Basic Science FRCS > Imaging Techniques > Flashcards

Imaging Techniques Flashcards

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1
Q

What are X-rays ?

A
  • Are a form of HIGH ENERGY RADIATION belonging to the electro magnetic spectrum, 10-10 lambda(m)
  • They pass thru the body undergoing Differential ABSORPTION by tissues
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2
Q

How do X-rays in radiography work?

A
  • X-rays fall onto fluorescent film- screen combinations -> black silver crystals ( film blackening) these processed to give an image.
  • The tissues containing high atomic no- e.g. calcium in bone, absorb a higher amount of X-rays (high beam attenuation) -> resulting in less crystals formation on the film & therefore a white radiodense appearance.
  • Tissues where less absorption - e.g. fat- ( low beam attenuation- give black radiolucent appearance as more crystals are formed on the film. Muscles appear grey as intermediate attenuation
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3
Q

How are X-rays produced?

A
  • Heating a fine filament usually made of Tungsten ( negative cathode) to INCANDESCENCE in a VACUUM to around 220oC-> Emission of ELECTRONS ( thermionic emission)
  • Electrons leave cathode move to positive anode- a smooth metal fragment of tungsten.
  • Electrons hit tungsten at about half speed of light on the FOCAL SPOT where they either interact with
    • 1) outer electrons of target nucleus -> heat
    • 2) inner electrons- knocking them off orbit-> X-RAYS
    • 3) nucleus - >electrons slow down! change direction -> X-rays from BRAKING radiation ( 80% production of xray made this way)
    • As X-rays pass thru pt they can be Transmitted, absorbed, scattered -> attenuation After passing thru pt fall onto X-ray plate
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4
Q

What is the X-ray cassette made up of?

A
  • Carbon fibre/ aluminium front - filter to minimise beam attenuation , remove low energy X-rays , reduce pt exposure
  • Lead sheet back- decrease backscatter of radiation
  • Film- polyester based coated with fine photographic emulsion - silver iodiobromide- sensitive to X-rays
  • Two intensifier screens on each side of the film-polyester base coated with dense layer of phosphor crystals that absorb X-rays and converts them into visible light which exposes the film
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5
Q

What is involved with processing of the film?

A
  • Development by alkaline immersion
  • Fixative in acid immersion
  • Wash Dry
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6
Q

What is the difference with digital imaging?

A
  • Digital Xrays is a form of X-ray imaging, where digital X-ray sensors are used instead of traditional photographic film.
  • Indirect FPDs. Amorphous silicon (a-Si) is the most common material of commercial FPDs.
  • Combining a-Si detectors with a scintillator in the detector’s outer layer, which is made from caesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S), converts X-rays to light.
  • Because of this conversion the a-Si detector is considered an indirect imaging device.
  • The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal.
  • The digital signal is then read out by thin film transistors (TFTs) or fiber-coupled CCDs. The image data file is sent to a computer for display.
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7
Q

What are the advantages of digital radiography?

A
  • Advantages include time efficiency through bypassing chemical processing
  • ability to digitally transfer and enhance images.
  • Also less radiation can be used to produce an image of similar contrast to conventional radiography.
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8
Q

What are the advantages of X-ray radiology?

A
  • Good for assessing bone
  • Cheap
  • Easily obtained
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9
Q

What are the disadvantages of X-ray radiology ?

A
  • Not sensitive to subtle bony destruction or abnormalities
  • Difficult to interpret in areas of complex anatomy
  • Form of ionising radiation -aim to reduce dose
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10
Q

What does Pacs stand for ?

A
  • Picture archiving and comminution systems
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11
Q

What are the disadvantages of digital X-rays ?

A
  • Do not have the high spatial resolution of radiographic film- screen combinations
  • Expensive though costs reducting
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12
Q

What is ultrasound ?

A
  • An imaging modality which used HIGH frequency (3-50Mhz) SOUND Waves generated by a PIZOELECTRIC crystal transducer.
  • The waves are reflected and refracted at the tissue interface
  • Reflected waves return to the Transducer where they are converted into electrical signals in order to -> image
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13
Q

How does ultrasound work?

A
  • Sound waves produced from PIZOELECTRIC CRYSTAL.
  • A DC VOLTAGE is applied & then reversed across the crystal which expands/ contracts- changes shape -> compression sound waves
  • When Coupled to the skin using lubricating gel sound waves are transmitted into the body and reflected back.
  • When sound waves arrive back at the transducer-> distort shape of PIZOELECTRIC crystal -> voltage
  • The more sound waves reflected the greater the voltage -> brighter the image
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14
Q

How does higher frequency of the wave effect the overall image?

A
  • The higher the frequency the higher spatial resolution , the attenuation within the tissue is higher- so limiting visualisation to specific structures
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15
Q

What is uss used for?

A
  • Assessing tendons
  • Assessing masses
    • cystic= hypoechoic appearance
    • solid= hyperechoic appearance
  • Confirming joint effusions
    • with guidance of injections
  • Screening and evaluation of DDH
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16
Q

What are the advantages?

A
  • NO IONISING RADIATION
  • No side effects
  • Painfree
  • Non invasive
  • Machines
    • SMALL
    • ** INEXPENSIVE**
    • PORTABLE DOPPLER added to observe Vascularity and blood supply- dynamic
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17
Q

What are its disadvantages?

A
  • High operator dependance for acquision and intrepretation
  • Field of view is often limited
  • Difficult to characterise the imaged tissue
  • Cannot penetrate cortical bone
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18
Q

What do the additional letters A,B,M mode stand for ?

A
  • A-mode= amplitude mode
  • B- mode= Brightness mode
  • M-mode= time motion mode
  • B mode mostly commonly used in clinical practice -> 2 d images
  • A mode-> 1 dimensional image used in opthalmolgy
  • M- used to asses cardiac valve motion
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19
Q

What does CT stand for?

A

Computerised Tomography

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20
Q

What is CT?

A
  • Is an imaging modality that uses x-rays to create pictures of cross-sections of the body.
  • X-rays are within a fan shaped x ray beam which rotates around the patient and sensitive detectors opposite which record the attenuated X-rays as they pass thru the body.
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21
Q

What are ct scans made up of?

A
  • Comprise of Scanning gantry - X-ray generator and curvlinear detector - opposite each other
  • Pt couch
  • Computer processor
  • Display system
  • Each rotation of the gantry produces an axial slice thru the patient
  • . Modern scanners use helical rotation to allow continuous acquisition of the data
  • The tissues that attenuate the beam to a higher degree, such as bone, appears denser on CT image than tissues such as muscle
  • EAch tissue is assigned an attenuation coefficient ( in Hounsfield units, HU)
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22
Q

What are the CT images made up of?

A
  • Pixels- it not only presents a 2d image but a thickness (in VOXELS)
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23
Q

What happens to the ct image before we see it?

A
  • It has to be manipulated by changing the window levels & widths so that the variable areas of attenuation in the grey scale that we cannot determine by the naked eye can be seen
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24
Q

What is ct used for ?

Study These Flashcards
A
  • Assessment of fractures
  • With contrast to assess joints
  • Bone densitometry
25
What tissues have a higher attenuation? How are this graded?
* Bone - appear denser on ct image- white cf areas of low attenuation- fat appear dark * Each tissue is given a **ATTENUTATION COEFFICIENT** in **HOUNSFIELD UNITS** relative to the **Value of WATER** ( zero HU)
26
What are CT advantages ?
* **3D imaging and x sectional anatomical data** * **Better than xrays** for soft tissue imaging but not as gd as MRI * **Quantiative data on composition** * Good for **assessing cortical bone** * **Guide interventional proceedures**- biopsy
27
What are its disadvantages ?
* Significant use of **ionizing radiation** * **Inferior soft tissue contrast** resolution to MRI * **Claustrophic** for pt
28
What is the advantage of spiral CT?
* Allow rapid multi slicing as the gantry rotates continuously
29
What are the advantages of multidetector Ct?
* Multiple rows of detectors opposite side of X-rays source- so one rotation allow multiple slices thru the body to be imaged at once
30
What does MRI stand for?
* Magnetic resonance imaging
31
What is MRI?
* **An image modality using _STRONG MAGNETIC FIELDS_ and _EXCITATION RADIOFREQUENCY PULSES_ to the patient which results in _emission of RADIOFREQUENCY signals by the tissues_ of interest which are used to build up an image.**
32
What do MRI DETECT?
* **Hydrogen nuclei- PROTONS** in water * Images reflect the _relative concentration of protons in tissues_ by measuring the **strength of signals from individual VOXELS In a slice of the patient.**
33
How do MRI work?
* Each proton spins like a top- aka **nuclear spin** * Without a magnetic field the axes are **RANDOMLY ORIENTATED** so produced no net magnetic field. * The MRI applies a **STRONG STATIC MAGNETIC FIELD -\> Protons AXIS ALIGN WITH LONG AXIS OF THE MAGNET/SCANNER -\> net magnetic field** * The protons also wobble around their axis w a fixed frequency= **PRECESSION** * A **RADIOFREQUENCY PULSE is applied for a few MILLISECONDS-\>** _protons realign at an angle to the longitudinal axis of the scanner_ & also causes precession of each proton to be pulled into step = phase with each other * They induce currents in the recieving coils * the changes in the currents/ signals are used to characterise the tissue * When the pulse stops- * the protons realign with the long axis of the scanner-\> long magnetization vector, it increases to max * The precessional then fall out of step **(dephasing)**-\> the transverse magnetization vector to decrease from its max
34
Define T1 in an MRI image?
* The time taken for the **LONGITUDINAL MAGNETIZATION VECTOR to recover to 63% of its max value**
35
Define t2?
* The time taken for the **TRANSVERSE MAGNETIZATION VECTOR to fall to 37% of its max value.**
36
How are t1 and t2 images used?
* T1 and t2 vary for different tissues and therefore are used to form the image * T1- fat bright signal - gd for anatomy * T2- fluid has bright signal- gd for pathology
37
What is MRI used for?
* In orthopaedics * Osteonecrosis * Infection * Trauma - occult fractures * Tumours * Regional disease- Rc pathology/ Intervertebral disc disease
38
What are its advantages?
* **Image in any plane**- multi planar * **High contrast resolution**- gd soft tissue, bone marrow imaging * **Non ionising radiation** * Non invasive imaging of blood vessels
39
What are the disadvantages of MRI?
* Limited by contraindications- think clinic SHEET!! * Absolute * **pacemaker** * **defribrillators** * **cerebral aneurysmal clips** * **vascular clips(\<2 wks old)** * **metal in orbit** * **internal hearing aids** * **dorsal column stabilisers** * Relative - 1st/2nd trimester of pregnancy , claustrophia, obesity, penile prostheis, children to sit within scanner * Other * **Not as good as Ct for imaging bone** * **High cost of equipment** * **Image artefact** - motion and ferromagnetic objects * **Claustrophia**
40
What is a bone scan?
* **A image modality involving the intravenous injection of technetium 99m phosphonates, which localises at the sites of _OSTEOBLASTIC Activity_**
41
How does bone scanning work?
* Radionuclides are **UNSTABLE nuclei** with **neutrons excess or deficit**. * The nuclei distingretate to form other atoms with the release of alpha, beta or gamma radiation. * **Technetium 99m is a radionuclide with a short half life and pure _GAMMA emitter_** * It is coupled with **methylene disphosphonate** and is Administered iv. * It is **chemical absorbed onto hydroxyapitite crystals in bone.** * It's uptake is reflection of **OSTEOBLASTIC activity.** * within 24 hrs around 70% of administered dose is excreted thru kidneys hence bladder exposure to radiation must be minimised by pt hydration adn frequent micturition
42
How is the gamma radiation detected ?
* **Photo emission recorded by Scintillation gamma camera** ( contains *crystals of sodium iodide that absorb Tc 99 gamma rays* * Signal is enhanced by **PHOTOMULTIPLIER TUBES and processed by computer**
43
Describe the 3 phases of a bone scan?
* 1)**Flow/ Dynamic phase**- immediate images **1-2mins**= arterial blood flow and hyper fusion * 2) **Blood pool/ Equilibrium images**- images at _3-5mins_ showing **EC FLUID VOL**- so extent of bone and soft tissue hyperaemia * 3) **STATIC/ Delayed images**- approx _4 hours_ after injection- when _soft tissue activity has cleared_, highlighting _skeletal activity_.anterior and post scans available.
44
What are bone scans used for?
* Assessment of occult bone pain * Assessment of metastatic disease * Infection - flow and blood pool- hot in cellulitis * blood flow, pool and static phases all hot osteomyelitis * Trauma- stress facture * Tumour- osteoid osteoma osteoblastoma especially spine * Arthritis- activity and extent * Post-op- paeudoaethrothesis, painful prosthesis * Assessment of Paget's disease
45
What are the advantages of bone scan?
* **Very sensitive** to increases in bone turnover * **Good general survey** for skeletal pathology * Helps to **localise local bone lesions** after which further imaging can occur
46
What are the disadvantages of bone scans ?
* **Poor spatial resolution** * **Non specific appearance** for elucidating causes if increased activity * may give **False negative in myeloma and lytic metastasis** due to _decreased blood flow or lack of osteoblastic response_ * **Relatively high radiation dose** - ESP to bone marrow and in children
47
What is SPECT?
* Single photon emission computer tomography * **Applies tomography technology to radionuclide scanning**- enabling x sectional images to be obtained with enhances the conspicity of the lesion and hleos in their localisation * the gamma head rotates 360 degrees around the body obtaining multiple planar images which are then reconstructed in sagittal, coronal and axial planes
48
What use is INDUM 111?
* It used to **label white cells and platelets** and can be used to **LOCALISE AREAS OF INFECTION AND INFLAMMATION**
49
What is the use of gallium 67?
* It binds to **plasma proteins ** * can be used to localise areas of inflammation e.g abscess and neoplasia * frequently used in combo with technetium bone scan and is less dependent on vascular flow than a technetium scan
50
What is dexa?
* **Dual energy X-ray absorptiometry**
51
What is used for?
* To measure the **BONE MINERAL DENSITY in the central skeleton -FEMUR AND LUMBAR SPINE**
52
How does it work?
* It used **TWO X-RAYS OF DIFFERENT ENERGIES** **which are absorbed in different proportions by bone and soft tissues** * The pt lies on the table for **2 mins** while **ap of femur **pt supine, ties together heels separated 25cm) and **lateral of spine** * lower ribs cover L2 and iliac crests cover L4 on lateral so only **L3** maybe the only vertebra scanned.
53
What is a dexa scan used for?
* **Assessing bone mineral density in primary and secondary osteoporosis**- for diagnosis and monitoring - allows for fracture risk * **Evaluating the effect of preventive interventions on metabolic diseases**- primary hyperparathyroidism * **Measuring periprosthetic bone loss-** in pt with cementless thr- measure magnitude and rate of change in bm content
54
How are the results interpretated?
* **T score** when bone density values (g/cm3) are matched **FOR SEX AND RACE** and **COMPARED TO PEAK BONE MASS OF NORMAL ADULTS** = * pt bmd - population bmd/ standard deviation of population bmd
55
What is the z score?
* When matched for **SEX, RACE AND *_AGE_*** = * (pts bmd- pop age related bmd)/ Standard deviation of population age related bmd
56
Define osteoporosis ?
* is a progressive bone disease that is characterized by a decrease in bone mineral density which can lead to an increased risk of fracture. * **diagnosed If the T score IS OVER -2.5 sd according to the WHO criteria** - kanis et al 1994 * Nb- **normal** t score is \>/** -1 ** * **Osteopenia** is a score **-2.5 to \<-1** * **osteporosis \\<-2.5** * severe oestoporosis = **\<-2/5 and presence of one or more fragility fx** * **_In premenopausal women Z scores are used _**
57
What is z score used for?
* Compare pts **BMD** with the mean value for individuals of the **same age** * A **LOW Z SCORE -= an aetiology other than age related bone loss** * z score used in premenopausal women
58
What are the advantages of dexa scan?
* Assessment of fracture risk in osteoporosis and the need for treatment
59
What are the disadvantages of dexa scan?
* **Doesn't distinguish between CORTICAL and CANCELLOUS bone density** * In the spine -AP PROJECTION fasley increase bmd by intervertebral oa- so prefer **LATERAL** * _False high density values_ maybe seen with _fractured vertebra and calcification_ * _False low density_ values found if _posterior element surgical resection_
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