How accurate do dosages have to be?

Older regulations used to require a therapeutic dosage of radionuclides must be within 10%. Updated regulations for the NRC say that injected dosages must be withing 20% of the prescription (or withing the specified range).

Note that this 20% requirement applies to both diagnostic and therapeutic dosages of nuclear material.

For further reading see:

What is the HU of bone?

For board exams and as a rule of thumb we say bone is 1000 HU; however that is not the complete truth. While a compact bone is typically high in HU, making a measurement of cancellous bone is much more difficult. It inherently incorporates other types of low HU tissue that is averaged into the measurement and reduces the average HU value. This measurement can be as low as 200 HU depending on the placement and size of the ROI (see image below)

As an analogy think about how your dish sponge might be yellow in color, but it is inherently filled with colorless air and so any measurement of its “yellowness” will be brought down by the colorless air inside of it.

Mean is 314HU and even max is only 560HU due to partial volume average within the pixel.
Placing the ROI only in compact bone gives us a mean value of 1441 HU which is in line with values published in text books

Contrast to Noise Ratio (CNR)

Recommended Reading with Nice Images:
Radiographic Techniques, Contrast, and Noise in X-Ray Imaging

Contrast can be thought of as the percent difference in values between two things in an image. The bigger the difference in value between two objects, the greater the contrast. For example consider a white square (thing 1) on a gray background (thing 2). If the brightness value of the background is 10 and the brightness value of a square is 25, then the contrast is (25-10)/10 = 1.5 or 150%. If contrast is 0, you cannot detect a difference between the two objects.

Noise is the amount of variation present when the signal should be exactly the same. Noise is random and a simplistic way of measuring it is to take the standard deviation of the brightness values. As noise increases, things become harder to detect.

Contrast-to-Noise Ratio (CNR):
CNR attempts to give a single number combining the effects of contrast and noise. CNR is typically a relative thing, thus we say if CNR increases it means that it will be easier to detect the difference between two things and if CNR decreases it will be harder. If the contrast between things is large, then you will still be able to see a difference if there is a lot of noise. If contrast is small, then noise must also be small in order to detect the difference between things.

Tissue Half-Value Layer

For our purposes, the half-value layer (HVL) is the thickness of material needed to reduce an x-ray beam to half its intensity. This value is determined by two factors: 1) Energy of the beam and 2) Material used.

Energy: As beam energy increases so does the penetration of the beam (i.e., more photons make it through the material). Thus, generally, increasing beam energy will also increase the HVL. For example, the tissue HVL at radiograph energies (~3cm) is ~3x greater than the HVL at mammography energies (~1cm).

Material: As a material increases in atomic number (Z) and density, more photons interact and thus less make it through the material. Thus, generally, higher Z materials and denser materials have a smaller HVL. For example, at diagnostic energies, the HVL for Lead is ~0.1mm, for Aluminum is ~7mm, and for Tissue is ~30mm (3cm).

Positron Emitter Production

Q: Are there any Rules of Thumb about how various isotopes are produced so I do not have to memorize all of them?

A: Yes, a good rule of thumb is that cyclotrons produce isotopes that decay by positron emission or electron capture while fission products often undergo beta minus decay. Cyclotrons bombard a target with protons, giving them too much positive charge and making them unstable. There are then two ways to get rid of the extra protons. 1) positron emission and 2) Electron capture.

Both In-111 and Ga-67 are produced in a cyclotron and decay by electron capture. F-18, O-15, N-13, and C-11 are produced in a cyclotron and decay by positron emission. It should be noted that it is possible for positron emitters to be produced by other means (e.g., Rb-82 and Ga-68 are produced from a generator). That is why it is a rule of thumb and not an absolute truth.