15.5 Non-Imaging Studies: Specimens, Counting, and Error Analysis
Key Takeaways
Non-imaging nuclear medicine includes plasma-clearance GFR, effective renal plasma flow, blood and plasma volume, red cell survival, absorption studies, thyroid probe uptake, and wipe-test counting—each depends on exact specimen timing rather than camera technique.
Every well-counter measurement requires matched geometry: same tube type, same sample volume, same position in the well, and the same count time for samples, standards, and background.
Net counts equal gross minus background, and the uncertainty of a background-corrected count is the square root of the sum of gross and background counts, so low-count samples over high backgrounds carry large percent error.
Percent error from Poisson statistics is approximately 100 divided by the square root of the counts, so quadrupling accumulated counts halves the percent error—count longer before reporting a borderline result.
Dilution-based volume measurements follow the principle that volume equals the total injected activity divided by the activity concentration in a well-mixed sample, which makes pipetting accuracy and standard preparation the dominant error sources.
15.5 Non-Imaging Studies: Specimens, Counting, and Error Analysis
Quick Answer: Non-imaging work is timing, geometry, and arithmetic. Net = gross − background. Uncertainty of a net count = √(gross + background). Percent error ≈ 100/√N. Volume measurements are dilution problems: volume = total activity ÷ concentration. One mistimed tube invalidates the study more thoroughly than any camera error.
The blueprint devotes two separate statements to this work: obtaining samples and data for non-imaging studies and evaluating the results of non-imaging studies. Section 15.4 introduced the topic within processing; here it gets full treatment.
The Study Families
| Study | Principle | Specimen pattern |
|---|---|---|
| Plasma-clearance GFR | Rate of disappearance of a filtered tracer (Tc-99m DTPA, I-125 iothalamate) from plasma | Timed post-injection blood samples (single- or multi-sample protocols) |
| Effective renal plasma flow (ERPF) | Clearance of a tracer extracted by tubular secretion (I-131 or Tc-99m MAG3 class) | Timed plasma samples ± camera-based curve |
| Plasma volume | Dilution of I-125 or I-131 human serum albumin | Injected standard plus a post-equilibration plasma sample |
| Red cell volume / mass | Dilution of Cr-51 or Tc-99m labeled autologous red cells | Standard plus post-equilibration whole-blood sample with hematocrit |
| Red cell survival | Serial disappearance of Cr-51 labeled cells | Blood samples over days to weeks |
| Vitamin B12 absorption (Schilling-class) | Urinary excretion of an orally absorbed labeled tracer | Timed 24-hour urine collection |
| Thyroid uptake | External probe counting (Section 12.4) | Neck, thigh, standard, room background |
| Wipe tests | Removable surface contamination | Wipe media counted against background |
Specimen Collection Technique
| Element | Rule |
|---|---|
| Timing | Record the actual clock time of injection and of every draw. Compute elapsed time from recorded times, never from the schedule you intended |
| Container | Use the anticoagulant the protocol specifies (heparin, EDTA) or a plain tube for serum; the wrong tube can hemolyze, clot, or alter the plasma-to-cell split |
| Volume | Fill to the marked volume; well-counter geometry assumes a consistent sample height |
| Labeling | Patient identifiers, draw time, and sample sequence on every tube before it leaves the room |
| Handling | Avoid hemolysis (no forceful aspiration through a small needle), mix gently, centrifuge per protocol, and store at the required temperature |
| Standard | Prepare the standard from the same injected preparation with a calibrated pipette, and record the dilution factor |
| Residual | Assay the syringe before and after injection; the injected activity is the difference, and an infiltrated or partially delivered dose invalidates every clearance number |
Contamination discipline: a hot glove touching the outside of a tube can add more counts than the sample contains. Change gloves between the standard bench and the patient specimens.
Well-Counter Geometry and Background
| Requirement | Reason |
|---|---|
| Same tube type and sample volume for samples, standards, and background | Counting efficiency depends on sample geometry in the well |
| Same position/depth in the well | Efficiency falls off toward the mouth of the well |
| Same count time | Compare counts, not mixed counts and rates |
| Correct energy window for the nuclide | Cr-51 (320 keV), I-125 (~27–35 keV), Tc-99m (140 keV) each need their own peak |
| Background count each session, and after any suspected spill | Room background drifts; a contaminated well silently inflates every result |
| Awareness of dead time | Very high-activity samples must be diluted, not counted "as is" |
Background Correction and Error Propagation
Net counts: N_net = N_gross − N_background (with matched count times).
Uncertainty of a net count: σ_net = √(N_gross + N_background) — the variances add even though the counts subtract.
Worked example. A wipe is counted for 1 minute: 2,500 gross counts. A 1-minute background is 400 counts.
- Net = 2,500 − 400 = 2,100 counts
- σ_net = √(2,500 + 400) = √2,900 ≈ 53.9
- Percent error = 53.9 ÷ 2,100 × 100 ≈ 2.6%
Now the same wipe with only 500 gross counts over the same 400 background:
- Net = 100 counts; σ_net = √900 = 30; percent error = 30%
Same instrument, same procedure, wildly different confidence. A "positive" wipe at 30% error is a count-longer decision, not a report.
The general rule: percent error ≈ 100/√N. Four times the counts halves the percent error. Reach the required precision by increasing count time or sample activity, not by rounding.
| Total counts | Approximate percent error |
|---|---|
| 100 | 10% |
| 1,000 | 3.2% |
| 10,000 | 1% |
| 40,000 | 0.5% |
Dilution Calculations
The dilution principle: volume = total activity administered ÷ activity concentration in a well-mixed sample.
Worked example — plasma volume. A standard is prepared by diluting 1 mL of the injected preparation to 1,000 mL and counting 1 mL of that dilution: 1,850 counts/min. The full injected activity therefore corresponds to 1,850 × 1,000 = 1,850,000 counts/min. A post-equilibration plasma sample (background corrected) counts 370 counts/min per mL.
Plasma volume ≈ 1,850,000 ÷ 370 ≈ 5,000 mL, adjusted for any protocol correction factors.
Error sources here are systematic, not statistical: an inaccurate pipette, an incompletely mixed dilution, an unrecorded residual in the syringe, or a sample drawn before equilibration will bias the answer far more than counting noise.
Evaluating and Escalating Results
| Red flag | Action |
|---|---|
| Sample drawn outside the protocol time window | Document the actual time; recompute or repeat rather than report to a standard reference range |
| Wrong tube or hemolyzed specimen | Redraw; do not "adjust" the number |
| Net count with unacceptably high percent error | Count longer or recount; escalate if the sample is exhausted |
| Result physiologically implausible (e.g., a plasma volume double the expected) | Check the dilution factor and residual assay first — arithmetic beats biology as an explanation |
| Suspected contamination of well or bench | Survey, decontaminate, recount background, and repeat affected samples |
Bottom line: the technologist owns the timing, the tube, the geometry, the background, and the error bar. Report the number only when you can defend all five.
A wipe sample yields 900 gross counts in one minute and the one-minute background is 500 counts. What is the approximate percent error of the net count?
About 3%
About 9%
About 2%
About 5%
Which practice is essential when counting patient specimens and a standard in a NaI well counter?
Use whatever tube is available as long as the activity is high enough
Count the standard on a different day so the counter has time to stabilize
Count the background only once per month to save time
Use identical tube type, sample volume, well position, and count time for samples, standards, and background
A plasma volume study uses a standard prepared by diluting 1 mL of the injected preparation to 1,000 mL; 1 mL of that dilution counts 1,500 cpm. The background-corrected plasma sample counts 500 cpm per mL. What is the calculated plasma volume?
3,000 mL
333 mL
1,500 mL
750,000 mL
Sections you finish are checked off in the contents.
You've completed this section
Continue exploring other exams