Radio Principles 10a & b

Question 1

What are the indications for MRI imaging?

Answer 1

Objective neurological signs
Joint derangement
-meniscal tear
-rotator cuff disease
Malignancy
-characterisation
-staging
Suspected bone marrow changes
-oedema
-reconversion

Question 2

Can you explain the 10 things about MRI’s

Answer 2

1. It is a non-iodizing radiation that utilises magnetic fields and radiofrequencies to create sectional images in any body plane.
2. The basic principle behind MRI is that hydrogen atoms in the body generate small magnetic fields, which are randomily orientated.
3. When patients are placed in a powerful magnetic field (0.3T-1.5), this aligns most H protons within the field spin in sequence with each other but are out of phase with each other due to the different tissue types.
4. Then a surface coil is placed on or near the patient which emmits a radiofrequency into the patients body which causes the H atoms to spin ‘into phase’.
5. When the RF pulse is removed, this coil measures energy emitted as the H atoms relax back into a more random phase of spin.
6. Different tissue will relax at different rates, which creates the basis of different tissue signals (appearances/ shades)
7. By manipulating the:
   TR (time of repetition between subsequent RF pulses;
   TE (time allowed for collection of the emitted signal, i.e., time of echo); and
   FA (flip angle, or net magnetisation vector)the appearance of the different tissues can be altered, emphasised, or de-emphasised
8. Typically it is the fat or water component of tissues hat have the greatest change in appearnace from altering the TR?TE and FA factors.
9. As with CT, image ‘slices’ are computer generated, producing multiple images to view in whichever plans or sequences have been aquired.

Question 3

Terminology:
Hypointense
Isointense
Hyperintense

T1
PD
T2
What you use each for?

Answer 3

Tissue signal
Hypointense = dark or low signal
Isointense = signal similar to reference tissue
Hyperintense = bright or high signal
Image sequences: Spin echo imaging
Uses 90° flip angles – standard imaging sequence
T1 = Fat scans / anatomy scans (due to high signal of fat and good image resolution)
PD = proton density. A balanced scan between T1 and T2 (good disc / CSF distinction)
T2 = Water scans / pathology scans (due to high signal of fluids and most pathological tissues)

Question 4

Image Sequences: Gradient echo imaging does what?

Answer 4

Image sequences: Gradient echo imaging

Uses

Question 5

Image Sequences: STIR imaging

Answer 5

Uses > 90° flip angles (180°)
Adds an additional factor: TI (time of inversion). Once the 180° pulse is implemented, at some later time a spin echo sequence (as described previously) is started. The TI is the time between the 180° pulse and the initiation of the SE sequence. The SE sequence uses TR and TE parameters similar to proton density imaging.

The value of IR imaging is that specific tissue signals can be eliminated (darkened); with STIR, fat signal is substantially eliminated and water-based tissues are the only ones with high signal (similar techniques, eg, “fat saturation” or “fat suppression” imaging, are also available).
Done to assess for pathologies in which the higher fat signal would interfere with identification of pathology.

Question 6

Advantages of MRI?

Answer 6

Soft tissue contrast
Direct acquisition in any plane
No radiation

Question 7

Disadvantages of MRI?

Answer 7

-Dense bone is very low signal (calcifications, osteophytes)
-Long imaging times (this is improving)
-Limited availability
-Geographically (there is just not a scanner on every corner out in the Goldfields or up in the Kimberley)
-Chronologically (through a national health system, triage, not cash, determines priority)
-Cost
-Claustrophobia
-Difficult to interpret
-Cross sectional views are just weird
New protocols developing continually

Question 8

Limitations of usage or MRI?

Answer 8

Patients with electrically, magnetically or mechanically activated implants may be in for a nasty surprise
Pacemakers
Insulin pumps, cochlear implants
Neurostimulators
Bone-growth stimulators
Implanted drug-infusion pumps

Patients with other metallic foreign bodies may experience burns, electric shock or sudden dislodging
Catheters
Vascular/aneurysm clips
Skin staples
Projectiles
Metallic fragments in the eye (metal workers)
Pregnancy

Harrington rods and joint prostheses are not contraindications
Most heart valves seem to be okay
Local pressure overpowers magnetic effects

Question 9

IMAGING DIFFERENT POPULATIONS.

The populations to consider include:

Answer 9

Children
Elderly or frail individuals
Muscular
Obese
Pregnant

Question 10

Children:

Answer 10

Children 12 years of age and under
-Bones are less developed
-Soft tissues are generally less dense
-Usually require less dose to achieve adequate density x-ray, compared to an adult of similar thickness
For film/screen systems
-Reduce mAs by one cell on the technique chart (i.e. 2cm of thickness)

Use shortest exposure time possible
-to reduce risk of motion artefact
-automatic exposure systems shouldnt require modifications
Digital systems may only require reducing exposure time.

Very young children may need immobilisation, either with device or parent guardian.
-if adult is use, they must be shown how to perform the procedure, be provided with a lead apron, and if female, should be question regarding potential for pregnancy.

Question 11

Eldery or frail patients.

What do we have to keep in mind

Answer 11

Older patients or those that are undernourished or have systemic diseases will have higher liklihood or degree of osteoporosis, as well as higher proportion of fat: muscle mass.
-as with children, reduces mAs by one cell on the technique chart.

Elderly may have difficulty holing position- recumberant may help

• if they were glasses dont remove untill just before exposure is taken
• be very gentle when positioning them coz their skin can damage.

Question 12

Muscular Patients

Answer 12

Musclular tissue has high density

• increase mAs by one cell on the technique chart (i.e 2cm) to maintain film density
• for lateral cervical spine- get weights to pull shoulders down, or take a swimmers view.

Question 13

Obese patients

Answer 13

Lumbar films
-compression band or recumbent position
-make sure table can handle weight.
The greater thickness has these effects:
-more scatter, decreasing image detail.
-requires higher mAs, which increases pt dose
-increase exposure time which increase change for motion artefact,
-it is not usually neccessary to use larger cassette as they still have same size bones.
-however if its difficult to find the bony landmarks,bigger cassette could be option to avoid cutting of anatomy.
-for projections with angulation , the surface location may be inaccurate for setting central ray.

Question 14

Pregnant patients

Answer 14

Key issue: foetal dose
overarching rule is: if the benefit to the patient is greater than the risk of obtaining the image, then it should be taked- ALARA principle in mind.
-if your imaging area remote to abdomen (eg neck and hand) with appropriate shileding the abdomen and pelvis, the risks are greatly reduved and pregnancy shouldnt preclude imaging being performed.

Question 15

Pregnant patients

10-day rule vs 28 day ruke vs isk/benefit rule:

Answer 15

• in the past, general rule was that imaging potentially pregnancy females was the 10 day rule
• The 10-day rule a potentially fertile female pt should only be x-rayed during the first 10 days from the onset of menstration (pregnancy less likely during this time)
• however we know organogenesis doenst occur untill 3rd week of gestation.
• in most cases we would have a clear idea pf wethere or not a pt is pregnant before we take image. If she is risk/benefit considerayion takes center stage and guises the decision making process.
• if a lumbar image is taken when preggas w/o knowing, risk of malformation is extremely low so shoudlnt be terminated.