions

  • When sound waves meet a boundary (interface) between two different media they may suffer reflections or refractions.
  • Reflection and absorption of sound waves varies according to the tissue type and density
  • Images are created by interpreting sound reflections.
  • Images are made up of a mosaic of white or grey dots.
  • Each dot represents an echo of a structure in a patient
  • Ultrasound is performed in ‘real-time’ .
  • If the incident beam is at right angles to the interface, the sound will be reflected back and some may be transmitted

Ultrasound History

  • Development of sonar in WWII was one of the first applications of ultrasound.

Ultrasound Machine Components

  • Housing with electronics and controls
  • Monitor for display
  • Transducer
  • Recording device - Separate or integrated

Transducer

  • Sound waves are produced by a transducer and sent into the body. These sound waves interact with the tissues of the body and reflections are sent back to the transducer. It is these reflected sound waves which create the image

  • Transducer houses the piezoelectric crystal which has a dual function as transmitter of pulses and receiver of echoes

    • Converts electrical energy into mechanical energy (OUT) (creating the physical u/s wave) - transmits pulses
    • Converts mechanical energy into electrical energy (IN) (records it digitally to become an image) - receives echos

Attenuation

  • Attenuation is a general term meaning the reduction in intensity (of power or amplitude) of an ultrasound beam as it passes through a medium

  • These attenuation processes include:

    • absorption
    • divergence
    • scattering
    • reflection

  • The degree of attenuation depends on the material involved, the distance travelled and the frequency of the beam

Sonographer

  • Is a person trained and qualified in ultrasound.
  • Sonographer performs the examination and records representative images
  • Not all sonographers have a radiography background
  • Sonographers have high degree of decisional latitude: they make independent decisions on the course of the examination.
  • Sonographers record their findings and submit these to the Radiologist.

Ultrasound Terminology

  • Echogenicity is a description of how bright the tissue or structure is – i.e. how intense the echoes are. A structure could be described as:

    • Anechoic – This term is used to describe an area that has no echoes – i.e. it appears completely Black on the image. eg. clear fluid, urine, blood, cysts Synonyms - echolucent or sonolucent
    • Hypoechoic – An area where the echo intensity is low – i.e. it is dark on the screen (Grey). However it is not anechoic – there are echoes present. eg. Tendon Synonyms - echopenic
    • Hyperechoic (Echogenic) - This describes an area where the echoes are more intense – i.e. they are brighter on the screen. White. eg. Bone, calcification, renal/gall stones Hyperechoic - a synonymous term, however echogenic is more commonly used.

  • Echotexture is a description of the pattern of echoes –Described as: fine or coarse

    • Homogenous (very even pattern)
    • Heterogenous (a mixed pattern)

  • An area on an U/S image is described in terms of its: Echogenicity and Echotexture

Scan Planes

  • Transverse scan plane
    • When the transducer is on the anterior aspect of the patient, the image is viewed with the right side of the patient on the left side of the screen.
    • Note that on the image, the transducer is ALWAYS at the top of the screen.
  • Sagittal
    • The image is always viewed with the patient’s head to the left of the screen.
  • Coronal
    • With this view, it is the lateral aspect of the patient at the top of the screen (that is where the transducer is placed).

Doppler Studies

  • Colour Doppler shows directional information about the vascular flow

    • E.g., foetal heart rate arterial and venous blood flow vascularity of tumours etc
    • In this case flow that is blue is in the direction of the transducer and flow that is red is way from the transducer
    • It is shown by the colour bar on the side of the image

Ultrasound Advantages

  • No ionising radiation
  • Mobile equipment
  • Relatively quick and cost effective
  • Well tolerated by patients
  • Good soft tissue information
  • Good for musculoskeletal applications
  • Real-time (can see motion)
  • Accurate measurement of structures
  • Good localisation for interventional studies

Ultrasound Disadvantages

  • Many areas are not suitable to image with ultrasound: bone, lungs, abdominal structures covered by bowel, adult cranium and G-I tract.
  • Limited by patient body habitus for some examinations
  • Handheld transducer: This results in scan plane variability, and it is very difficult to reproduce an exact scan plane.
  • Highly operator dependent
  • Interpretation of images difficult

Clinical Indications

  • Obstetric patients
  • Gynaecological patients
  • Organ imaging
  • Vascular imaging
  • Cardiac studies – echocardiography
  • Neonatal studies
  • Ophthalmology
  • Small Parts – Breast, Thyroid, Testes
  • Musculoskeletal imaging
  • Interventional Procedures

Nuclear Medicine

  • Nuclear Medicine attaches radioisotopes to compounds which are taken up by specific organs (kidneys, heart, bone) and areas of high metabolic activity (bone tumours, arthritic changes, fractures) for both diagnostic and therapeutic purposes.
  • Functional imaging - imaging the distribution of a radiopharmaceutical in an organ
  • Relies on microcirculation to transport a pharmaceutical to the organ
  • The resolution is poor compared with other modalities and hence anatomical detail is poor
  • Is a sensitive imaging modality for detection of some pathologies
  • Provides complementary information to the other imaging modalities and can be used in registration with other modalities

Molecular Imaging

  • Molecular imaging is an imaging modality that provides metabolic and functional information.

  • To see how the body is functioning and to measure its chemical and biological processes.

  • Other diagnostic imaging procedures—such as x-rays, computed tomography (CT) and ultrasound—predominantly offer anatomical information

  • When disease occurs, the biochemical activity of cells begins to change.

    • E.g., cancer cells multiply at a much faster rate and are more active than normal cells and brain cells affected by dementia consume less energy than normal brain cells.

  • Molecular imaging detects the cellular changes that occur early in the course of disease, often well before structural changes can be seen on CT and MR images

Molecular Imaging Applications

  • Identify disease in its earliest stages
  • Determine the exact location of a tumor, often before symptoms occur or abnormalities can be detected with other diagnostic tests
  • Determine information that would require more invasive procedures such as biopsy or surgery
  • Provide treatment direct to cellular level
  • Demonstrate response to treatment

Nuclear Medicine Technologies

  • Single photon emission computed tomography (SPECT)

    • uses gamma emitting radioisotope (tracer):
      • technetium-99m
      • iodine-123
      • iodine-131
    • poorer contrast and spatial resolution (cf. PET)
    • usually one large crystal based detector

  • Positron emission tomography (PET)

    • uses positron emitting radioisotope (tracer)
      • F-18 fluorodeoxyglucose (FDG)
    • better contrast and spatial resolution (cf. SPECT)
    • ring of multiple detectors

SPECT Scans

  • SPECT scans involve two steps: a radioactive injection (called a tracer) and using a SPECT machine to scan a specific area of body to provide 3D images.
  • the radioactive tracer highlights areas of blood flow most common uses of SPECT are to help diagnose or monitor brain disorders, heart problems and bone disorders.
  • SPECT scans can also diagnose and track the progression of cancer that has spread to the bones

Gamma Camera

  • Conventional image receptor is NOT sufficiently sensitive for detection of the relatively ‘high’ energy gamma-rays used in NM. Instead, a “gamma camera (or “scintillation camera)” is used to record the ‘image’ of the activity distribution

PET: Positron Emission Tomography

  • Molecular Imaging Technique
  • Uses radioactive tracers that emit positrons during the radioactive decay process eg. FDG (fluorodeoxyglucose),
  • Demonstrates the metabolism and biochemical function of organs and tissues and identifys changes at a cellular level
  • Allows the identification of areas of metabolic activity which enables escalation of the radiation dose for the most aggressively growing tumors
  • Radioactive compounds specificity for distinct tissues e.g high grade glioma or prostrate cancers.

Bone Scans

  • A radioactive isotope T99 (Technetium) is injected into the body which accumulates in the bone
  • This uptake process takes about 3 hours
  • The Gamma ray emissions from the isotope in the body are detected by a special camera (called a Gamma Camera) placed over the patient. The radiation pattern is converted to an image
  • 3 hours post injection images will demonstrate the absorption of the radioisotope into the bone as ‘hot spots’ indicating metabolic uptake

Nuclear Medicine Advantages

SPECT
  • High sensitivity of modality to allow for very early detection of disease (molecular imaging)
  • Excellent quantitative ability
  • More Accessible and Available: SPECT scanners are more widely available than PET scanners.
  • Less Expensive : SPECT scans are generally less expensive than PET scans.
  • Tracers- more readily available.
  • Longer half-lifes so longer imaging scope
PET
  • Higher Spatial Resolution better detail and can detect smaller abnormalities
  • Higher Sensitivity detect smaller amounts of activity
  • More Accurate Quantification enables more precise measurements of drug uptake and other physiological processes
  • More Sophisticated Tracers – short half-lifes
  • Improved Image Quality

Nuclear Medicine Disadvantages

  • Low resolution images
  • Although highly sensitive, NM is not specific
  • NM departments are highly specialised with limited locations outside major population areas.
  • Due to the long uptake time the whole procedure takes about 4 hours to complete (SPECT)

Clinical Indications

  • Bone Scans:

    • Cancer of the breast, prostate or other forms of cancer that can spread to bone
    • Trauma
    • Stress fractures
    • Shin splints
    • Bone Infections
    • Atypical bone pain
    • Bone scintigraphy: study of skeletal system

  • Genitourinary studies: anatomic and functional studies of kidneys

  • Brain scan: to evaluate for stroke, Alzheimer’s disease, and Parkinson’s disease

  • Thyroid uptake study: sodium iodide (123I) taken orally (evaluates for hyperthyroid and hypothyroid)

  • Cardiac studies: myocardial perfusion—stress/rest cardiac imaging

  • Lung studies: ventilation/perfusion studies

Contraindications

  • Pregnancy
  • Breastfeeding
  • Inability to lie flat
  • Inability to keep still (may take up to 60mins)
  • Claustrophobia

Nuclear Medicine Team

  • Nuclear medicine technologist

    • Handling, assessment, and administration of radionuclides

  • Nuclear medicine physician

    • Interpretation of nuclear medicine procedures

  • Radiation safety officer (RSO)

    • Reviews imaging protocols and dosimetry records

  • Health physicist

    • Calibration and maintenance of equipment

Hybrid Techniques

  • PET/CT
  • mMR (PET/MRI)

Hybrid Imaging System

  • Provides details of disorders at a molecular level (PET) and a detailed image of the body’s anatomy (CT).
  • CT scan and PET scan performed using different aspects of the modality.
  • Fusion / co registration to enable information from CT and PET to be viewed together.

PET /CT

  • Hybrid imaging combining NM/CT scan
  • PET demonstrates biological function
  • CT gives anatomical information
  • Similar to PET scan though less expensive due to the type of radioisotope used.
  • Lower resolution images than PET
  • Higher radiation dose to the patient
  • Used mainly in cardiac and brain imaging
  • SPECT scan - single photon emission CT

Clinical Applications:

  • Oncology

  • Cardiology

  • Neurology

  • Detect cancer and metastatic disease

  • Monitor treatment response

  • Assess blood flow

    • Brain
    • heart