mdct.net’s Guide to Technology and Protocols – Technical Considerations

MDCT Technology – Introduction Technical Considerations Preparing Patients for CE Exams CM Safety

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Technical Considerations

Spatial Resolution

Spatial resolution is the capability of an imaging system to resolve closely placed objects or to image and display fine details.

When Should You Favor Spatial Over Temporal Resolution?

• When the area of interest is small
  • Smaller arteries (coronary, renal, mesenteric, peripheral, pulmonary, carotid)
  • Ducts (bile, pancreatic, salivary)
  • Inner ear
• When looking for small lesions
  • Subcentimeter lesions (e.g., peripheral pulmonary nodule or embolus)
  • Fine abnormality (e.g., fracture, arterial stenosis, or pulmonary interstitial disease)

Trade-offs When Favoring Spatial Resolution

• Reduced coverage
• Reduce temporal resolution
• Reduced low-contrast resolution (increased noise)
• Longer exams and/or higher radiation dose, especially if high spatial resolution is accompanied by extended coverage

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Radiation Dose

Radiation Dose vs Radiation Exposure

• Radiation exposure is related to the amount of ionization of air produced by an x-ray beam. It is a radiation source-related term and is a measured quantity.
• Radiation dose is a body-related term and is calculated from the exposure. For a given measure of radiation exposure, we can specify the amount of radiation energy deposited in the patient’s body as a result of that exposure.

Factors Affecting Radiation Dose

Radiation dose must be optimized. Insufficient radiation dose results in increased noise and degradation of image quality. An increase in radiation dose above a certain level does not further improve image quality; it merely deposits more radiation in the patient’s body. Radiation dose can be modified by adjusting the tube voltage, tube current, scan time, scan coverage, collimation or detector configuration, pitch, and table movement.

 

Tube voltage and radiation dose

• The amount of tube voltage (kVp) affects radiation dose.
  • Reduction of the tube voltage will decrease the output of the x-ray tube and reduce the radiation dose to the patient.
  • Inappropriate reduction of the voltage may result in a marked increase in CT tissue attenuation and noise, particularly in large patients.
• Most CT scanners offer a limited number of choices for kVp values when performing CT scans.
  • Routine body CT for adult patients is generally performed at 120 to 140 kVp.
  • The use of 80 kVp is a well accepted level when attempting to reduce radiation doses in pediatric patients.

Tube current and radiation dose

• The tube current (mA) is usually adjustable over a wide range; from 10 mA to 800 mA.
• The effect of tube current on image quality is more straightforward than that of tube voltage.
  In general, image quality improves with an increase in tube current.
• Radiation dose and image noise are directly affected by the product of tube current and scan time.

Pitch and radiation dose

• For single-detector–row CT scanners, overall radiation dose and scan duration decrease proportionally with increasing pitch (or table speed) if the tube voltage and current are kept constant.
  Noise remains constant with increasing pitch.
• For multidetector-row CT scanners, the relationship between pitch and radiation dose is not straightforward.
  • The tube current can be increased to compensate for increased noise at higher pitch values.
  • Increased pitch does not translate directly into reduced radiation dose.

 

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CM Administration

Patient Variables that Affect Contrast Enhancement

Body Weight and Contrast Enhancement

• In many CT applications, for optimal enhancement, tailor the iodine dose to body weight.
• Body weight affects both vascular and parenchymal enhancement.
  • The greater the weight, the lower the enhancement.
  • Time-to-peak enhancement is unchanged.
• The greater the weight, the greater the iodine dose needed to achieve the same degree of enhancement.
• Contrast media with higher iodine concentrations will be useful to administer the iodine dose needed.

Simulated contrast enhancement curves with four different body weights.

Simulated enhancement curves of the (a) aorta and (b)liver based on injection of 125 mL of contrast medium at 5 mL/sec. The magnitude of contrast enhancement is inversely proportional to the body weight. Modified with permission (Bae KT, Heiken JP. Computer modeling approach to contrast medium administration and scan timing for multislice CT. In: Marincek B, Ros PR, Reiser M, Baker ME, eds. Multislice CT: A Practical Guide, Proceedings of the 5th International SOMATOM CT Scientific User Conference, Zurich, June 2000.Heidelberg: Springer-Verlag; 2000:28-36).

Body Weight Used to Calculate Iodine Dose

If a standard iodine dose is calculated for a patient weighing 70 kg/150 lbs (=100%), the following dose adjustments would be needed:

Patient Weight in kg (lbs)	% Standard Iodine Dose
45 (100)	75
90 (200)	140
115 (250)	175
135 (300)	220

Cardiac Output and Contrast Enhancement

• A reduced cardiac output leads to:
  • Delayed enhancement
  • Greater enhancement
  • Prolonged enhancement
  • Therefore
    • Use bolus tracking for scan timing
    • Use a lower iodine dose
    • Use a slower injection rate

Simulated contrast enhancement curves at baseline and reduced cardiac outputs (C.O.) .

Simulated enhancement curves of the aorta, based on injection of 120 mL of 370 mgI/mL contrast medium at 4 mL/sec. A set of (a)aortic and (b) hepatic contrast enhancement curves were generated by reducing the baseline cardiac output, i.e. 6500 mL/min, by 20%, 40%, and 60%. Modified with permission (Bae KT. Technical aspects of contrast delivery in advanced CT. Appl Radiol.Dec. 2003; 32 (suppl):12-19) .

Vascular Access

• High contrast flow rates (≥3 mL/s) can be used in larger caliber veins (e.g., antecubital vein)
• However, lower contrast flow rates should be used when vascular access is limited or a central line is employed

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Scan Timing

Scan Delay

• If volume, flow rate, and iodine concentration are kept constant, delaying the scan becomes more critical as the:
  • Number of slices per rotation increases (4 to 8 to 16)
  • Scan becomes faster
  • Scan duration shortens
• It is essential to increase scan delay to image during peak arterial enhancement

Scan delay: how to optimize image quality during arterial enhancement

• When using shorter scan durations (16-row scanners), consider
  • Decreasing the contrast volume
  • Using contrast solutions with high iodine concentration (the higher, the better)
  • Using a high flow rate (≥4 mL/s) for contrast administration
  • Increasing the scan delay compared with 4-row scanners
  • Using bolus tracking
• When using longer scan durations (4-row scanners or extended coverage), consider
  • Increasing the contrast volume
  • Using a lower flow rate to increase the injection duration
  • Using contrast solutions with a high iodine concentration to increase the enhancement level for a given injection rate
  • Using bolus tracking or automated settings provided by the scanner manufacture

Scan delay: how to optimize image quality during hepatic parenchymal enhancement

• Iodine dose is critical
  • Use higher volumes of lower iodine concentration
  • Use lower volumes of higher iodine concentration
  • Iodine dose should not be reduced even if faster scanners are used
• Flow rate is not as critical as in arterial enhancement
• Scan delay should be increased if faster scanners are used

  

Representative scan timing and scan duration curves for single-slice, and 4- and 16-slice MDCT scanners are shown for (a) aortic and (b) hepatic enhancement.

As scanners become faster, scan durations become shorter, and therefore it is critical to delay scanning to capture images during peak enhancement, as illustrated in Panel b. Images courtesy of Kyongtae Ty Bae, MD, PhD, Mallinckrodt Institute of Radiology, St. Louis, MO.

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KT Bae

Multidetector CT Protocols
Developed for GE, Philips, Siemens,Toshiba Scanners
Springer 2005, 2006