Renal MAG3 Scan — Dictation, Appropriateness, and Dose for Residents
The transplant team is on the phone. They have a 3-day post-op renal allograft patient with rising creatinine, and they need to know if it’s acute tubular necrosis (ATN) or rejection. The MAG3 scan just finished. Your attending is expecting a report with clear differential function, perfusion indices, and washout curves — and they want the key takeaway in the first line of the impression. You’ve got this.
When I was a fellow, the nucs rotation felt like a different language. Time-activity curves, T-max, T-half washout… it’s a lot to synthesize under pressure. The key is having a solid template that forces you to look for the right data points every time. This guide is built for that moment. For more tools like this, check out the residents and fellows resource hub we’ve put together with free calculators and references.
What a NM Renal Scan (MAG3) Covers and What Attendings Look For
A Technetium-99m MAG3 (mercaptoacetyltriglycine) renal scan is a dynamic functional study. Unlike CT or ultrasound which show anatomy, this study answers questions about physiology: how well is each kidney working, and is there a blockage? MAG3 is excreted via tubular secretion, which makes it the go-to tracer even in patients with poor renal function where DTPA (which relies on glomerular filtration) might fail.
Your attending expects the report to definitively answer the specific clinical question, which is usually one of the following:
- Differential Function: What percentage of the total renal function does each kidney contribute? This is critical before a partial nephrectomy.
- Obstruction vs. Dilation: Is that hydronephrosis on CT causing a real physiologic blockage? The Lasix challenge gives a quantitative answer.
- Transplant Evaluation: Is the new kidney working? We can differentiate between common complications like ATN, acute rejection, or a urinary leak.
- Renovascular Hypertension: The captopril challenge can unmask a hemodynamically significant renal artery stenosis.
The final report needs to be quantitative. Don’t just say “delayed excretion.” Say “T½ washout is greater than 20 minutes, consistent with obstruction.”
Radiology Report Template for NM Renal Scan (MAG3)
This template is a solid starting point for your personal macro. It includes the key quantitative elements that should be in every MAG3 report.
Technique
Radiopharmaceutical: [5-10] mCi of Technetium-99m MAG3 administered intravenously.
Hydration: The patient was hydrated with [500 mL of oral water / IV saline] prior to the study.
Diuretic Challenge: At [20] minutes post-injection, [0.5 mg/kg, up to 40 mg] of furosemide (Lasix) was administered intravenously.
Captopril Challenge: [e.g., A baseline study was performed, followed by administration of 25 mg of oral captopril. A post-captopril study was performed 60 minutes later.]
Imaging: Dynamic images were acquired in a 128×128 matrix for 30 minutes, with subsequent imaging for an additional 20 minutes following diuretic administration. Data was processed to generate time-activity curves and calculate differential function and washout times.
Findings
Perfusion: Both kidneys demonstrate prompt, symmetric perfusion.
(Or: Perfusion is asymmetrically decreased to the [left/right] kidney.)
Function (Cortical Uptake): Both kidneys demonstrate prompt, symmetric cortical uptake and concentration of the radiotracer.
(Or: The [left/right] kidney demonstrates [delayed/decreased] cortical uptake. The peak activity (Tmax) for the right kidney is [e.g., 4 minutes] and for the left kidney is [e.g., 15 minutes]. Normal is 3-5 minutes.)
Excretion (Clearance):
Pre-diuretic: There is prompt clearance of tracer from the renal parenchyma into the collecting systems bilaterally. [There is retention of tracer in the dilated left collecting system.]
Post-diuretic: Following Lasix administration, there is prompt washout from the right collecting system with a T½ of [e.g., 5 minutes]. There is delayed washout from the left collecting system with a T½ of [e.g., greater than 20 minutes].
(Normal T½ <10 min; Equivocal 10-20 min; Obstructed >20 min)
Differential Renal Function:
Right kidney: [%]
Left kidney: [%]
Impression
1. Differential renal function: Right kidney contributes [%], left kidney contributes [%].
2. Evidence of high-grade obstruction of the [left/right] kidney, with a post-diuretic T½ washout time of greater than 20 minutes.
(Or: No evidence of urodynamically significant obstruction. The dilated [left/right] collecting system demonstrates prompt washout after Lasix administration with a T½ of less than 10 minutes.)
(Or for transplant: Findings of delayed cortical uptake but preserved excretion are most consistent with acute tubular necrosis (ATN). No evidence of urinary leak.)
(Or for renovascular HTN: The post-captopril study demonstrates significantly reduced uptake and prolonged retention in the [left/right] kidney compared to baseline, a finding highly suggestive of hemodynamically significant [left/right] renal artery stenosis.)
Free Template Sources
Building your own macro library is a rite of passage. But you don’t have to start from scratch. Two great free repositories exist that are worth bookmarking. The RSNA maintains RadReport.org, which has a huge library of peer-reviewed templates. A similar excellent resource maintained by Australian radiologists is Radiology Templates.
The Next-Level Move: AI-Assisted Structured Reporting
The real bottleneck isn’t finding a template, it’s populating it accurately while you’re trying to read the scan. You see the delayed Tmax and poor washout, but then you have to stop, find the right fields in the template, and type in the numbers. It breaks your flow.
This is where new tools can make a huge difference. Instead of clicking through a structured template, you can simply dictate your positive findings in free form—”Left kidney shows delayed Tmax at 15 minutes with T-half washout greater than 20 minutes after Lasix, right kidney is normal. Split function 30 percent left, 70 percent right.” The AI then parses this and generates a perfectly structured report using pre-loaded ACR and SIR templates. It also helps surface relevant Clinical Decision Support (CDS) frameworks when they apply to a study, ensuring you don’t miss a key classification. For a MAG3 scan, the focus is less on a specific CDS popup and more on getting the quantitative data into the right structure, which the AI is designed to streamline.
When Should You Order a NM Renal Scan (MAG3)? ACR Appropriateness Criteria
The American College of Radiology (ACR) provides evidence-based guidelines to help clinicians choose the right test. For a patient presenting with renal failure, the decision often comes down to whether the question is anatomic or functional.
Per the ACR Appropriateness Criteria for Renal Failure, a renal scan is usually appropriate when the primary clinical questions are about differential renal function or potential obstruction. For instance, if ultrasound shows hydronephrosis, a MAG3 scan with Lasix is the definitive next step to determine if it’s truly obstructive. Similarly, before a surgeon performs a partial nephrectomy, they need to know the split function, which this study provides.
However, it’s not the first choice for everything. If the goal is to evaluate for renal scarring from pyelonephritis, a DMSA scan provides better cortical detail. For pure anatomic evaluation of the collecting system or to look for masses, a non-contrast CT, renal ultrasound, or MR Urography are better choices. MRU has the advantage of providing both anatomic and functional information without ionizing radiation, but it’s a longer and more expensive exam.
How Much Radiation Does a NM Renal Scan (MAG3) Deliver?
Patients often ask about radiation, and it’s our job to give them an accurate, contextualized answer. A typical Tc-99m MAG3 renal scan delivers an effective dose of 0.7 to 1.5 mSv.
This is a very low dose. To put it in perspective, it falls into the 1-10 mSv tier, which is comparable to the amount of natural background radiation a person receives over several months to a few years. It’s significantly less than a standard abdominal CT scan. The dose is optimized for the clinical question, with pediatric doses adjusted based on weight to adhere to the ALARA (As Low As Reasonably Achievable) principle.
| Imaging Study | Typical Effective Dose (mSv) | Comparison to Background Radiation |
|---|---|---|
| NM Renal Scan (MAG3) | 0.7 – 1.5 mSv | ~6 months |
| Chest X-ray (2 views) | ~0.1 mSv | ~10 days |
| CT Abdomen/Pelvis | ~10 mSv | ~3 years |
NM Renal Scan (MAG3) Imaging Protocol — Phases, Contrast, and Analysis
The protocol for a MAG3 scan is standardized but has key decision points depending on the clinical question. The core of the study is a dynamic acquisition that tracks the tracer as it is taken up by the kidneys and excreted into the bladder. Proper patient preparation, especially hydration, is critical for an accurate study.
| Phase | Key Parameters |
|---|---|
| Patient Prep | Hydration: 500 mL PO water or 250-500 mL IV saline 30 min prior. Patient must empty bladder immediately before scan. Hold ACE inhibitors if captopril challenge is planned. |
| Injection | Agent: Tc-99m MAG3. Activity: 5-10 mCi IV (adult). Detector: Posterior (or anterior for transplant kidney in iliac fossa). |
| Dynamic Acquisition | Perfusion phase: 1-2 sec/frame for 60 seconds. Functional phase: 15-30 sec/frame for 20-30 minutes. Matrix: 128×128. |
| Lasix Challenge | For obstruction. F+20 protocol: Give furosemide (0.5 mg/kg, max 40 mg) at 20 min. F0 protocol: Give furosemide with tracer for severely dilated systems. Image for another 20-30 min. |
| Captopril Challenge | For renovascular HTN. Perform baseline scan. Administer captopril (25-50 mg PO). Perform second scan 60 minutes later and compare. |
| Analysis | Place ROIs on kidneys. Generate time-activity curves. Calculate split function (from 1-3 min phase), Tmax (peak time), and T½ washout (post-Lasix). |
A common pitfall is choosing the wrong diuretic protocol. The F+20 protocol (giving Lasix 20 minutes after the tracer) is standard, as it ensures the collecting system is full of tracer before the diuretic challenge. However, in a patient with a massively dilated collecting system and very poor function, the tracer may never adequately fill the system; in these cases, an F0 protocol (giving Lasix at the same time as the tracer) may be preferred.
3+ months free for radiology residents and fellows
Look like a rockstar on your reports — dictate positive findings in free form, and the AI generates a structured report using ACR + SIR templates with the appropriate clinical decision support firing automatically. We’re offering extended free access to trainees to help you master your reporting workflow.
All we ask is feedback so we can keep improving the product for you. The signup is simple. No credit card, no long forms. To get set up, just provide these three items:
- Your PGY year (e.g., PGY-2, PGY-4)
- Your training type (radiology residency or fellowship specialty)
- Your training program / hospital name
Ready to give it a try? You can apply for the residents free-access program here.
Free GigHz Tools That Pair With This Article
Three free tools that complement the material above:
- ACR Appropriateness Criteria Lookup — Type an indication or clinical scenario in plain language and get the imaging studies the ACR rates for it, with adult and pediatric radiation levels. Built directly from 297 ACR topics, 1,336 clinical variants, and 15,823 procedure ratings.
- GigHz Imaging Protocol Library — A searchable library of 131 imaging protocols with the physics specs surfaced and the matching ACR Appropriateness Criteria alongside. Plain-English narratives readable in 60 seconds, organized by modality.
- GigHz Radiation Dose Calculator — Pick the imaging studies a patient has had and see total dose in millisieverts (mSv) with comparisons to natural background radiation, transatlantic flights, and chest X-rays. Useful for shared decision-making.
Frequently Asked Questions
Is it HIPAA-compliant?
Yes. The platform is designed for de-identified workflows by default. No PHI is required to generate a structured report from your findings. It operates as a co-pilot, not as part of the permanent medical record or PACS.
Do I need my hospital’s IT department to set this up?
No. It’s a secure, browser-based tool. There’s no software to install. It works on any computer, including the call-room PC or your personal laptop or iPad.
Does this replace PowerScribe or other dictation systems?
No, it works alongside them. You can dictate your findings, use the tool to structure them, and then copy/paste the final, clean report into your hospital’s official dictation system. It augments your workflow, it doesn’t replace it.
Can I use this on my phone or iPad?
Yes, the tool is fully responsive and designed to work on mobile and tablet browsers, which is perfect for reviewing a template or checking a classification on the go.
Can I customize the templates?
Yes. While the system comes pre-loaded with ACR and society-recommended templates, you can create, modify, and save your own templates to match your personal style or your institution’s specific requirements.
What happens after I finish residency or fellowship?
The extended free access is specifically for trainees. After you graduate, you can transition to a standard plan. Many attendings find the time savings and reporting consistency are well worth it in practice.
Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 7, 2026
