Week 4: Injection Techniques

Question 1

Method of injecting iodinated CM will vary according to:

Answer 1

• Vascular access available
• Type of examination
• Specific clinical indication

Question 2

Injection parameters may/will vary:

Answer 2

If the injection is performed by hand or using a mechanical injector
- Contrast volume and concentration
- Flow rate at which the contrast will be injected
- The delay between injection and scanning
- Whether a saline flush is used

Question 3

CVAD (central venous access device)

Answer 3

• a venous catheter designed to deliver medications & fluids directly into SVC, IVC, or right atrium
• several types (PICC, nontunneled, tunneled, implantable)
• not optimal for CM but sometimes only option

Question 4

Starting a Peripheral IV

Answer 4

• obtain pt consent and use aseptic technique
• use catheter with flexible cannula when mechanical injector used

Question 5

Using established IV catheter

Answer 5

• evaluate if site appropriate for CM
• verify latency with saline flush
• Turn off existing medication only long enough to complete the injection
• Once the CM injection is completed, flush the line with saline and restart medications at the identical pre-examination rate

Question 6

PICC (Peripherally Inserted Central Catheter)

Answer 6

• long catheter that is inserted through the large veins of the upper arm & advanced so that its tip is located in the lower one third of the SVC
• some PICC cannot tolerate pressure required for CM injection so instead use separate IV for CM
• If no other IV possible use slowed injection rate and hand bolus instead of mechanical injector

Question 7

nontunneled and tunneled CVCs (central venous catheter)

Answer 7

• when possible avoid uses these CVCs for CM and instead starting a standard IV line
• NEVER use a dialysis CVC for CM

Question 8

Documenting CM Administration

Answer 8

• documentation of IV contrast administration is a legal necessity
• should include: name of agent, dose (volume and concentration), flow rate, injection site and and adverse effects and their treatment

Question 9

Phases of Tissue Enhancement

Answer 9

• CE (contrast enhancement) is determined by the rate the CM is delivered and the time elapsed from injection start and when scanning initiated
• three general phases include: bolus (arterial) phase, non-equilibrium (venous) phase, equilibrium (delayed) phase
• timing and start of each of the phases is affected by injection parameters (shapes of the bolus) and pts cardiac output

Question 10

Bolus (Arterial) Phase

Answer 10

• Immediately follows an IV bolus injection
• arterial venous iodine difference (AVID) of 30 HU or more
• Arterial structures are filled with CM, venous structures not yet filled
• a few seconds-30 seconds after injection, 30 sec is optimal time

Question 11

Equilibrium (delayed) Phase

Answer 11

• Last phase of tissue enhancement.
• It can begin as early as 2 mins after bolus phase.
• AVID is less than 10 HU.
• CM is mostly emptied from arteries, greatly diluted in the veins, and has soaked the organ parenchyma
• typically 2 minutes or more, some up to 8 mins

Question 11

Drip Infusion

Answer 11

• uncommon
• CM is allowed to drip in during a period of several minutes.
• Scanning begins after most, or all, of the CM is in.
• Can be used for routine brain scans, but not appropriate for other examinations
• This is an exception to the general rules of CM injection.
• Injection rate is not important. Brain is imaged during equilibrium phase.
• Scan delay is 4 minutes typically.
• CM must have time to cross the blood-brain barrier.

Question 11

Non-equilibrium (venous) Phase

Answer 11

• Follows the bolus phase
• begins approximately 1 minute after the
  start of the injection
• AVID of 10 to 30 HU
• CM is still in arteries, but also in veins
• typically 1-2 minutes, 60-70 seconds being optimal

Question 12

Hand Bolus

Answer 12

• Variable flow rate: because of syringe size, contrast viscosity, IV catheter size, and operator strength
• Results in inconsistent images that are not reproducible
• primarily used when injecting into PICC, tunneled or nontunneled CVCs

Question 13

Pharmacokinetic Factors

Answer 13

• factors that affects contrast enhancement
• CM characteristics (concentration, osmolality, viscosity), volume, flow duration, scan delay time and total scan time
• largely controllable

Question 14

Mechanical Injectors

Answer 14

• CT injectors may have a single or dual heads for affixing the syringe
• Include a programmable pressure limit
• CM is administered at the selected rate(s), unless pressure reaches the maximum psi set
• Safety precautions must be strictly adhered to precent CM extravasation and air embolism

Question 15

Patient and Equipment Factors Affecting Contrast Enhancement

Answer 15

• pt factors: cardiac output, weight
• equipment factors: scanner speed, determined by number of detector rows

Question 16

Test Bolus

Answer 16

• method to individualizing the scan delay to adjust to pt factors
• 10 - 20 ml of CM is injected & several trial scans are taken
• To determine the length of time from injection to peak enhancement in a target region (ex. aortic arch for chest, descending aorta for and/liver/pancreas)
• trial scans done on very low mAs and begin from 8-15 seconds after injection start
• scan delay calculated

Question 17

Bolus Triggering

Answer 17

• method to individualizing the scan delay to adjust to pt factors
• Use contrast bolus itself to initiate the scan
• A series of low-radiation scans monitor the
  progress of the contrast bolus
• Measurement of HU in ROI measured before contracts and when ROI reaches peak HU the scan starts
• Once adequate HU is reached, the table moves to the starting level & scanning begins.
• A drawback is that a technologist cannot stay with the patient to monitor the injection site.

Question 18

Saline flush benefits

Answer 18

• Flushes out CM that would otherwise be left behind in the injection tubing
• Eliminates the extra step of clearing the vascular access site of residual contrast after injection
• Pushes the contrast bolus forward, so may create a more desirable bolus shape
• Increases the amount of contrast available for use in image acquisition and may reduce artifact
• Decreases amount of CM needed