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P3 notes to study..., Lecture notes of Computed Tomography

Open University of Hong KongComputed Tomography

CT P3 notes to study before cfe

Typology: Lecture notes

2025/2026

Uploaded on 03/26/2026

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COMPUTED TOMOGRAPHY
RTE 022
SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING
CHAPTER 11: PX PREPARATION
EXAMINATION INITIATION
- CT exams must be initiated by qualified
clinicians
OutPx Settings:
- Order submission → from clinician’s office
- Scheduling → coordinated by clerical staff
- Order transmission → sent to ct department
- Order verification → confirmed by ct tech
| Ensure electronic order transmission to reduce
transcription errors
InPx Settings:
- May also include opportunities for
transcription errors
| Most accurate method for transmitting orders is
through CPOE → Computerized Physician Order
Entry
PROTOCOL SELECTION
| Select the most appropriate examination protocol
to address clinical questions with minimal px risk
Factors Considered:
- Px safety → minimize radiation, avoid
contrast reactions
- Px tolerance → ensure px comfort
- Scanner capabilities → adapt protocols to
equipment
Decision Process:
1. Radiologist review
2. Algorithm-based selection
3. Technologist oversight
ROOM PREPARATION
Before Px Arrival:
1. Calibration and Warm-up → ensure proper
scanner function
2. Cleanliness → sanitize and restock supplies
3. Equipment setup → prepare necessary
accessories
4. Safety measures → ensure availability of
protective gear
MEDICAL HISTORY COLLECTION
| Ensure px safety and accurate dx
Px Identification:
Verify using at least two methods
- Full name
- Date of birth
Key Medical History Elements:
- Renal function → BUN, Creatinine
- Allergies → Contrast sensitivity
- Thyroid conditions
- Pregnancy status
- Coagulation ability
PX EDUCATION AND INFORMED CONSENT
Px Education:
- Procedure overview → explain process and
duration
- Contrast use → discuss administration and
side effects
- Expectations → emphasize px cooperation
Informed Consent:
| Required for invasive procedure and contrast
administration
- Procedure details
- Alternatives
- Risks and benefits
RTE022: COMPUTED TOMOGRAPHY
P3 NOTES | YSA2526
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SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING CHAPTER 11: PX PREPARATION EXAMINATION INITIATION

  • CT exams must be initiated by qualified clinicians OutPx Settings:
  • Order submission → from clinician’s office
  • Scheduling → coordinated by clerical staff
  • Order transmission → sent to ct department
  • Order verification → confirmed by ct tech | Ensure electronic order transmission to reduce transcription errors InPx Settings:
  • May also include opportunities for transcription errors | Most accurate method for transmitting orders is through CPOE → Computerized Physician Order Entry PROTOCOL SELECTION | Select the most appropriate examination protocol to address clinical questions with minimal px risk Factors Considered:
  • Px safety → minimize radiation, avoid contrast reactions
  • Px tolerance → ensure px comfort
  • Scanner capabilities → adapt protocols to equipment Decision Process:
  1. Radiologist review
  2. Algorithm-based selection
  3. Technologist oversight

ROOM PREPARATION

Before Px Arrival:

  1. Calibration and Warm-up → ensure proper scanner function
  2. Cleanliness → sanitize and restock supplies
  3. Equipment setup → prepare necessary accessories
  4. Safety measures → ensure availability of protective gear MEDICAL HISTORY COLLECTION | Ensure px safety and accurate dx Px Identification: Verify using at least two methods
  • Full name
  • Date of birth Key Medical History Elements:
  • Renal function → BUN, Creatinine
  • Allergies → Contrast sensitivity
  • Thyroid conditions
  • Pregnancy status
  • Coagulation ability PX EDUCATION AND INFORMED CONSENT Px Education:
  • Procedure overview → explain process and duration
  • Contrast use → discuss administration and side effects
  • Expectations → emphasize px cooperation Informed Consent: | Required for invasive procedure and contrast administration
  • Procedure details
  • Alternatives
  • Risks and benefits

SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING

  • Px understanding
  • Voluntary agreement SPECIAL CONSIDERATIONS Special Cases:
  • Pediatric px → parental/guardian consent required
  • Sedated px → obtain consent before sedation IMMOBILIZATION AND PX RESTRAINT DEVICES | Variety of immobilization and restraint devices are used in CT for both safety and to improve quality Common Immobilization Devices:
  • Straps → prevents px from falling and a reminder to stay still
  • Bean bags → placed alongside lower limbs to prevent motion during the scan Guidelines for Using Restraints:
  1. Px communication → always explain the need for the device
  2. Consent exceptions → in cases where the px cannot provide consent, a clinician's order is technically required. However, short terms use for imaging is often performed without prior consultation Key Principles When Using Restrains:
  • Allow as much mobility as safely possible
  • Pad areas under restraints to prevent skin injury
  • Maintain normal anatomical position
  • Avoid knots that tighten with movement
  • Ensure the restraint can be quickly removed in emergencies
  • Do not impair circulation or respiration
  • Use wrist immobilizers if leg restraints are applied to prevent self injury ASSESSMENT AND MONITORING VITAL SIGNS | Techs should monitor px closely throughout the CT examination to detect any adverse effects | Initial observations should include breathing patterns, skin coloration, and overall health Px Monitoring Protocols:
  • Use the scanner’s intercom system to communicate with px during the procedure
  • Observe px visually for signs of distress
  • For unstable px, a nurse or trained health professional should accompany the px during the scan | Vital signs (temp, pulse, respiration and bp) are the best early indicators of physiologic changes Body Temperature: measured using different methods
  • Oral, rectal, tympanic→ higher than axillary readings due to mucous membrane contact
  • Axillary → lower readings, commonly used for px unable to tolerate other methods Pyrexia: Medical term for fever. A natural response to infections Hyperthermia: Increase in body temperature Hyperpyrexia: Extreme high fever, usually over 106 degree F (41C). Can occur with severe infections Pulse: Alternate expansion and recoil of any artery with each heartbeat
  • Temporal → anterior to the ear

SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING INTRAVASCULAR CONTRAST AGENTS | Iodinated agents are universally used because they are water soluble and have a high safety index | Increase beam attenuating ability of the blood, making structures with blood supply more visible

  • Has high attenuation compared with human soft tissue
  • Whenever it distributes it increases the ability of the enhanced structure to attenuate the beam
  • Relatively high Z → 53 - Adequate blood supply → increase in attenuation and displayed as a change from dark to light
  • Tissues must differ by at least 10HU to be visibly different on a CT scan
  • Can enhance tissue differentiation by 40-75HU PROPERTIES OF IODINATED AGENTS OSMOLALITY | The number of particles in solution (measured in mOsm/kg of water ) Significance: high osmolality increases the risk of adverse reactions. | IOMC is safer for px at risk of renal complications Types of Contrast Media by Osmolality:
  1. High Osmolality (HOCM)
  • 1,300-2,140 mOsm/kg (4-7x the osmolality of blood plasma)
  • Older and less commonly used due to higher risk of adverse reactions
  1. Low Osmolality (LOCM)
  • 600-850 mOsm/kg (2-3x the osmolality of blood plasma
  • Safer and widely used today
  1. Isosmolar (IOCM)
  • Same osmolality as blood (~ mOsm/kg)
  • Introduced in 1990 and preferred for px at risk of renal complications VISCOSITY | Thickness or flow resistance of contrast agent Factors:
  • Molecular structure
  • Iodine concentration (higher iodine increases viscosity)
  • Temperature (warming reduces viscosity) IONICITY | Whether a contrast agent’s molecules separate into charged particles (ions) in solution Types of Contrast Media by Ionicity:
  1. Ionic Contrast Media
  • Molecules dissociate into ions in solution
  1. Nonionic Contrast Media
  • Molecules do not dissociate into ions CLEARANCE | Once injected, all types of iodinated contrast media undergo very rapid distribution throughout the entire extracellular spaces
  • Not metabolized and excreted by the body nearly exclusively by kidney
  • Half life it takes for px with normal renal function → 2hours

SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING CONTRAST DOSAGE | Accurate dosing considers both iodine concentration and volume | Different protocols may require specific doses based on injection rate and scan timing Pediatric Dosing;

  • Typically using 2mL/kg Adult Dosing: | Historically, adult dosing has been uniform → 150mL / px - Weight based dosing is more precise and may improve image quality while reducing costs and overdose risks IODINATED CONTRAST MEDIA DURING PREGNANCY AND LACTATION Pregnancy: | CT examinations are rarely performed during pregnancy due to concerns about fetal exposure to ionizing radiation | Iodinated contrast agents may be used with caution | Iodinated contrast media cross the placenta and enter the fetus but no definitive evidence suggests teratogenic or mutagenic effects in humans The ACR Recommends:
  • Consulting with the referring physician to document the necessity of contrast-enhanced imaging.
  • Ensuring the imaging affects the care of the mother and fetus.
  • Obtaining informed consent from the pregnant patient to document understanding of risks and alternatives. Lactation:
  • Less than 1% of administered iodinated contrast media is excreted into breast milk.
  • Of the contrast media ingested by the infant, less than 1% is absorbed from the
  • gastrointestinal tract.
  • It is generally safe for mothers to continue breastfeeding after receiving contrast agents.
  • If concerned, mothers can choose to abstain from breastfeeding for 24 hours while expressing and discarding milk. ADVERSE EFFECTS OF IODINATED CONTRAST MEDIA Mechanism of Adverse Reactions: | Rare but can be unpredictable and vary in severity Classification: 1. Chemotoxic Reactions
  • The physicochemical properties of the contrast dose, and injection rate 2. Idiosyncratic Reactions
  • Unpredictable
  • Often occurring within 1 hour of administration
  • Not true allergies but resemble anaphylactic reactions Types of Idiosyncratic Reactions: 1. Mild
  • Self limiting symptoms (cough, itching, rash)
  • Monitor for 20-30 min as symptoms can progress 2. Moderate
  • Require treatment
  • Not immediately life threatening

SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING

  • The central dose for a body scan is approximately one-third to one-half that of the peripheral dose | To a great degree, this phenomenon accounts for the fact that, for a given set of machine parameters (mAs, slice thickness, and pitch), organ doses are clearly higher for children compared with (larger) adults CT DOSE INDEX Computed Tomography Dose Index (CTDI)
  • Another type of dose measurement
  • Only be calculated if slices are contiguous, no overlapping or gapped slices
  • Med physicists use a special dosimeter called pencil ionization chamber to measure CTDI CTDIvol
  • A measure of exposure per slice and is independent of scan length
  • Preferred expression of radiation dose in CT dosimetry FACTORS AFFECTING DOSE Radiation Beam Geometry:
  • Theoretically, a rotation arc of only 180° is all that is required to satisfy most construction algorithms.
  • Most scanners use a 360° tube arc to compensate for radiation beam divergence and patient motion.
  • The extra scanning information improves image quality but increases radiation dose. Filtration:
  • Filtration affects the radiation dose by removing some of the soft x-rays.
  • These low-energy x-rays are quickly absorbed by the px
  • Photons are needed to penetrate the patient and strike the detectors, but some of them must be absorbed to produce radiographic contrast. Sliced Width and Spacing:
  • Considering a single cross-sectional slice, as slice thickness increases, the volume of tissue irradiated increases, and the dose may increase slightly to the adjacent slices.
  • For multiple slice examinations, decreasing slice thickness and using contiguous slices will increase the MSAD because of the increased amount of scatter radiation to adjacent slices.
  • To maintain image quality at the same level, additional radiation is needed for thinner slices. | Multiple slice examinations using overlapping slices will produce a higher overall dose, whereas gapped slices will produce a lower overall dose. Pitch:
  • Selecting a pitch greater than 1 will spread the radiation more thinly over the slices.
  • That is, although there are no areas along the z axis that are skipped completely, the gantry will not make a full rotation in a given slice location.
  • The pitch has a direct influence on patient radiation dose because as pitch increases, the time that any one point in space spends in the x-ray beam is decreased Scan Field Diameter: | Holding all technical factors constant, a scan of the head phantom will result in a higher radiation dose than that of the body phantom.
  • The primary difference in results between the head and body phantoms is size.

SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING

  • Each of the tables shows that the smaller object always absorbs the higher dose and that the difference is at least a factor of two.
  • Smaller px would be expected to absorb much higher amounts of radiation than larger px. Radiographic Technique: | In general radiography, the technique used to create the CT image affects radiation exposure to the px
  • The higher the mAs and kVp settings used to create the image, the higher the dose to the patient.
  • The relationship between mAs and dose is linear. | A reduction in dose is associated with a subsequent increase in image noise.
  • X-ray tube potential (or kVp) also affects the radiation dose, although the effect is not linear.
  • With the mAs kept constant, changing from 120kVp to 140kVp increases the radiation dose approximately 30% to 45%. Px Size and Body Thickness:
  • Large px or thick body parts require radiographic techniques that increase the radiation dose to avoid an unacceptable level of image noise. | Px size and body composition may affect the degree of scatter radiation. Repeat Scans:
  • Areas of the patient that are rescanned to visualize various stages of intravenous contrast enhancement or for other technical or clinical reasons receive additional radiation.
  • The effect is cumulative. Collimation:
  • Lead collimators are used near the x-ray tube to control the size of the beam striking the patient.
  • If the beam were not controlled to match the detector size, there would be additional scatter radiation to degrade the image; this scenario would result in a higher radiation dose to the patient. | Collimators may also be used near the detectors for scatter rejection and aperture use. Localization Scans:
  • Often referred to as the scout image, it delivers a very low dose.
  • The radiation dose for the scout image is much lower than that used to produce cross-sectional slices. SPECIAL CONSIDERATIONS FOR THE PEDIATRIC POPULATION | There are three primary factors of special relevance to the use of CT in pediatric radiology: increased sensitivity, higher effective dose, and increasing use. Increased Sensitivity:
  • Children are much more radiosensitive than adults Reasons for Increased Sensitivity:
  1. Younger age, children have more time to develop cancer than do adults.
  2. Exposure is cumulative | Children seem to be inherently more sensitive to radiation simply because they have more dividing cells

SECOND SEMESTER | A.Y 25-26 | PROF. KLENT CAHAMBING STRATEGIES FOR DOSE REDUCTION IN PEDIATRIC PX

  • Appropriate patient selection
  • Appropriate technical parameters that will minimize the dose without compromising diagnostic quality. **_- Use CT only when clinically indicated
  • Customize the CT Examination
  • Px shielding Technical Parameters:_** - Adjust mAs → imperative in children
  • Px size → primary criteria in selection if mAs. ROI and Clinical indications are also considerations Mao ba ni CAT Scan?