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- Iterative Reconstruction
- Iterative reconstruction techniques have recently been introduced which reduce image noise
- This noise reduction allows studies to be acquired at significantly lower dose, but with image quality comparable to FBP images acquired at higher doses
- Thin-section images, which are normally difficult to interpret when reconstructed with FBP (as a result of image noise), are less noisy and of better diagnostic quality when reconstructed with iterative reconstruction. - Iterative Reconstruction
- “Correction” loops are extremely time-consuming
- Particularly true with early algorithms, which performed reconstructions only in “raw data space”
- Algorithms now much faster as a result of performing reconstructions in both “raw data space” and “image space”
- Markedly speeds up reconstruction time and makes IR clinically viable
- Some manufacturers “blend” FBP with IR to reduce reconstruction times and to make images look more like traditional FBP images - Patient Positioning
- Improper centering of patients (either vertically or laterally) can increase surface dose by 23% and noise by 7%.
- Even a small amount of miscentering can have dramatic effects on dose
- Particularly a problem if AEC is used, as doses will be increased to compensate for noise. - Patient Positioning
- Must be especially careful with small and pediatric patients.
- Effect of miscentering increases if you are using your scanner in dual-source mode.
- Scoliotic patients and unconscious ICU patients also problematic
Patient Positioning
- Bowtie filters normally compensate for patient attenuation during tube rotation
- increased x-ray intensity to the thickest parts of the patent (i.e. center of the patient)
- decreased intensity to the thinnest parts of a patient (i.e. the patient surface).
- Bowtie filter functions under the assumption that the patient is correctly centered within the gantry
- When patient is improperly positioned, the mathematical assumptions underlying this filter break down, and doses to the surface of the body increase.
- CTDIvol
- Measures the radiation output or energy delivered by the scanner to tissue
- Not truly radiation dose
- CTDIvol is measured on plastic cylinder phantoms (16 or 32 cm)
- CTDIvol measurements are made by the manufacturer on phantoms
- Calculated at scanner based on the scan parameters being used - CTDIvol
- Allows comparison of different protocols with different scan parameters
- Described in units of Gray (Gy) or milligray (mGy) – energy per kg of mass
- Would only be a reasonable estimate of dose if patient were composed of plastic and was same size as phantom
- Does not take into account scan length
- Does not take into account patient size, attenuation, or shape - Dose-Length Product (DLP)
- Better accounts for overall energy delivered by any given protocol
- DLP(mGy.cm) = CTDIvol x scan length (cm)
- Hence, unlike CTDIvol, DLP takes into account scan length
- CTDIvol might be same for CT Abd and CT Abd/Pelvis, but DLP would be higher in the latter exam - Dose-Length Product (DLP)
- Changing CT scan parameters will change both CTDIvol and dose-Length Product (DLP).
- Changing scan length will only change DLP
- DLP is a better measure of the radiation risk of an examination
- Allows you to calculate effective dose - Effective Dose (E)
- Biologic effects of radiation depend not only on radiation imparted, but also on biologic sensitivity of an organ
- Unlike DLP and CTDIvol, better answers question: “What are the chances the patient wil be harmed by scan?”
- Allows comparison with other radiation risk (i.e. background)
- Units of effective dose is Sievert (Sv) or millisievert (mSv) - Effective Dose (E)
- E = DLP x k (weighting factor)
- Annual background radiation is roughly 3 mSv
- Natural radiation in rocks, soil, radon gas, space, etc. - Tube Current (mAs) and Automatic Tube Current Modulation (AEC)
- Increases in product of tube current and scan time (mAs) improve image quality and reduce noise
- Linear relationship between tube current and dose
- Can be manually controlled, but AEC is now available on virtually all scanners.
- Increases mAs in parts of the body with higher attenuation (i.e. shoulder & hips)
- Decreases mAs in parts of body with lower attenuation (i.e. abdomen & thorax). - Automatic Exposure Control (AEC)
- Patient size modulation varies the mAs based on a global evaluation of the overall size of the patient as seen on the scout radiograph.
- Z-axis modulation changes the mAs constantly along the Z-axis of the patient depending on the patient attenuation at each point, as determined using the scout image.
- Angular (x, y) modulation changes the mAs as the x-ray tube rotates 360 degrees around the patient to account for different attenuations in different projections of the x-ray beam.
- Combined x-y-z modulation adjusts the mAs in all three axes based on the patient’s attenuation. - Automatic Exposure Control (AEC)
- Be careful with small or pediatric patients
- Improper centering can result in increased image noise and low mAs
- Check to see if your vendor’s software can take into account metallic hardware
- Acceptable noise levels should be varied depending on the type of scan
- Be careful with obese patients
- “Cap” the tube current or else mAs may be increased markedly with increased dose - Tube Potential (kVp)
- Radiation dose changes with the square of the tube potential
- Reducing kVp from 120 to 100 reduces dose by 33%
- Reducing kVp from 120 to 80 reduces dose by 65%
- Reductions in kVp can increase attenuation of iodine
- Approach k-edge of iodine and photoelectric effect
- Can potentially improve contrast-to-noise ratios in some cases - Tube Potential (kVp)
- No predictable relationship between kVp and image noise
- Can result in non-linear exponential increases in image noise
- Will require increases in mAs in most cases to preserve diagnostic quality
- Must adjust reference mAs or noise index to increase tube current - Tube Potential
- Low kVp protocols most useful in thin-non-obese patients
- Very useful in vascular or angiographic studies
- Improved conspicuity of iodine
- Requires some trial-and-error, as there is no formula to determine when to reduce kVp
- SCCT recommends kVp of 100 in cardiac CT for patients weighing under 90 kg - Reconstruction Algorithm
- “Filtered back projection” (FBP) is the traditional method of CT reconstruction
- Used to reconstruct “Projection data” (i.e. raw data from the scanner) into a final image
- Assumes a perfect relationship between raw data and final image
- This assumption only holds true with high radiation doses and low image noise - Pitch
- Pitch = Table travel per rotation
- Pitch < 1 : Overlap between acquisitions
- Pitch > 1 : Gaps between acquisitions
- Pitch =1 : Acquisitions are contiguous
- Smaller pitch, with increased overlaps and sampling at each location, increases radiation dose
- Improved signal-to-noise and contrast-to-noise
- Larger pitch decreases dose in a linear fashion. - Pitch
- For most body applications, a pitch of roughly 1 is acceptable
- Higher pitches (i.e. > 1.5) result in interpolation artifacts and increased image noise
- Lower pitches (~ 0.5) in cardiac applications
- Overcomes motion artifacts and necessitated by reconstruction algorithms - High Pitch Mode
- Higher pitches (i.e. FLASH mode) now available on latest scanners (dual-source)
- Pitches up to 3.4 now available
- Fast scan times and decreased motion artifacts
- Significant dose reductions compared to retrospective or prospective gating for vascular studies
- Apfalterer et al found dose reduction of 45-50% for vascular studies
- “ Recent advances in CT scanning techniques, such as automated tube current modulation, use of optimal tube voltage, and improved utilization of interative image reconstruction, have allowed the reduction of CT radiation dose while maintaining diagnostic image quality.”
Emerging Techniques for Dose Optimization in Abdominal CT
Kaza RK et al.
RadioGraphics 2014; 34:4-17 - “ Radiologists need to understand the latest dose optimization strategies and should incorporate them into clinical practice by collaborating with physicists and CT technologists.”
Emerging Techniques for Dose Optimization in Abdominal CT
Kaza RK et al.
RadioGraphics 2014; 34:4-17 - What influences percieved image quality-
- Image noise
- Contrast to noise ratio (ability to distinguish CT number differences from background noise)
- Spatial resolution
- Image artifacts - Dose Reduction Techniques
- Automated tube current modulation (based on reference patient selected)
- Noise index and slice thickness
- Optimal tube voltage (120-80 kV)
- Iterative image reconstruction (ASIR, SAFIRE, iDose)
"In summary, it is important and necessary to individually adapt the scan protocol, with the use of every possible strategy for dose reduction. A combination of several dose saving algorithms is often feasible and leads to an efficient reduction in the overall radiation dose for a cardiac CT study." "A combination of several dose saving algorithms is often feasible and leads to an efficient reduction in the overall radiation dose for a cardiac CT study."
Trends in radiation protection in CT: Present and future status
Bischoff B et al.
J Cardiovasc Comput Tomogr (2009) 3, Supplement 2, S65-S73- Education and Training for the "Team"”: The Best Radiation Dose Reduction Plan Must Be Explained in Detail Including;
- The goal of your department is the lowest dose scans possible. If they have any thoughts or ideas please feel free to make any suggestions
- Follow the requested protocols for the patient. It is "not OK" to get an extra run if the patient moved or breathed during the study without having radiologist approval-after all the study with motion "may be good enough"
- Be careful when selecting the top and bottom lines on the scoutview or topogram. By choosing overly generous scan volumes you can increase dose by 30% or more
- If they are uncertain of a protocol or scan parameters take a “time-out” and speak to their supervisor or other appropriate professional (including the radiologist). Do not scan if you are concerned about dose or any other issue. Being careful can not be overemphasized . Education and Training for the"Team": The Best Radiation Dose Reduction Plan
In the article Image Gently: Ten Steps You Can Take to Optimize Image Quality and Lower CT Dose for Pediatric Patients Strauss KJ et al. AJR 2010; 194:868-873 the authors clearly made step number one that of training and education was for the radiologic technologist. "1.Increase awareness and understanding of CT radiation dose issues among radiologic technologist"is a critical point that must not be overlooked. At the end of the day it is "your" technologist who selects or modifies the final scan protocol that will define dose. Some of the things to discuss with your technologist should include (but not limited to);
" The U.S. per capita annual effective dose from medical procedures has increased about sixfold (0.5 mSv (1980) to 3.0 mSv (2006)."
Radiologic and Nuclear Medicine Studies in the United States and Worldwide: Frequency, Radiation Dose, and Comparison with Other Radiation Sources-1950-2007
Mettler FA et al.
Radiology 2009; 253:520-531"Worldwide estimates from 2000-2007 indicate that 3.6 billion medical procedures with ionizing radiation (3.1 billion diagnostic radiologic, 0.5 billion dental, and 37 million nuclear medicine examinations) are performed annually."
Radiologic and Nuclear Medicine Studies in the United States and Worldwide: Frequency, Radiation Dose, and Comparison with Other Radiation Sources-1950-2007
Mettler FA et al.
Radiology 2009; 253:520-531"In the United States in 2006, about 377 million diagnostic and interventional radiologic examinations and 18 million nuclear medicine examinations were performed. The United States accounts for about 12% of radiologic procedures and about one-half of nuclear medicine procedures performed worldwide."
Radiologic and Nuclear Medicine Studies in the United States and Worldwide: Frequency, Radiation Dose, and Comparison with Other Radiation Sources-1950-2007
Mettler FA et al.
Radiology 2009; 253:520-531- Limiting Scan Dosage to the Patient: Pearls
- Limit the field of view to the study ordered. There is no need to scan the lung above the liver or to scan beneath the symphysis in routine cases
- In multiphase studies determine what areas need the multiple acquisitions and which do not
- Choose the right protocol depending on patient size and body habitus "In summary, it is important and necessary to individually adapt the scan protocol, with the use of every possible strategy for dose reduction."
Trends in radiation protection in CT: Present and future status
Bischoff B et al.
J Cardiovasc Comput Tomogr (2009) 3, Supplement 2, S65-S73"Cumulative CT radiation exposure added incrementally to baseline cancer risk in the cohort. While most patients accrue low radiation induced cancer risks, a subgroup is potentially at higher risk due to recurrent CT imaging."
Recurrent CT, Cumulative Radiation Exposure, and Associated Radiation-induced Cancer Risks from CT of Adults
Sodickson A et al.
Radiology 2009; 251:175-184"Our results suggest that radiation dose and cancer risk of CT coronary angiography to pediatric patients are not negligible, more so in Hong Kong children than in U.S. children. Therefore, these examinations should be well justified clinically."
Pediatric 64-MDCT Coronary Angiography With ECG-Modulated Tube Current: radiation Dose and Cancer Risk
Huang B et al.
AJR 2009; 193:539-544"Ongoing efforts to ensure that CT examinations are both medically justified and optimally performed must continue, and education must be provided to the medical community and general public that put both the potential risks—and benefits– of CT examinations in proper perspective."
In Defense of Body CT
McCollough CH et al.
AJR 2009; 193:28-39"Conservative estimations of potential risk show that the potential risk of dying from undergoing a CT examination is less than that of drowning or of a pedestrian dying from being struck by any form of ground transportation, both of which most Americans consider to be an extremely unlikely event."
In Defense of Body CT
McCollough CH et al.
AJR 2009; 193:28-39"In the final analysis, physicians must request the imaging examination that best addresses the specific medical question without allowing worries about radiation to dissuade them or their patients from obtaining needed CT examinations."
In Defense of Body CT
McCollough CH et al.
AJR 2009; 193:28-39"Our purpose here is to discuss medical justification of the small potential risk associated with the ionizing radiation used in CT and to provide perspectives on practice specific decisions that can maximize overall patient benefits."
In Defense of Body CT
McCollough CH et al.
AJR 2009; 193:28-39"LNT was a useful model half a century ago. But current radiation protection concepts should be based on facts and on concepts consistent with current scientific results and not on opinions. Preconceived concepts impede progress; in the case of the LNT model, they have resulted in substantial medical, economic and other societal harm."
The Linear No-Threshold Relationship Is Inconsistent with Radiation Biologic and Experimental Data
Tubiana M et al
Radiology 2009; 251:13-22"Among humans, there is no evidence of a carcinogenic effect for acute radiation at doses of less than 100 mSv and for protracted irradiation at doses less than 500 mSv."
The Linear No-Threshold Relationship Is Inconsistent with Radiation Biologic and Experimental Data
Tubiana M et al
Radiology 2009; 251:13-22"The fears associated with the concept of LNT (linear no-threshold model) and the idea that any dose, even the smallest, is carcinogenic lack scientific justification."
The Linear No-Threshold Relationship Is Inconsistent with Radiation Biologic and Experimental Data
Tubiana M et al
Radiology 2009; 251:13-22"Irradiated cells protect themselves (a) by immediate defense, repair and damage removal mechanisms and (b) by delayed and temporary protection also renewed DNA damage, irrespective of its causes – that is through adaptive responses."
The Linear No-Threshold Relationship Is Inconsistent with Radiation Biologic and Experimental Data
Tubiana M et al
Radiology 2009; 251:13-22"In summary, excess cancer risks obtained in the Japanese atomic bomb survivors and in many medically and occupationally exposed groups exposed at low or moderate doses are generally statistically compatible. For most cancer sites the dose response in these groups is compatible with linearity over the range observed."
Risks Associated with Low Doses and Low Dose Rates of Ionizing Radiation: Why Linearity May Be (almost) the Best We can Do?
Little MP et al
Radiology 2009; 251:6-12Risks Associated with Low Doses and Low Dose Rates of Ionizing Radiation: Why Linearity May Be (almost) the Best We can Do?
Little MP et al
Radiology 2009; 251:6-12VS
The Linear No-Threshold Relationship Is Inconsistent with Radiation Biologic and Experimental Data
Tubiana M et al
Radiology 2009; 251:13-22- Predicting the Future
- More detailed informed consent to patients (problem-do we really know the truth)
- Credit card collection of patient exposures over a lifetime become part of the medical record
- Legal suits begins over lack of true informed consent, monitoring of dose and “unnecessary studies”